Bermuda Weather Service Glossary

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1 or 2 Literally 'one or two' during the forecast period.
300/500mb height Upper air patterns are analysed and forecast at these heights.  Wind and height fields at these heights, around 30000ft and 20000ft respectively, are used to establish weather patterns at the surface. The UKMO generally use 300mb while the US use 500mb. This is partly due to the different weather regimes they experience (the US generally experiencing more extremes).
850 hPa temperature This is the air temperature at around 5000ft (850 hPa)
A few Literally 'a few' (a small number of) during the forecast period.
Absolute Instability A condition that exists where the environmental lapse rate exceeds the dry adiabatic lapse rate
Advection (noun), advect (verb) The process of transport of an atmospheric property solely by the mass motion (velocity field) of the atmosphere. Often, particularly in synoptic meteorology, advection refers only to the horizontal or isobaric components of motion, that is, the wind field as shown on a synoptic chart. Regarding the general distinction between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere, while convection describes the predominantly vertical, locally induced motions.
Afternoon 12 noon to 6pm.
Aircraft turbulence Irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion, caused by a rapid variation of atmospheric wind velocities. Usually associated with larger cumulus clouds e.g. cumulonimbus.
Airmass A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region.
Aloft Refers to weather characteristics which are above the boundary layer (not in contact with the Earth's surface).
Altocumulus A layer of cumulus cloud at medium levels, 7000ft and above (formed largely of water droplets with some ice crystals)
Altocumulus Castellanus A type of altocumulus cloud with tower-like projections that billow upwards from the base of the cloud. They are evidence of mid-level instability and can be the precursor to the development of cumulonimbus clouds and associated thunderstorms (generally a summertime phenomenon).
Altostratus Medium layer cloud, roughly 7000ft to 20000ft (formed of water droplets and ice crystals), often bringing overcast conditions.
Ascat/Oscat wind field This is a diagram of a wind field over a swath of the earth, captured by a polar orbiting satellite.  It is a very useful tool for forecasters when wind data is hard to come by, such as the data sparse regions of the Atlantic Ocean. These diagrams are often used to perform a streamline analysis of the wind field (see streamlines for more information).
Atlantic Hurricane Season This is the time when tropical systems (e.g. tropical storm, hurricane) are most likely to occur. In the Atlantic, the season officially runs from 1st June until the 31st November. Note that tropical systems can and have occurred outside this period.
AWOS Automatic weather observing system, consisting of several automatic sensors across the Island. These are available to the public on the web
Backing winds Winds which are changing direction in an anti-clockwise fashion.
Baroclinic A baroclinic atmosphere is one for which the density depends on both the temperature and the pressure; contrast this with barotropic atmosphere, for which the density depends only on the pressure. In atmospheric terms, the barotropic zones of the Earth are generally found in the central latitudes, or tropics, whereas the baroclinic areas are generally found in the mid-latitude/polar regions.
Baroclinic leaf A synoptic-scale cloud pattern frequently observed in satellite imagery just prior to the onset of cyclogenesis.
Barotropic A barotropic atmosphere is one in which the pressure depends only on the density and vice versa, so that isobaric surfaces (constant pressure surfaces) are also isopycnic surfaces (constant density surfaces).
Beaufort wind scale A system of estimating and reporting wind speeds using a numerical scale ranging from 0 (calm) to 12 (hurricane). It was invented in the early nineteenth century by Admiral Beaufort of the British Navy and was originally based on the effects of various wind speeds on the amount of canvas that a full- rigged frigate of the period could carry. It has since been modified and modernized and in its present form for international meteorological use it equates 1) Beaufort force (or Beaufort number), 2) wind speed, 3) descriptive term, and 4) visible effects upon land objects or sea surface.
Bermuda-Azores High, also Bermuda High A semi-permanent high pressure system, usually at its strongest over Bermuda late Spring through early Autumn.  It brings Bermuda its most settled weather pattern, generally characterised by gentle south-easterly winds and fair weather cumulus.
Boundary layer The bottom layer of the troposphere that is in contact with the surface of the earth. It is often turbulent and is capped by a statically stable layer of air or temperature inversion
Broadscale (analysis) Looking at weather systems/patterns such as ridges and troughs on a planetary scale rather than a localised scale.  Useful for gaining an insight into the general weather trend over the next few days
BRWFS Bermuda Regional Wave Forecasting System.  A local wave model initialized using the WW3/GFS models or the GFDL model in possible tropical cyclone conditions
BUFKIT A product used to display weather models in the highest horizontal and vertical resolution
Calm seas (WMO code 0) (glassy) 0m or 0ft
Calm seas (WMO code 1) (rippled) 0-0.1m or 0-0.33ft
CAPE Convective Available Potential Energy, used to diagnose atmospheric instability
Cape Verde hurricane season The Cape Verde hurricane season is so named due to a greater likelihood of tropical waves (organised clusters of thunderstorms) moving off the coast of Africa and developing into tropical cyclones (e.g. tropical storms and hurricanes) as they move westwards past the Cape Verde Island and across the tropical Atlantic. This season typically begins in the latter part of August and continues through much of September. The height of the North Atlantic hurricane season occurs within this period.
CIMSS This acronym stands for the 'Cooperative Institute for Meteorological Satellite Studies'. This institute uses satellite data in order to measure several elements of the atmosphere, such as winds, vorticity, divergence etc.
Cirrostratus A layer of cirrus, often thick enough to obscure the sun, giving hazy conditions.
Cirrus High cloud, generally 20000ft or more (made up of ice crystals), often thin, patchy and wispy in nature.
Clear air turbulence (CAT) A higher altitude (6–15 km) turbulence phenomenon occurring in cloud-free regions, associated with wind shear, particularly between the core of a jet stream and the surrounding air.
Cloudy or overcast 8 oktas of cloud
Cloudy with sunny/clear periods Generally 6-7 oktas with occasional periods of 4 oktas or less.
CMC/GEM Canadian global weather model.
Col The point of intersection of a trough and a ridge in the pressure pattern of a weather map. It is the point of relatively lowest pressure between two highs and the point of relatively highest pressure between two lows. It is often (but not always) associated with relatively quiet weather and light winds.
Cold air advection (CAA) Usually refers to colder air moving into an area, based on 1000-500mb thickness or 850hPa temperature.
Collier Index An index designed to diagnose instability with respect to a water surface
Conditional Instability A condition that exists where the environmental lapse rate exceeds the moist/saturated adiabatic lapse rate but not the dry adiabatic lapse rate
Confluence The rate at which adjacent flow is converging along an axis oriented normal to the flow at the point in question.
CONUS CONtinental United States.
Convective Convective is mainly used in relation to showers or convection.  Showers develop due to convection, that is water heated, allowing it to evaporate and rise.  It eventually reaches an altitude where the water vapour is cooled to its dew point, producing a shower cloud.  Convection is the main source of rainfall during the summer months.
Convergence This is an area where winds/windfields converge. It can be at the surface or aloft. Convergence at the surface usually indicates cyclonic development.
Coriolis force The Coriolis effect is an apparent deflection of moving objects (air parcels) when they are viewed from a rotating reference frame (the spinning planet Earth).
Cumulonimbus A cumulus type of deep vertical extent, often reaching in excess of 30000ft. The depth of cloud inevitably generates a heavy shower (sometimes of hail), which often is accompanied by thunder and lightning due to oppositely charged water particles (in form of water and ice) at the base and top of the cloud. Strong gusts are often recorded at the surface, and the extreme weather this cloud produces is a significant hazard to aircraft.
