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Weather Glossary

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A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

-A-

Acid Rain -

Acid deposition - commonly called acid rain - is caused by emissions of sulphur dioxide and nitrogen oxides. Although natural sources of sulphur oxides and nitrogen oxides do exist, more than 90% of the sulphur and 95% of the nitrogen emissions occurring in eastern North America are of human origin. These primary air pollutants arise from the use of coal in the production of electricity, from base-metal smelting, and from fuel combustion in vehicles. Once released into the atmosphere, they can be converted chemically into such secondary pollutants as nitric acid and sulfuric acid, both of which dissolve easily in water. The resulting acidic water droplets can be carried long distances by prevailing winds, returning to Earth as acid rain, snow, or fog.

Advection -

Transport of an atmospheric property by the wind. See cold advection, moisture advection, warm advection.

Air Mass -

An air mass, by definition, is a large dome of air which has similar horizontal temperature and moisture characteristics. Often, a front separates two different air masses. Fronts are very narrow zones of transition. In other words, temperatures can change dramatically with short horizontal distances near fronts. Fronts are usually anywhere from 10 kilometers to hundreds of kilometers wide, while air masses can be thousands of kilometers wide.

Air Pressure - or atmospheric pressure.

Air pressure is the force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.

Anemometer -

An instrument to measure wind speed. Wind directions are always reported as the direction winds are coming from - a southerly wind pushes air from the south to the north.

Anticyclonic Rotation -

Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.

Atmosphere -

The air surrounding and bound to the earth. The mass of air held close to the earth by gravity. The atmosphere is subdivided into four sections: the troposphere- from the earth's surface to an altitude of about 10 km; the stratosphere - from 10 km to 50 km; the mesosphere - from 50 km to 80 km; and the thermosphere- beyond 80 km.

Aurora Borealis - Also known as the northern lights -

The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colours.

Automated Weather Station -

An unmanned station with various sensors that measure weather elements such as temperature/wind/pressure and transmit these readings for use by meteorologists.

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-B-

Backing Winds -

Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.

Baroclinic Zone -

A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems.

Barometer -

An instrument used to measure air pressure. The international standard of measurement is the kilopascal although millibars and inches of mercury are also commonly used.

Beaufort Scale -

A scale assigned to wind force. See this link for an exact definition.

Blizzard -

A severe storm lasting four or more hours. It is characterized by low temperatures, strong winds and poor visibility due to blowing snow. True blizzard conditions are most common on the prairies of Canada and the United States. Blizzards are a rare occurrence on the west coast and in Atlantic Canada. The Environment Canada has listed the following general criteria for issuing blizzard warnings in Canada:

visibility less than 1 kilometer,
wind speeds greater than 40 kilometers per hour,
high wind chill values.

Boundary Layer -

In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.

There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.

Bust -

[Slang], an inaccurate forecast ~ usually a situation in which significant weather is expected, but does not occur.

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-C-

Cb -

Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil shaped top. Also known as a "thunderhead".

Chinook -

Chinooks occur when a mountain range is exposed to a strong winds blowing at right angles, or near right angles to the direction of the mountain ridge. Moist air is forced up the mountains bringing both cloud and precipitation to the windward side. The descending air then becomes warmer and drier as it is forced down the leeward side of the mountains. The relatively warm, dry gusty winds that occasionally occur to the leeward side of mountain ranges around the world are known by many names. In Canada and the northern United States, they are referred to as Chinooks. In the southern states, they are known as Santa Ana and in parts of Europe, foehn winds.

Climate -

The prevalent or characteristic meteorological conditions, and their extremes, of any place or region.

Cloud -

A visible cluster of tiny water and/or ice particles in the atmosphere.

Cloud Formations -

Please see this online cloud atlas for photos and descriptions of all the various cloud types.

Cirrus -

High-level clouds (5,000 metres or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

Cold Advection -

Transport of cold air into a region by horizontal winds.

Cold-air Funnel -

A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Cold Front -

The leading edge of a cooler airmass.

Cold Pool -

A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

Comma Cloud -

A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.