Cumulus One of Bermuda's most common cloud types, consisting of individual, detached elements that are generally dense and possess sharp nonfibrous outlines. It is a low cloud which in its shallow form (cumulus humilis) provides fair weather (dry) conditions. As it increases in vertical extent (due to surface heating for example) it becomes cumulus mediocris, cumulus congestus and finally cumulonimbus. This increase in cloud depth brings an increasing risk of a rain shower, which can be very notable in the case of cumulonimbus (see definition).
Cut-off low A low (often in the upper levels of the atmosphere) that has grown out of a trough and become displaced out of the basic westerly current and lies equatorward of this current.
Cyclogenesis Any development or strengthening of cyclonic circulation in the atmosphere.
Dew Point A measure of the absolute humidity at the surface. Also the temperature to which a given air parcel must be cooled at constant pressure and constant water vapour content in order for saturation to occur (i.e. water vapour condensing into droplets or dew).
Diffluence The rate at which adjacent flow diverges along an axis oriented normal to the flow at the point in question; the opposite of confluence
Divergence This is an area where winds/windfields diverge. It can be at the surface or aloft. Divergence at the surface usually indicates anti-cyclonic development.
Double-barrelled low A closed cyclone that contains two smaller closed cyclones in the centre instead of the usual one.
Drizzle Very small, numerous, and uniformly distributed water drops that may appear to float while following air currents. It usually falls from low stratus clouds and is frequently accompanied by low visibility and mist or fog. By convention, drizzle drops are taken to be less than 0.5 mm (0.02 in.) in diameter. Larger drops are considered raindrops.
Drought An extended period of time with significantly below average rainfall and thus a deficiency in the water supply.
Dry adiabatic lapse rate (DALR) The dry adiabatic lapse rate (DALR) is the negative of the rate at which a rising parcel of dry or unsaturated air changes temperature with increasing height, under adiabatic conditions. Unsaturated air has less than 100% relative humidity, i.e. its temperature is higher than its dew point. The term adiabatic means that no heat transfer (energy transfer due to a temperature difference) occurs into or out of the parcel. Air has low thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small.
Dynamic Usually used in the context of dynamic rain, it is a description of rain generated by broadscale dynamics. Broadscale dynamics refer to weather generated by different air masses on a large scale, e.g. a weather front in the mid-latitudes.  Dynamic rain is the main source of precipitation in the winter months.
Easterly wave (or African easterly wave) Easterly waves, also known as African easterly waves in the Atlantic region due to the area they develop in, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which move from east to west across the tropics causing areas of cloudiness and thunderstorms. They are essentially another term for a tropical wave, and these features can act as precursors for tropical cyclone development.
ECMWF This is the European Centre for Medium range Weather Forecasting. This centre runs a global spectral model, which is one of the most powerful and computer-hungry models in the world. It has recently performed well when forecasting the development of tropical storms.
Ensemble Forecasting Numerical Weather Prediction models can occasionally provide poor forecasts due to inaccurate measurements regarding the initial state of the atmosphere. Ensemble forecasting attempts to mitigate this issue by running the model with several different initial conditions and then comparing the results of the ensemble "members". Higher confidence can be assumed when the spread of solutions is more tightly clustered. Taking the mean of the ensemble members can also yield a skillful forecast, especially when looking beyond 3 days.
Evening 6pm to midnight.
Explosive cyclogenesis Rapid extratropical cyclone development, called explosive cyclogenesis, is often associated with major winter storms and occurs when surface pressure falls by more than about 24 millibars per day.  It is typical of storms developing off the US east coast in winter time.  Cyclones associated with this are sometimes referred to as 'bombs', due to the rapid fall in pressure.
Extratropical storm A storm system/ cyclone which has not tropical or subtropical components to it. It is purely generated by upper air dynamics and baroclinicity (strong temperature gradients).
Eye The roughly circular area of comparatively light winds that encompasses the centre of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.
Eyewall An organised band or ring of cumulonimbus clouds that surround the eye, or light-wind centre of a tropical cyclone.
Fair Fair weather means mostly cloudy to cloudy skies, but with no precipitation.
Fair visibility 5/8 to 5 SM.
Fine Fine weather means mostly sunny to sunny skies with no precipitation.
Fog Water droplets suspended in the atmosphere close to the earth's surface (generally from surface to a few hundred feet) that affect visibility.  Usually reported when the visibility falls below 1km with humidity 90% or higher.
Forcing A term used to describe the situation where a change or an event in one part of the atmosphere causes a strengthening change in another part of the atmosphere. It is usually used to describe connections between upper, middle or lower levels (such as upper-level divergence causing lower level convergence in cyclone formation).
Frequent On average, 1 hour or less between weather events, over a period of at least 3 hours.
Frontogenesis The initial formation of a front or frontal zone.
Frontolysis The dissipation of a front or frontal zone.
FROPA Shorthand form for 'frontal passage'.
Fujita Scale The Fujita scale (F-Scale) is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determined by meteorologists and engineers after a ground or aerial damage survey, or both. The estimated wind speed is calculated using the following formula: V = 6.30(F+2) to power of 1.5 ms-1. A six-point scale has been developed that corresponds to the following wind-speed estimates: F0 (light damage): 18–32 m s-1 F1 (moderate damage): 33–49 m s-1 F2 (considerable damage): 50–69 m s-1 F3 (severe damage): 70–92 m s-1 F4 (devastating damage): 93–116 m s-1 F5 (incredible damage):117–142 m s-1. Although extremely dependent on the design of a structure and the tree type, the following visual characteristics of the damage have been assigned to the F-scale. F0 - Some damage to chimneys; branches broken; shallow-rooted trees knocked over. F1 - Surface of roofs peeled off; mobile homes pushed off foundations or overturned; moving autos pushed off road. F2 - Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted. F3 - Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown. F4 - Well-constructed houses leveled; structures with weak foundations blown off; large missiles generated. F5 - Strong frame houses lifted off foundations and carried considerable distances; automobile- sized missiles flying through the air for distances in excess of 100 m; trees debarked. Most tornadoes that occur in Bermuda would be rated F2 or less.
Fujiwhara effect The Fujiwhara effect or Fujiwara interaction is a type of interaction between two nearby cyclones (low pressures), causing them to appear to "orbit" each other.W hen the cyclones approach each other, their centers will begin orbiting cyclonically about a point between the two systems. The two cyclones will be attracted to each other, and eventually spiral into the center point and merge. When the two cyclones are of unequal size, the larger cyclone will tend to dominate the interaction, and the smaller cyclone will orbit around it. This effect can be related to tropical cyclones.
Funnel cloud A condensation cloud, typically funnel-shaped and extending outward from a cumuli-form cloud, associated with a rotating column of air that may or may not be in contact with the ground. If the rotation is violent and in contact with the ground, the vortex is a tornado.
Gale force Winds, either sustained or gust, of a magnitude of 34-47 knots.
Gale force wind 34 to 47 knots
Gale warning When winds of mean speed 34 to 47 knots are forecast to begin affecting the marine area within the next 36 hours a Gale Warning will be issued.
Geopotential The potential energy of a unit mass relative to sea level, numerically equal to the work that would be done in lifting the unit mass from sea level to the height at which the mass is located; commonly expressed in terms of dynamic height or geopotential height.
Geostrophic wind & balance The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effect and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars (lines of constant pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground or the centrifugal force from curved fluid flow. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation.
GFDL This is a high resolution model derived from the GFS used in potential or actual tropical cyclone situations.
GFS Global Forecasting System - one of the main US models used by NOAA.
Good visibility 5 SM or greater.
Gradient wind Any horizontal wind velocity tangent to the contour line of a constant-pressure surface (or to the isobar of a geopotential surface) at the point in question. This wind effectively follow the isobars.
Graupel Also known as soft hail, this refers to precipitation that forms when supercooled droplets of water are collected and freeze on a falling snowflake, forming a 2–5 mm (0.079–0.197 in) ball of rime. Graupel or small hail occasionally affects Bermuda in the winter time.
Grid point model A model where the prognostic field variables are represented on a grid, as gridpoints.  It is generally less accurate than a spectral model for the reasons explained under 'spectral model'.  However, it is less data/hardware hungry, ie more efficient to run.
Gustnado A specific type of short-lived, low-level rotating cloud that can form in a severe thunderstorm. The name is a portmanteau of "gust front tornado", as gustnadoes form due to non-tornadic cyclonic features in the downdraft from the gust (outflow) front of a strong thunderstorm, especially one which has become outflow dominated.
Hail Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. Hail measures between 5 and 200 millimetres (0.20 and 7.9 in) in diameter. A relatively rare phenomenon in Bermuda.
Halo Any of a family of coloured or whitish rings, arcs, pillars or spots of light that appear in the sky and are explained by the reflection or refraction of light by ice crystals (often in the form of cirrus cloud). They are usually found in the vicinity of the light source, the most important of which are the sun and moon. However, the colour is usually fairly pale, being best on a red edge next to the light. The exceptions, in having very good colors, are the circumhorizontal and circumzenithal arcs, the positions of which are not determined by the minimum angle of refraction. Halos that are white, or show the same color as the light source itself, are explained by the reflection of light off the crystal faces. Whether explained by reflection or refraction, the pattern that emerges depends upon the crystal type, crystal orientation (actually the probability of various orientations within a population of crystals), and the elevation angle of the light (sun).
Hazy sunshine or thin overcast Indicates thin cirrus or cirrostratus.
Heavy Over 10 mm/hr and/or Poor Visibility <5/8 SM
High (Anticyclone) An area of high pressure, referring to a maximum of atmospheric pressure in two dimensions (closed isobars) in the synoptic surface chart, or a maximum of height (closed contours) in the constant-pressure chart.
High seas (WMO code 7) 6-9m or 19.8-29.7ft
HORACE The name of Bermuda Weather Service's forecaster workstation, designed and supported by the UK Met Office.
hPa (Hectopascal) A measure of pressure. It is the equivalent measure to millibars (mb).
Hurricane A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.
Hurricane Awareness Week (HAW) This is a community outreach event, held annually, usually the last week of May, ahead of the official start to the Atlantic Hurricane Season which begins on 1st June. Various media tools and channels are used (video, briefings, TV, radio and internet) to reach out to the community and stake holders regarding all aspects of tropical season, e.g. season ahead, reminders on how to read TUBs, hurricane preparedness etc.
Hurricane Category 1 A hurricane with sustained winds 74-95 mph, 64-82 kt, or 119-153 km/hr. Very dangerous winds which will produce some damage.
Hurricane Category 2 A hurricane with sustained winds 96-110 mph, 83-95 kt, or 154-177 km/hr. Extremely dangerous winds which will cause extensive damage.
Hurricane Category 3 (Major) A 'major' hurricane with sustained winds 111-130 mph, 96-113 kt, or 178-209 km/hr. Devastating damage will occur.
Hurricane Category 4 (Major) A 'major' hurricane with sustained winds 131-155 mph, 114-135 kt, or 210-249 km/hr. Catastrophic damage will occur.
Hurricane Category 5 (Major) A 'major' hurricane with sustained winds greater than 155 mph, greater than 135 kt, or greater than 249 km/hr. Catastrophic damage will occur.
Hurricane force wind 64 knots and greater
Hurricane warning A Hurricane Warning shall be issued when: Sustained winds of 64 knots or more are expected to affect Bermuda within 36 hours, associated with a tropical cyclone or a subtropical cyclone.
Hurricane watch A Hurricane watch is issued when: Sustained winds of 64 knots or more may possibly affect Bermuda, within 48 hours, associated with a tropical cyclone or a subtropical cyclone.
HWRF High resolution tropical storm/hurricane model (see WRF for more details).
In the vicinity Not necessarily affecting the area but visible to the naked eye.
Infra-red satellite This satellite data is useful at night as it does not depend on sunlight. The satellite measure the temperature of the cloud and ground, and this produces a similar image to visible satellite imagery, although usually more coarse.  High cloud will appear white (very cold) while warm land/sea appear black.
Instant occlusion This is a cloud feature which forms when a Comma-like cloud feature merges with a frontal cloud band. The Comma is situated within a cold air mass behind a Cold Front, so it is in unstable environment, which leads to the formation of convective cloudiness. The process is one where stratified frontal cloudiness and convective cloudiness merge. It is very much a winter-time phenomenon.
Intermittent this is generally used to describe rain which is not falling continuously. The rain is stopping and starting repeatedly with dry periods in between.
Inversion An inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a "temperature inversion", i.e., an increase in temperature with height, or to the layer ("inversion layer") within which such an increase occurs. An inversion can lead to pollution in highly populated urban areas, such as smog being trapped close to the ground, with possible adverse effects on health. An inversion can also suppress convection by acting as a "cap". If this cap is broken for any of several reasons, convection of any moisture present can then be released into thunderstorms.
Inverted trough An atmospheric trough which is oriented opposite to most troughs of the mid-latitudes. Most (but not all) inverted troughs are tropical waves (also commonly called easterly waves). Most troughs of low pressure in the mid-latitudes of the Northern Hemisphere are characterized by decreasing atmospheric pressure from south to north while inverted troughs are characterized by decreasing pressure from north to south. All troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.
Invest A weather system for which a tropical cyclone forecast centre (e.g. NHC) is interested in collecting specialised data sets (e.g. microwave imagery) and/or running model guidance. Once a system has been designated as an invest, data collection and processing is initiated on a number of government and academic web sites, including the Naval Research Laboratory (NRL) and the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS). The designation of a system as an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone. Operational products should be consulted for this purpose.
Isobar A line of equal or constant pressure; also known as an isopleth of pressure.
Isolated Individual features that are isolated from each other, but may affect the area intermittently.
Jet entrance/exit This is an area observed in upper air data, usually using the wind field, where the upper winds accelerate and decelerate notably.  This acceleration and deceleration is around the core of the jet stream or a jet streak.  These areas have a direct impact on the weather at the surface.  A right jet entrance area and a left jet exit area indicate cyclogenesis at the surface, while a left jet entrance and a right jet exit indicate anticyclonic development at the surface.
Jet stream Relatively strong winds concentrated within a narrow stream in the atmosphere. While this term may be applied to any such stream regardless of direction (including vertical), it is coming more and more to mean only a quasi-horizontal jet stream of maximum winds embedded in the midlatitude westerlies, and concentrated in the high troposphere. Mini jet streams are sometimes referred to as jet streaks.
Kalman Filter A 4-D data assimilation method that provides an estimate of the model state by evolving explicitly the error co-variance of the state estimate. This method is applied to atmospheric data assimilation problems (e.g. variables such as wind and temperature in weather forecasting). The filter estimate is based on all data observed up to and including the current time.
K-Index (KI) An index designed to measure atmospheric instability. In Bermuda, a value of 30 or higher suggests at least a moderate risk of airmass thunderstorms.
Knot The knot is a unit of speed equal to one nautical mile (which is defined as 1.852 km) per hour, approximately 1.151 mph.