Convection -

The physical process by which vapour becomes liquid or solid; the opposite of evaporation.

Convection -

Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Coriolis Effect -

In synoptic scale weather systems (hurricanes and large mid-latitude storms), the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. The air flowing around a hurricane spins counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere (as does the earth, itself). In both hemispheres, this rotation is called cyclonic. If the earth did not rotate, the air would flow directly in towards the low pressure center, but on a spinning earth, the Coriolis force results in the are arcing in towards the low pressure center. The coriolis force is of much too small a magnitude to have any relevance to the direction of rotation in a sink or toilet.

Cumulus -

Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus.

Cyclogenesis -

Development or intensification of a low-pressure center (cyclone).

Cyclone -

In the Northern Hemisphere, a closed counter-clockwise movement of air-known as a circulation - around a low pressure centre; usually called a low The term is frequently misused to describe a tornado.

*Cyclonic Circulation (or Cyclonic Rotation) -

Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all storms and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, sometimes rotate anticyclonically (clockwise). Compare with anticyclonic rotation.

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-D-

Dew Point -

As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower.

Dew Point (or Dew-point Temperature) -

A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

Diurnal -

Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night).

Doppler Radar -

Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).

Downdraft -

A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm.

Downstream -

In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Drizzle -

Precipitation from stratus clouds consisting of numerous minute, fine water droplets which appear to float. In drizzle, the droplets are much smaller than in rain.

Drought -

An extended period of dry weather.

Dust Devil -

A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather.

Dust Whirl -

A rotating column of air rendered visible by dust.

Dynamics -

Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

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-E-

El Nino -

Spanish for the "Christ Child". Fishermen in Peru and Ecuador used to use this term to refer to a warming of coastal waters around Christmastime that occurs most years. It has come to be used as a term for abnormal warming events which occur, on an average of two or three times a decade and typically last for a few seasons. It has important consequences to climate as well as for ocean states (fishing etc). The US government has a very interesting site dedicated to El Nino.

Environment Canada -

The federal government department responsible for issuing weather forecasts and weather warnings in Canada.

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-F-

Flurry -

Precipitation in the form of snow from a convective cumulus- type cloud. Flurries are characterized by the suddenness with which they start and stop, by their rapid changes in intensity, and usually by rapid changes in the appearance of the sky.

Fog -

A cloud based at the earth's surface consisting of tiny water droplets or, under very cold conditions, ice crystals or ice fog; generally found in calm or low wind conditions. Under foggy conditions, visibility is reduced to less than one kilometer.

Forecast -

A forecast provides a description of the most significant weather conditions expected during the current and following day. The exact content depends upon the intended user, such as the Public or Marine forecast audiences.

F Scale -

See Fujita Scale.

Freezing Rain -

Rain which freezes on impact to form a coating of ice upon the ground and on the objects it strikes. A freezing rain warning is usually issued when slippery driving and walking conditions are expected, and/or when freezing rain may damage trees, power lines, or other structures.

Front -

A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

Front -

A front is the boundary between two different air masses. A cold front is the leading edge of an advancing cold air mass, while a warm front is the trailing edge of a retreating cold air mass.

Frost -

Water vapour which deposits directly as a solid on a surface colder than the surrounding air and which has a temperature below freezing. It is not frozen dew. A Killing Frost is a frost severe enough to end the growing season.

Fujita Scale (or F Scale) -

A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:

F0 (weak):	40- 72 mph, light damage.
F1 (weak):	73-112 mph, moderate damage.
F2 (strong):	113-157 mph, considerable damage.
F3 (strong):	158-206 mph, severe damage.
F4 (violent):	207-260 mph, devastating damage.
F5 (violent):	261-318 mph, (rare) incredible damage.

All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

Funnel Cloud -

Cloud extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.

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-G-

Gale -

A strong wind. A gale warning is issued for expected winds of 34 to 47 knots (63-87km/h) over the water.