Landfall As defined by the National Hurricane Center - The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water
Late night 12 midnight-6am
Lee trough A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced. It can also be apparent across the higher ground so some of the Caribbean region, including Hispaniola. Its formation may be explained thermodynamically by the warming due to adiabatic compression of the sinking air on the lee side of the mountain range, or dynamically by generation of cyclonic circulation (cyclogenesis) by the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope. Alternatively, the latter viewpoint is often expressed as the conservation of potential vorticity, where the vertical stretching of the columns is compensated by an increase in their relative vorticity.
Lift/ascent Upward vertical motion in the atmosphere, typically associated with unsettled weather
Lifted Index [LI] An index designed to measure atmospheric instability. When the index falls to zero and below, there is an increasing risk of airmass thunderstorms.
Light (intensity) Up to 2 mm/hr rainfall rate and/or Good Visibility >5 SM (statute miles).
Light wind 1 to 9 knots
Low (Cyclone) An area of low pressure, referring to a minimum of atmospheric pressure in two dimensions (closed isobars) on a constant-height chart or a minimum of height (closed contours) on a constant-pressure chart.
Macroscale Meteorological expression referring to synoptic events occurring on a scale of thousands of kilometers, such as warm and cold fronts.
Madden-Julian oscillation (MJO) The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall. The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30–60 days. The wet phase can enhance tropical cyclogenesis in the Atlantic basin as the MJO pulse moves eastwards across this region.
Mainly or mostly sunny MAINLY or MOSTLY SUNNY/CLEAR is equal to 2 to 3 oktas.
Major Hurricane This is a tropical cyclone that possesses wind speeds of at least 96 kt (178 km/h, 111 mph) and is classified as a Category 3 hurricane or above.
Meridional flow A term used when describing the upper air pattern. Meridional means strong troughs and ridge spanning many degrees of latitude. This correlates to a very changeable weather pattern at the surface.
Mesoscale Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers, including thunderstorms, squall lines, fronts, precipitation bands in tropical and extratropical cyclones
Mesoscale convective system (MCS) A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one  direction.
METAR This is the abbreviation for a Meteorological Terminal Air Report; also known as Aviation Routine Weather Reports. At LF Wade International airport these reports are recorded every hour + 55 mins e.g. 1155Z, 1255Z etc.
Meteorogram A chart in which meteorological variables are plotted against time.
Microburst A microburst is a very localised column of sinking air (a downdraft from the base of a shower cloud, such as cumulonimbus), producing damaging divergent and straight-line winds at the surface that are similar to, but distinguishable from, tornadoes, which generally have convergent damage. There are two types of microbursts: wet microbursts and dry microbursts. They go through three stages in their life cycle: the downburst, outburst, and cushion stages. The scale and suddenness of a microburst makes it a significant hazard to aircraft due to the low-level wind shear caused by its gust front.
Microscale Atmospheric motions with spatial scales of 2 km or less (e.g. an individual cumulonimbus cloud).
MIDS (Map Info Display System) A computer interface that allows the forecaster to see where lightning is occurring in real time across the western Atlantic - a very useful tool for approaching thundery fronts/troughs or a developing summer-time cumulonimbus situation.
Millibars/hPa/Inches Mercury A universal measure of air pressure, either in millibars or inches of mercury.  Some European countries use hectopascals, hPa.
Mix of sun & cloud (day), variable cloudiness (night) Generally equal periods of 3 oktas or less of cloud and periods of 6 oktas or more.
Moderate >2 to 10 mm/hr and/or Fair Visibility 5/8 to 5 SM.
Moderate seas (WMO code 4) 1.25-2.5m or 4.1-8.3ft
Moderate wind 10 to 19 knots
Moist or saturated adiabatic lapse rate (SALR) When the air is saturated with water vapour (at its dew point), the moist adiabatic lapse rate (MALR) or saturated adiabatic lapse rate (SALR) applies. It varies strongly with the moisture content, which depends on temperature, and lightly with pressure from +3 °C/km (high temperature near surface) to +9.78 °C/km (very low temperature). However, at temperatures above freezing it is usually near +4.9 °C/km (+2.7 °F/1000 ft or +1.51°C/1000 ft). The reason for the difference is that latent heat is released when water condenses. Even though there are no more than 10 grams of water in a kilogram of air at 15 degrees Celsius, water's high heat of vaporization creates a significant release of the energy when it condenses (and is an important source of energy in the development of thunderstorms). Until the moisture starts condensing, the parcel of air cools at the dry adiabatic lapse rate. Thus, any air that is unsaturated can be assumed to be 'dry'.
Morgan's cloud This is a local weather effect which occurs when winds blow across the length of the Island, i.e. an east-northeasterly wind or a west-southwesterly wind. Even in a relatively dry airmass, the heat of the Island will help generate cumulus cloud, which may become thick enough to produce an isolated shower at the far end of the Island to which the wind is blowing to. In summer time this effect, coupled with copious amounts of moisture in the lower atmosphere, can be pronounced enough to produce a heavy shower and even thunderstorm.
Morgan's Cloud (Story) "Morgan's Cloud" is an old Bermudian name for a towering cumulus cloud formed in moist, unstable air over the sun-heated Island in the summertime. The cloud often produces heavy showers or even a few rumbles of thunder. It usually forms in a light southwest wind, and can be especially dark and ominous over the east end. These clouds are also fairly common over other tropical islands in the trade winds, and are known as "island streamers." They can continue downwind from their source for many miles.

The local name involves the American Revolution, stolen British gunpowder, and a certain ship captain named Morgan. The local supporters of George Washington and his Continental Congress found a way to "trade" the British powder, stored near Tobacco Bay in St. George's, for food and supplies which were badly needed locally due to a British embargo of trade with America.

On a hot August night in 1775, Captain Morgan (not to be confused with Henry Morgan the pirate of the previous century) and his associates, using Morgan's ships the "Charleston and Savannah Packet" and "Lady Catherine" safely transported 100-odd barrels of stolen British gunpowder to George Washington which helped the American patriotic army win the Revolution.

A local resident, upon hearing afterward of the powder theft, proclaimed that it would be a dark cloud that would hang over Bermuda.

So, on hot summer days, when that dark cloud hangs over St. George's, oldtime Bermudians say it is Captain Morgan's ghost returning to haunt the scene of the crime, perhaps even to apologize.

The story is told at length in William Zuill's book "Bermuda Journey" and is also mentioned in Wilfred Brenton Kerr's "Bermuda and the American Revolution: 1760-1783".

Wayne Little, Meteorological Technician
Morning 6am to 12 noon.
MOS MOS (model output statistics) - a type of statistical post-processing and a class of techniques used to improve numerical weather prediction models' ability to forecast various weather parameters by relating model outputs to observational or additional model data.
Mostly cloudy (with sunny breaks for daytime) 6-7 oktas of cloud.
NAM-WRF North Americal Mesoscale Model/Weather Research and Forecasting, a regional gridpoint model, run to 84 hours, used in weather prediction
NCEP National Centre for Environmental Prediction - it delivers national and global weather, water, climate and space weather guidance, forecasts, warnings and analyses to its Partners and External User Communities, and is part of NOAA.
NHC National Hurricane Centre, based in Miami, Florida. This has the main responsibility for forecasting tropical weather in the Atlantic and east Pacific basins. Bermuda Weather Service takes it lead from the NHC when issuing tropical advisories and is in constant discussion with NHC when a storm looks like being a potential threat.
No threat (for Tropical Cyclone) The threat parameter 'No Threat' is issued in tropical update bulletins when a Tropical cyclone has past its closest point of approach to Bermuda and is moving away under the influence of a well defined upper southwesterly steering flow.