Greenhouse Effect -

The greenhouse effect is the name applied to the process which causes the surface of the earth to be warmer than it would have been in the absence of an atmosphere because it receives energy from two sources:

The sun and the atmosphere. Radiation is not trapped, and the atmosphere does not behave as a greenhouse and the greenhouse gasses do not behave as a blanket ~ the name "greenhouse effect" is somewhat of a misnomer..

Gust -

A sudden, brief increase in wind speed, generally less than 20 seconds.

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-H-

Hail -

Precipitation in the form of lumps of ice associated with thunderstorms. Hail size usually ranges from that of a small pea to the size of cherries, but has been observed as large as oranges. Hail occurs most frequently during the summer when thunderstorm activity is at a peak. Extensive damage in the agricultural industry is caused each year by hailstorms.

Halo -

In ages past, the huge rings or haloes around the sun or the moon were thought to portend everything from storms to great personal disasters. We now know that they are the optical result of the refraction of light from the sun or moon by ice crystals in the very high cloud (9,000 meters) called cirrus or cirrostratus.

On occasion, only two bright spots on either side of the sun can be seen. These are known as sun dogs and are caused when the ice crystals occur in a certain uniform arrangement.

Haze -

Fine dust or salt particles dispersed through a portion of the atmosphere which reduce visibility. Haze is distinguished from fog by its bluish or yellowish tinge.

High Pressure -

In the Northern Hemisphere, an area of high atmospheric pressure with a closed, clockwise movement of air (circulation). Also known as an anticyclone.

Humidity -

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Hurricane -

(also known as: Typhoon, Tropical Cyclones and Willy-Willies)

Tropical storms with wind speeds of 64 knots (117km/h) up to 240 knots (414 km/h) that can be thousands of square kilometers in size. Such systems usually have a lifespan of several days.

In the North Atlantic, the hurricane season is from May to November, but the majority of storms occur in August, September and October. Although the east coast is the area of Canada most frequented by hurricanes, these storms still average less than one per year over the Atlantic Provinces and coastal waters. While there have been as many as five in one year, several years can pass with no tropical storms.

A hurricane warning is issued in the Marine Forecast if winds are expected to exceed 64 knots (115 km/h).

Hygrometer -

An instrument used to measure humidity.

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-I-

Insolation -

Incoming solar radiation. Solar heating; sunshine.

Instability -

The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for thunderstorms to develop - the greater the instability, the greater the potential for severe thunderstorms. See lifted index.

Inversion -

Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs.

Isobar -

A line connecting points of equal pressure.

Isodrosotherm -

A line connecting points of equal dew point temperature.

Isohyet -

A line connecting points of equal precipitation amounts.

Isopleth -

General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

Isotach -

A line connecting points of equal wind speed.

Isotherm -

A line connecting points of equal temperature.

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-J-

Jet Stream -

Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.

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-K-

Kilopascal -

The internationally recognized unit used by the Atmospheric Environment Service for measuring atmospheric pressure.

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-L-

Leeward -

Situated away from the wind; downwind - opposite of windward.

Lifted Index (or LI) -

A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

Lightning -

Generally, any and all of the various forms of visible electrical discharge produced by thunderstorms.

Low-level Jet -

A region of relatively strong winds in the lower part of the atmosphere.

Low Pressure Area -

or a "Low" An area of low atmospheric pressure that has a closed counter-clockwise circulation in the Northern Hemisphere. Also known as a cyclone.

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-M-

Marine Wind Warnings -

Small Craft Warning - issued if winds are forecast to be in the range of 20 to 33 knots inclusive.

Gale Warning - issued if winds are forecast to be in the range of 34 to 47 knots inclusive.

Storm Warning - issued if the winds are forecast to be in the range of 48 to 63 knots inclusive.

Hurricane Force Wind Warning - issued for winds of 64 knots or greater.

Maximum/Minimum thermometer -

a thermometer that marks the lowest temperature (minimum) or highest temperature (maximum) since the previous reading (usually 1 day).

Meridional Flow -

Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.

Mesoscale -

Size scale referring to weather systems smaller than synoptic-scale systems but larger than single storm clouds. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines are an example of mesoscale weather systems.