NOAA National Oceanic and Atmospheric Administration, a US government agency.
Nor'easter A low pressure system that forms off the US east coast, usually near Cape Hatteras (30 degrees N - 35 degrees N, and moves north or north-east towards the Canadian Maritimes. A nor'easter is so named because the winds in a nor'easter come from the northeast.
Not at this time (for Tropical Cyclone) The threat parameter 'Not at this time' is issued in tropical update bulletins when a Tropical cyclone has the potential to affect Bermuda, but not within the next 72 hours, or is forecast to pass by Bermuda within the next 72 hours but expected to miss by more than 400 nm.
NOTAM This is the aviation communications code word for 'notice to airmen'. It is a notice filed with an aviation authority to alert aircraft pilots of potential hazards along a flight route or at a location that could affect the safety of the flight.
Occasional More than 1 hour between weather events such as a shower.
Occlusion When a cold front catches up with a warm front the warm air is literally occluded out. This means the warm air is driven aloft.  Instead of the warm front providing a boundary between warm and cool air and the cold front providing a boundary between cool and warm air, the occluded front combines the two to provide just one boundary of cool and warm air.
Ocean-effect Showers or thunderstorms that develop due to absolute instability with respect to the ocean surface.
Okta A term used to describe amounts of cloud in the sky.  The sky is effectively divided into 8 equal portions.  The amount of cloud in the sky is then described as 1 okta (1/8th) to 8 oktas (8/8th), 1 okta being just a few clouds dotting the sky, 8 oktas being overcast conditions.
Omega block An upper level blocking pattern, preventing the normal west to east progress of cyclones and anticyclones where a closed anticyclone (cut off high) or amplified ridge remains nearly stationary with closed cyclones (cut off lows) on each side.
OPC Ocean Prediction Centre, part of NOAA.
Outbreaks used to describe rainfall as in 'outbreaks of rain'. This correlates to periods of rain affecting the area on and off during a given time. The rain is not 'continuous'.
Outflow boundary A surface boundary formed by the horizontal spreading of thunderstorm-cooled air. It generally occurs on the forward side of a line of thundery showers associated with a frontal trough. This boundary acts as a focus for new convection. Outflow boundaries may be short-lived, or last for longer than a day.
Overnight Midnight to 6am.
Partly sunny (daytime)/Partly cloudy (night) 4-5 oktas of cloud
Patchy Used to describe the spatial coverage of rain, as in 'patchy rain'. The rain is by definition patchy in its areal coverage and not continuous as in 'widespread rain'.
Periods Longer than 30 minutes in duration.
Phase space diagrams These are diagrams showing how much cold/warm air is associated with a particular low pressure system through all levels of the atmosphere. The cold air is referred to as cold core, the warm as warm core. Depending on the balance between the warm and cold air and the heights at which they occur, this technique can help to decipher whether a developing system is attaining tropical or sub-tropical characteristics.  Very useful tropical or extra-tropical transition of a weather system.  It is a technique developed by academics at Florida State and Penn State Universities.
Phenomenal seas (WMO code 9) >14m or >46.2ft
PIREP The aviation communications code word and commonly used contraction for pilot report.
Polar jet This is a jet stream running eastwards across the mid-latitudes. It is a boundary between high pressure in the Arctic and areas of low pressure running east along the mid latitudes.
Poor visibility <5/8 SM.
Porgy Holes A porgy is a well-known local food fish. Porgies are usually found in the deeper sand holes among the outer reefs. These sandy "porgy holes" normally show up as a darker blue among the brownish hues of the coral. Translated into local weather lore, a porgy hole is simply a patch of blue sky appearing in a thick cloud layer. The inference is that there may be a clearing trend, or even some sunny breaks developing.

Wayne Little, Meteorological Technician
Post-tropical cyclone A former tropical cyclone. This generic term describes a cyclone that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds. Note that former tropical cyclones that have become fully well as remnant lows...are two classes of post-tropical cyclones.
Potential Threat (for Tropical Cyclone) The threat parameter 'Potential Threat' is issued in tropical update bulletins when the centre of a Tropical system is expected to pass within 400 nm of Bermuda within 72 hours.
PRE (Predecessor Rainfall Event) PREs are coherent areas of heavy rainfall that occur well in advance of tropical cyclones (TCs), and are separate from the main precipitation shields. PREs tend to develop along pre-existing frontal boundaries a few hundred miles north of tropical cyclones, as prevailing winds funnel huge amounts of moisture northward from the cyclone and concentrate it along the frontal zone.
Precipitable water (also known as PWAT or precipitable water vapour.) The total atmospheric water vapour contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section. The total precipitable water is that contained in a column of unit cross section extending all of the way from the earth's surface to the “top” of the atmosphere. In actual rainstorms, particularly thunderstorms, amounts of rain very often exceed the total precipitable water vapor of the overlying atmosphere. This results from the action of convergence that brings into the rainstorm the water vapour from a surrounding area that is often quite large. Nevertheless, there is general correlation between precipitation amounts in given storms and the precipitable water vapour of the air masses involved in those storms.
Pressure gradient force The pressure gradient force is not actually a 'force' but the acceleration of air due to pressure difference (a force per unit mass). It is usually responsible for accelerating a parcel of air from a high atmospheric pressure region to a low pressure region, resulting in wind.
PROB30/PROB40 Code used in a TAF to indicate probability of a weather event when it is not certain, e.g. PROB30 of rain equates to a 30% chance of rain.
Progged Forecasted
Progressive Pattern This is generally when the jet stream is active, with troughs and ridges moving quickly eastwards across the globe. Can be associated with meridional flow.  Typical in winter months. 
PVA/NVA Positive Vorticity Advection/Negative Vorticity Advection, a diagnositic for vertical motion, which correlates directly to development at the surface. An area of PVA generally infers cyclonic (unsettled) development at the surface, while NVA infers anticyclonic (settled) development at the surface.
Quantitive precipitation forecast (QPF) A model prediction of the amount of precipitation that will fall at a given location in a given time interval.
Radar echoes/reflectivity, dBZ See notes about radar, already available on website:
Radiosonde An expendable meteorological instrument package, often borne aloft by a free-flight balloon, that measures, from the surface to the stratosphere, the vertical profiles of atmospheric variables and transmits the data via radio to a ground receiving system. Radiosondes typically measure temperature, humidity, and pressure.
Rain Precipitation in the form of liquid water drops that have diameters greater than 0.5mm, or, if widely scattered, the drops may be smaller. Rain shower, or showery rain, is a brief period of rainfall in which intensity can be variable and may change rapidly. Typically, any precipitation that is prolonged (longer than 15 minutes) is described as rain.
RAOB A radiosonde observation.
Relative humidity Used to describe how much moisture there is in the air.
Relax Usually used to describe a trough which is weakening, e.g. 'the trough will relax over the next few days leading to mainly fair weather'.
Retrograde the movement of a pressure system in a direction opposite to the mean flow in which it is embedded
Rex block An upper level blocking pattern, preventing the normal west to east progress of cyclones and anticyclones where a closed anticyclone (cut off high) is meridionally aligned with a closed cyclone (cut of low).
Ridge An elongated area of relatively high atmospheric pressure, almost always associated with and most clearly identified as an area of maximum anticyclonic curvature of wind flow. It is usually associated with settled weather conditions.
Rough seas (WMO code 5) 2.5-4m or 8.3-13.2ft
Saffir-Simpson Hurricane Wind Scale The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale – originally developed by wind engineer Herb Saffir and meteorologist Bob Simpson – has been an excellent tool for alerting the public about the possible impacts of various intensity hurricanes.
Saharan Air Layer (SAL) Dry and/or dusty air in the lower atmosphere (~600-850 hPa or ~4,500-1,500 m), usually emanating from the Sahara region of Africa. This layer of air can inhibit tropical development in the Atlantic.
Scattered Individual features that are widely distributed in extent and are likely to affect the area frequently.
Severe weather warning Thunderstorms can develop into severe thunderstorms producing a variety of severe weather. In addition, non-thunderstorm phenomena (e.g. ‘gustnadoes’) can produce severe weather. Severe weather is defined as one or more of the following: 1) Wind gusts greater than 50 knots 2) Hail greater than 3/4 inch 3) Tornado or tornadic waterspout (tornado over water) not to be confused with funnel clouds or waterspouts (which are not regarded as severe weather). 4) Severe Squall: A sudden increase of wind speed by at least 16 knots, the mean wind speed rising to 34 knots or more and lasting for at least one minute. Once severe weather conditions are identified on Doppler radar, or are strongly suspected to have developed in the proximity of the Island and are considered a definite threat, a Severe Weather Warning shall be issued.
Severe weather watch Thunderstorms can develop into severe thunderstorms producing a variety of severe weather. In addition, non-thunderstorm phenomena (e.g. ‘gustnadoes’) can produce severe weather. Severe weather is defined as one or more of the following: 1) Wind gusts greater than 50 knots 2) Hail greater than 3/4 inch 3) Tornado or tornadic waterspout (tornado over water) not to be confused with funnel clouds or waterspouts (which are not regarded as severe weather). 4) Severe Squall: A sudden increase of wind speed by at least 16 knots, the mean wind speed rising to 34 knots or more and lasting for at least one minute. If within the next six hours any of the above conditions are forecast as possible, a Severe Weather Watch is issued.
Sferic or spheric A broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Generally used as a term for where a lightning strike is located.
Shark oil barometer This is a traditional Bermuda barometer, using the colouration of the oil of a shark's liver to determine whether a storm is coming. For more information please go to 'shark oil barometers' on the following web page -
Sharpen Usually used to describe a trough which is strengthening, e.g. 'the trough will sharpen over the next day or so, leading to increasingly unsettled weather'.
Shower Precipitation from a convective cloud; showers are characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.
SIGMET SIGMET or Significant Meteorological Information, is a weather advisory that contains meteorological information concerning the safety of all aircraft. There are two types of SIGMETs - convective and non-convective. The criteria for a non-convective SIGMET to be issued are severe or greater turbulence over a 3,000-square-mile (7,800 km2) area, severe or greater icing over a 3,000-square-mile (7,800 km2) area or instrument meteorological conditions (IMC) over a 3,000-square-mile (7,800 km2) area due to dust, sand, or volcanic ash. A Convective SIGMET is issued for convection over the Continental U.S. Convective SIGMETs are issued for an area of thunderstorms affecting an area of 3,000 square miles (7,800 km2) or greater, a line of thunderstorms at least 60 nm long, severe thunderstorms or embedded thunderstorms affecting any area that are expected to last 30 minutes or longer. Severe thunderstorms are characterized by tornado(s), hail 3/4 inches or greater, or wind gusts 50 knots or greater. A Convective SIGMET is valid for 2 hours and they are issued every hour + 55 min.
Significant Wave Height The average height of the highest third (33 1/3%)of the waves. This is an internationally recognised standard for wave reporting and forecasting, representing the wave height that an observer would report from the deck of a vessel. not to be confused with dominant, maximum or average wave heights (which are less prevalent measures of wave height).
Significant Wave Period An arbitrary period generally taken as the period of 1/3 highest waves of a given wave group.
Skew-t plot/Tephigram/Profile This is a plot of the data received from a radiosonde balloon.  The two main variables plotted against height or altitude are air temperature and dew point temperature.  The plot is one of the forecaster's most useful tools, giving a detailed analysis of the cloud structure aloft.  It can also be used to predict fog, temperature and cloud heights.
Slight seas (WMO code 3) 0.5-1.25m or 1.65-4.1ft
Small craft warning When winds of mean speed 20 to 33 knots and/or seas of 9 feet or greater are forecast to begin affecting the marine area within the next 36 hours a Small Craft Warning will be issued.
Smooth seas (WMO code 2) (wavelets) 0.1-0.5m or 0.33-1.65ft
SPECI In the METAR observation program, a SPECI is a surface observation issued on a non-routine basis as dictated by changing meteorological conditions.
Spectral model A model in which the prognostic field variables are represented as sums of a finite set of spectral modes rather than at gridpoints. The spectral modes may be Fourier modes in the one-dimensional case or double Fourier modes or spherical harmonics in the two-dimensional case. The advantage of a spectral model is that horizontal derivatives can be calculated exactly for the spectral modes represented in the model and thus the model error is confined only to the unrepresented higher spectral modes beyond the model's spectral truncation.
Sprinkle Popular term for a very light shower of rain.
Squall A sudden increase of wind speed by at least 16 knots, the speed rising to 22 knots or more and lasting for at least one minute. It is often accompanied by showers or thunderstorms.
Stable airmass An airmass which is generally undergoing subsidence, and therefore has little chance of producing any significant shower activity. A ridge at medium levels can be associated with this.
Stagnant Pattern Jet stream is weak, with slow movement of weak troughs and ridges. Typical in summer months.
Stevenson's screen A type of instrument shelter. The shelter is a wooden box painted white with double louvered sides and mounted on a stand 122 cm (4 ft) above the ground. In addition to the dry- and wet-bulb thermometers, it usually contains maximum and minimum thermometers.
Storm force Winds, either sustained or gust, of a magnitude of 48-63 knots.
Storm force wind 48 to 63 knots
Storm Surge An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
Storm warning When winds of 48 knots mean speed or greater are forecast to begin affecting the marine area within the next 36 hours a Storm Warning will be issued.
Straight-line winds When the US National Weather Service conduct storm damage surveys they distinguish between straight-line wind and wind produced from a tornado. Straight-line wind damage will push debris in the same direction the wind is blowing (hence the term straight-line). Tornado damage will scatter the debris in a variety of different directions since the winds of a tornado are rotating violently. This type of survey can be used to determine if straight-line wind occurred instead of a tornado or vice versa. Straight-line wind intensity can be as powerful as a tornado. Because of this some people may believe a tornado occurred when it reality one did not occur.
Stratocumulus A layer of cumulus cloud, often with little or no breaks in it, giving the impression of overcast skies or sunny breaks at best.
Stratus Low layer cloud, generally around 1000ft or less (formed of water droplets), often bringing overcast conditions, mainly occurring in early Spring time.
Streamlines A method of analysing a windfield in order to clearly present converging or diverging winds. This technique is heavily used in summer shower (convection) situations which are poorly modelled by computer models. Converging streamlines indicate an area of lift (rising air) and thus shower development.
Strike As defined by the National Hurricane Center - For any particular location, a hurricane strike occurs if that location passes within the hurricane's strike circle, a circle of 125 n mi diameter, centered 12.5 n mi to the right of the hurricane center (looking in the direction of motion). This circle is meant to depict the typical extent of hurricane force winds, which are approximately 75 n mi to the right of the center and 50 n mi to the left.
Strong wind 20 to 33 knots
Subsidence/descent Downward vertical motion in the atmosphere, typically associated with fair weather
Subtropical cyclone A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.
Subtropical depression A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.
Subtropical jet A jet stream running eastwards in the subtropics.  It is a boundary between subtropical high pressure and low pressure area along the equator.