Meteorologist -

A person who studies meteorology. There are many different paths within the field of meteorology. For example, one could be a research meteorologist, radar meteorologist, climatologist, or (my personal favourite) operational meteorologist.

Meteorology -

Meteorology is the study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.

Mist -

Consists of microscopic water droplets suspended in the air which produce a thin greyish veil over the landscape. It reduces visibility to a lesser extent than fog.

Monsoon -

The word "monsoon" appears to have originated from the Arabic word mausim which means season. It is most often applied to the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other. As monsoons have come to be better understood, the definition has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia and Africa and the adjacent seas and oceans. It is within these regions that the most vigorous and dramatic cycles of weather events on the earth takes place.

- from The Elementary Monsoon by Peter Webster.

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-N-

Nocturnal -

Related to nighttime, or occurring at night.

Normal -

The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.

Northern Lights -

See aurora borealis.

Nowcast -

A short-term weather forecast, generally out to six hours or less.

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-O-

Orographic -

Related to, or caused by, physical geography (such as mountains or sloping terrain).

Orographic Lift -

Lifting of air caused by its passage up and over mountains or other sloping terrain.

Outflow Winds -

Winds that blow down fjords and inlets from the land to the sea. When cold arctic air flows from the interior of BC onto the coast, the wind speeds through mainland inlets can reach over 100 km/h.

Ozone -

A pungent-smelling, slightly bluish gas which is a close chemical cousin to molecular oxygen. About 90% of the earth's ozone is located in a natural layer far above the surface of the globe, in a frigid region of the atmosphere known as the stratosphere. Here in this outer region it protects the earth and its inhabitants from the harmful effects of ultraviolet radiation from the sun.

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-P-

Popcorn Convection -

Clouds, showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Precipitation -

The precipitation of water from the atmosphere in the form of hail, mist, rain, sleet, and snow. Deposits of dew, fog, and frost are excluded.

Probability of Precipitation -

Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period. Measurable means at least 0.2 mm of rain or the water equivalent of snow. For example, a 40% probability of rain today means there are 4 chances in 10 that it will rain.

-Q-

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-R-

Radar -

An entire radar glossary is available.

Radial Velocity -

Component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, cannot be detected.)

Radiosonde -

An instrument lifted into the air via balloon that measures various atmospheric parameters such as temperature, pressure wind and humidity.

Rain -

The wet stuff.

Rainbow -

Rainbows occur when sunlight is refracted and then reflected by raindrops. The raindrops act like a prism, breaking the light into the colours of a rainbow, with red on the outer, and blue on the inner edge. On occasion the light can be reflected from both the front and back of the raindrops and two rainbows are visible, with the colour bands in the second opposite to those in the primary rainbow.

Rainbows can be seen when the sun is shining and the air contains water spray or raindrops. This condition occurs frequently during or immediately following showers. Rainbows are always observed in the portion of the sky opposite the sun. The sun, the observer's eye, and the centre of the rainbow arc always fall on a straight line.

Rain Gauge -

An instrument used to measure rainfall amounts.

Reflectivity -

Radar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates.

Relative Humidity -

A ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

Report -

A weather report is a statement of the actual weather conditions observed at a specific time at a specific site.

Ridge -

An elongated area of relatively high atmospheric pressure; the opposite of trough.

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-S-

Satellite Photo -

A photograph of the earth taken by weather satellites that shows areas of cloud.

Sea Breeze -

Sea breezes occur during the daytime in warm sunny weather when the air over a land is heated more rapidly than that over an adjacent water surface. As a result, the warmer air rises and relatively cool air from the sea flows onshore to replace it. At night, the air over the land cools faster than that over the nearby ocean and causes the air circulation to be in the opposite direction - a land breeze. Land breezes are usually weaker than sea breezes and have a less noticeable effect upon the temperature.

Severe Thunderstorm -

A thunderstorm which produces tornadoes, large hail, or winds of 50 knots (80 kp/h) or more. Structural wind damage may imply the occurrence of a severe thunderstorm.