Subtropical storm A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 km/hr) or more.
Subtropics The subtropics are the geographical and climatical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S. The term "subtropical" describes the climatic region found adjacent to the tropics, usually between 20 and 40 degrees of latitude in both hemispheres.
Sunny with cloudy periods Generally 3 oktas or less of cloud with occasional periods of 6 oktas or more.
Sunny/clear SUNNY or, for night-time, CLEAR is equal to 0 to 1 okta of cloud.
Supercell An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min). Most rotating updrafts are characterized by cyclonic vorticity. The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells often propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, small hail, and possible tornadoes.
Super-geostrophic (wind) Any wind of greater speed than the geostrophic wind required by the pressure gradient.  Winds emanating from a high pressure cell are often super-geostrophic due to the additional centrifugal force (see geostrophic wind definition).
Swell Surface gravity waves on the ocean that are not growing or being sustained any longer by the wind. Generated by the wind some distance away and now propagating freely across the ocean away from their area of generation, these waves can propagate in directions that differ from the direction of the wind.
Synoptic chart (Also called surface map, sea level chart, sea level pressure chart.) An analyzed chart of surface weather observations. Essentially, a surface chart shows the distribution of sea level pressure, including the positions of highs, lows, ridges, and troughs and the location and character of fronts
Synoptic weather observation A surface weather observation, made at periodic times (usually at three-hourly and six-hourly intervals specified by the World Meteorological Organization), of sky cover, state of the sky, cloud height, atmospheric pressure reduced to sea level, temperature, dewpoint, wind speed and direction, amount of precipitation, hydrometeors and lithometeors, and special phenomena that prevail at the time of the observation or have been observed since the previous specified observation.
TAF Terminal aerodrome forecast - a coded forecast (24hr forecast duration) issued 4 times a day at 00z 06z 12z and 18z. It is used by the aviation industry.
The Reef For the purpose of the marine forecast, the reef can be interpreted as being the main northern reef, rather than the narrow fringing reefs on South Shore. Note that no forecast is specifically for sea state in enclosed waters, such as the Great Sound. These sea states would be covered by "Seas inside the reef....".
Theta-w A value in Celsius/Fahrenheit, which acts as a marker for how warm or cold an airmass is.  A warm, tropical airmass may have a theta-w of 18-20C or more, while a cold polar or continental maritime airmass may have a theta-w of 4C or less (for the Bermuda area).
Threat (for Tropical Cyclone) The threat parameter 'Threat' is issued in tropical update bulletins when effects from a tropical system are possible within 72 hours.
Thunderstorm In general, a local storm, invariably produced by a cumulonimbus cloud and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. It is usually of short duration, seldom over two hours for any one storm. A thunderstorm is a consequence of deep atmospheric instability. A strong convective updraft is a distinguishing feature of this storm in its early phases. A strong downdraft in a column of precipitation marks its dissipating stages. Thunderstorms often build to altitudes of 40000–50000 feet in midlatitudes and to even greater heights in the Tropics; only the great stability of the lower stratosphere limits their upward growth. A unique quality of thunderstorms is their striking electrical activity.
Thunderstorm advisory Some specialised agencies, together with mariners and many members of the public, require notification of the expected occurrence of thunderstorms well in advance. Accordingly, advisories are issued if there is a moderate to strong forecast probability of thunderstorms affecting Bermuda and/or the marine area (up to 25 miles offshore). TS advisories are issued up to 36 hours ahead of the expected occurrence, depending on the degree of confidence, but preferably at least six hours ahead as the main purpose is to enable recipients to pre-plan their activities and avoid potentially dangerous situations.
Today 6am-6pm
Tonight 6pm-6am
Tornado A violently rotating column of air, in contact with the surface, pendant from a cumuliform cloud, and often (but not always) visible as a funnel cloud. Wind speeds are sometimes estimated on the basis of wind damage using the Fujita scale.
Total Totals Index (TT) An index designed to measure atmospheric instability. In Bermuda, a value of 50 or higher suggests at least a moderate risk of airmass thunderstorms.
Trade winds The wind system, occupying most of the Tropics, that blows from the subtropical highs toward the equatorial trough; a major component of the general circulation of the atmosphere. The winds are northeasterly in the northern hemisphere.
Tropical cyclone A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).
Tropical depression A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.
Tropical disturbance A discrete tropical weather system of apparently organized convection -- generally 100 to 300 nmi in diameter -- originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
Tropical storm A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 km/hr) to 63 kt (73 mph or 118 km/hr).
Tropical storm warning A Tropical Storm Warning is issued when: Sustained winds of 34 to 63 knots are expected to affect Bermuda, within 36 hours, associated with a tropical cyclone or a subtropical cyclone.
Tropical storm watch A Tropical Storm Watch is issued when: Sustained winds of 34 to 63 knots will possibly affect Bermuda, within 48 hours, associated with a tropical cyclone or a subtropical cyclone.
Tropical Update Bulletins (TUBs) Tropical Update Bulletins (TUBs) include a map of the Tropical Cyclone current and forecast positions and a text summary on location, strength, movement, distance and threat, all relative to Bermuda. TUBs are normally produced four times per day (around 0300, 0900, 1500, 2100 UTC), on receipt of the US National Hurricane Center (NHC) bulletins. This information is also available on the NHC website. When a Tropical Cyclone threatens a land area, intermediate advisories are issued by NHC, however, intermediate TUBs will only be required if Bermuda is under ‘threat’ – (see threat definitions). The style of the map is essentially the same as the one utilized by NHC on their web site, showing the forecast ‘best track’ and the swath encompassing the area of potential track error, based on statistics. Additionally, since 2010, the radius rings of 34 knots, 50 knots and 64 knots are outlined on the initial position of the Tropical Cyclone and the swath of potential error is applied to the 34 knot wind forecast, in a lighter shading than that of the initial potential track error swath.
Tropical wave A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere. It is often the precursor of a tropical cyclone.
Tropics The tropics are a region of the Earth by the Equator. They are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23° 26′ 16″ (or 23.4378°) N and the Tropic of Capricorn in the southern hemisphere at 23° 26′ 16″ (or 23.4378°) S.
Troposphere That portion of the atmosphere from the earth's surface to the tropopause; that is, the lowest 10–20 km (6–12 mi) of the atmosphere; the portion of the atmosphere where most weather occurs.
Trough (also Trof) An elongated area of relatively low atmospheric pressure; the opposite of a ridge. It is usually associated with unsettled conditions.
Tsunami warning The procedure to issue a Warning will be followed when there is evidence proving the existence of a Tsunami (via the West Coast Alaska Tsunami Warning Centre or the Pacific Tsunami Warning Centre) and after consultation with the Bermuda Weather Service Director/Deputy Director and the EMO.
Tsunami watch In general, a Tsunami Watch will be issued before a Tsunami Warning. However this is not a hard rule, and under certain circumstances a Tsunami Warning may be issued without a related Watch. The procedure to issue a Watch will be followed when Bermuda Weather Service have been alerted to the possible existence of a Tsunami via the 'West Coast Alaska Tsunami Warning Centre' or the 'Pacific Tsunami Warning Centre'.
UKMET/UKMO The UK Meteorological Office's spectral global weather model, run to 144 hours, used in weather prediction
Unified Model or UM The Unified Model is a Numerical Weather Prediction and climate modelling software suite originally developed by the United Kingdom Met Office, and now both used and further developed by many weather-forecasting agencies around the world. The Unified Model gets its name because a single model is used across a range of both timescales (nowcasting to centennial) and spatial scales (convective scale to climate system earth modelling). The models are grid-point based, rather than wave based, and are run on a variety of supercomputers around the world. The Unified Model atmosphere can be coupled to a number of ocean models. At the Met Office it is used for the main suite of Global Model, North Atlantic and Europe model (NAE) and a high-resolution UK model (UKV), in addition to a variety of Crisis Area Models and other models that can be run on demand. Similar Unified Model suites with global and regional domains are used by many other national or military weather agencies around the world for operational forecasting.
Unstable airmass An airmass which is generally undergoing divergence aloft, allowing the air to rise and condense into cloud and showers. A low or trough at medium levels can be associated with this.
Upper air This is the upper atmosphere, generally 500mb and above.  Useful when making a broadscale analysis as large features such as upper ridges and troughs are very evident.
Upper low Also called upper-level low, upper cyclone, high-level cyclone or low aloft. It is a cyclonic circulation existing in the upper air, specifically as seen on an upper-level constant-pressure chart. Upper lows often support deep convection (showers, thunderstorms) due to the atmospheric instability associated with them.
Upper ridge A pressure ridge existing in the upper air.
Upper trough A pressure trough existing in the upper air.
UV UV (Ultraviolet radiation) - Wavelengths of UV radiation range from 5 to 400 nm, which may be further subdivided into the UV-A, UV-B, and UV-C ranges. UV radiation contains about 9% of the total energy of the solar electromagnetic spectrum. Such radiation has marked actinic and bactericidal action, and produces fluorescence in a number of substances. Ultraviolet radiation from the sun is responsible for many complex photochemical reactions characteristic of the upper atmosphere, for example, the formation of the ozone layer through ultraviolet dissociation of oxygen molecules followed by recombination to form ozone. The absorption of UV by stratospheric ozone and upper atmospheric oxygen is sufficiently strong that very little ultraviolet radiation with wavelengths shorter than about 300 nm reaches the earth's surface.
UV Index The ultraviolet index or UV Index is an international standard measurement of the strength of ultraviolet (UV) radiation from the sun at a particular place and time. The scale was developed by Canadian scientists in 1992, then adopted and standardized by the UN's World Health Organization and World Meteorological Organization in 1994. It is primarily used in daily forecasts aimed at the general public. The UV Index is designed as an open-ended linear scale, directly proportional to the intensity of UV radiation that causes sunburn on human skin. For example, if a light-skinned individual (without sunscreen or a suntan) begins to sunburn in 30 minutes at UV Index 6, then that individual should expect to sunburn in about 15 minutes at UV Index 12 — twice the UV, twice as fast. The purpose of the UV Index is to help people effectively protect themselves from UV radiation, of which excessive exposure causes sunburn, eye damage such as cataracts, skin aging, immunosuppression, DNA damage, and skin cancer. Public health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat) when the UV Index is 3 or higher. The UV Index is a linear scale, with higher values representing a greater risk level of sunburn (which is correlated with other health risks) due to UV exposure. An index of 0 corresponds to zero UV radiation, as is essentially the case at night. An index of 10 corresponds roughly to midday summer sun with a clear sky when the UV Index was originally designed (Toronto 1992). Now summertime index values in the teens are common for tropical latitudes, mountainous altitudes, and areas with above-average ozone layer depletion.
Vaisala Lightning Detector A local lightning detector with a radius of around 15 nm.
Veering winds Winds which are changing direction in a clockwise fashion.
Very high seas (WMO code 8) 9-14m or 29.7-46.2ft
Very rough seas (WMO code 6) 4-6m or 13.2-19.8ft
Virga Wisps or streaks of water or ice particles falling out of a cloud, but evaporating before reaching the earth's surface as precipitation. Frequently seen trailing from altocumulus and altostratus clouds, but also discernible below the bases of high-level cumuliform clouds from which precipitation is falling into a dry subcloud layer.
Visible satellite Satellite data which is available during daylight hours, it is a view of the earth from a satellite, available when sunlight lights the surface of the earth. It is very similar to the view of the earth from space, except it is generally black and white.
Visual Weather This is the forecasting computer platform which the Bermuda Weather Service meteorologists use.
Vorticity This is basically a measure of how much a column of air is spinning.  Depending on direction of spin and speed of spin, a variety of surface conditions can be surmised, such as the development of low or high pressure at the surface.
Warm air advection (WAA) Usually refers to warmer air moving into an area, based on 1000-500mb thickness or 850hPa temperature.
Warm Seclusion A relatively rare case of the process of occlusion, where the point at which the cold front first overtakes the warm front (or quasi-stationary front) is at some distance from the apex of the wave cyclone.  When an isolated mass of warm air is completely surrounded by colder air this is referred to as a warm seclusion.  Warm seclusions may have cloud-free, eye-like features at their centre (reminiscent of tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong convection. A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics.
Warm sector The sector, in a horizontal plane, between a warm front and a cold front. It is generally characterised by low cloud and warm, humid conditions. The low cloud can be thick enough to produce light showery rain or drizzle.
Water vapour satellite The water vapor satellite image displays the water vapor concentration in the atmospheric layer between 600 and 300 millibars, or approximately 4000 to 9000 meters above the surface of the earth. This is the middle and upper parts of the troposphere, a key region for storm development and growth.  It is useful as a tool as it displays the large scale features of upper troughs and ridges very well, as well as offering insight into possible storm development.
Waterspout In general, any tornado over a body of water. In its most common form, a non-supercell tornado over water. Such events consist of an intense columnar vortex (usually containing a funnel cloud) that occurs over a body of water and is connected to a cumuli-form cloud. Waterspouts exhibit a five-stage, discrete life cycle observable from aircraft: 1) dark-spot stage; 2) spiral pattern stage; 3) spray-ring stage; 4) mature or spray-vortex stage; and 5) decay stage. Waterspouts occur most frequently in the subtropics during the warm season. Funnel diameters range from a few up to 100 m or more; lifetimes average 5–10 minutes, but large waterspouts can persist for up to one hour.
Wave height The vertical distance between a wave crest and the preceding or following wave trough.
Wave period This is the time taken between when any part of a wave passes a fixed point and when the same part of the next wave passes that point.
Weather front (warm and cold) The boundary between two air masses of different characteristics, e.g. cold/warm air, clear/cloudy, dry/showers.
Widespread Large unbroken features leading to long periods of rain.
Wind shear Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear has a significant effect during take-off and landing of aircraft due to their effects on control of the aircraft
WMO The World Meteorological Organisation, based in Geneva, Switzerland. It is an intergovernmental organisation and a specialised agency of the United Nations with a membership of 188 Member States and Territories which include Bermuda.
WRF The Weather Research and Forecasting (WRF) model is a specific computer program with a dual use for forecasting and research. It was created through a partnership that includes the National Oceanic and Atmospheric Administration (NOAA), the National Center for Atmospheric Research (NCAR), and more than 150 other organizations and universities in the United States and abroad. WRF is one of the latest numerical program models to be adopted by NOAA's National Weather Service as well as the U.S. military and private meteorological services. It is also being adopted by meteorological services worldwide. A specialized version of the program, the Hurricane Weather Research and Forecasting (HWRF) model became operational in 2007.
WW3 A global wave model, run to 126 hours, initialized using GFS variables
Z (Zulu time) / UTC (Universal Time Constant) A standard of time based on GMT (Greenwich Mean Time), which is constant all over the world, ie. 1000Z in Bermuda is the same as 1000Z in the UK.
Zonal Zonal flow is when the upper pattern is relatively unperturbed, with weak troughs and ridges.