Shear -

Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Shortwave (or Shortwave Trough) -

A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

Snow -

The white stuff.

Sounding -

A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location ( Fig. 6). Soundings are used extensively in weather forecasting, e.g., to determine instability, locate temperature inversions etc.

Squall Line -

A solid or nearly solid line or band of active thunderstorms.

Steering Winds (or Steering Currents) -

A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Stratiform -

Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

Stratocumulus -

Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

Stratus -

A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.

Subsidence -

Sinking (downward) motion in the atmosphere, usually over a broad area ~ it often implies clearing skies.

Synoptic Scale (or Large Scale) -

Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale.

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-T-

Temperature -

In general, the degree of hotness or coldness measured against some definite scale by means of a thermometer.

Thermodynamics -

In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thunderstorm -

or thundershower...

A local storm, produced by a cumulonimbus cloud, and accompanied by thunder and lightning.

Tornado -

A tornado appears as a violent funnel-shaped wind vortex in the lower atmosphere with upward spiralling winds of high speeds - spawned by severe thunderstorms. The tornado usually appears from a bulge in the base of a cumulonimbus cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours. In area, it is one of the least extensive of all storms, but in violence, it is the world's most severe. More tornadoes occur in the United States than in any other country. In Canada, when they do occur, it is mainly in the Prairies and southern Ontario.

Towering Cumulus -

A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

Trade Winds -

The wind system, occupying most of the tropics, which 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 Nrthern Hemisphere and southeasterly in the Sothern Hemisphere; hence they are known as the northeast tardes and the southeast trades, respectively.

Tropopause -

The upper boundary of the troposphere, usually characterized by an abrupt change in temperature with height from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). See Fig of sounding.

Troposphere -

The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height. It's the layer of the atmosphere where most of the weather occurs.

Trough -

An elongated area of relatively low atmospheric pressure, usually extending from the centre of a low pressure region. The opposite of ridge.

Turbulence -

The vertical motion of the air, at times violent, which can cause the up-and-down movement of a plane, etc.

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-U-

Updraft -

A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

Upslope Flow -

Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream -

Toward the source of the flow, or located in the area from which the flow is coming.

UV (or Ultraviolet Index) -

The UV Index is a scale that measures the intensity of the sun's burning ultraviolet rays. UV Index readings typically range from 0 to 10 in Canada but may reach 14 in the tropics. Index values are classified as low (0 - 3.9), moderate (4.0 - 6.9), high (7.0 - 8.9) or extreme (9+). The UV Index is derived by weighting quantities of incoming UVB radiation according to how effective these are in reddening human skin.

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-V-

Veering Winds -

Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft).

Virga -

Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.

Vorticity -

A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

Vort Max -

(Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of an airmass.

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-W-

Warm Advection -

Transport of warm air into an area by winds.

Warm Front -

A front that moves in such a way that the warmer air replaces the colder air.

Warning -

A product issued by Environment Canada forecast offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. Warnings are broadcast by the media, on the Weatheradio Canada System and by the Coast Guard. They are issued for snowstorms, blizzards, heavy blowing snow, heavy rains, frost, cold waves, freezing rain, severe thunderstorms and strong winds - according to thresholds established for local and regional public needs.

In Vancouver a warning is issued for an expected snowfall of 5 centimeters or more while through most of Canada a warning would only be issued for 15 centimeters or more of snow.

Watch -

An Environment Canada product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).

Waterspout -

In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud.

Wave -

In meteorology, the intersection of warm and cold fronts.

Weather -

State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

Weather Balloon -

Large balloons filled with helium or hydrogen and carry radiosondes (weather instruments) aloft to measure temperature pressure and humidity as the balloon rises through the air. The whole contraption is attached to a small parachute so that when the balloon inevitably breaks, the radiosone doesn't hurtle back to earth dangerously quickly.

Wind -

The horizontal movement of air relative to the earth's surface.

Wind Chill -

The combined cooling effect of wind and temperature is called wind chill. The wind chill factor is a measure of this cooling effect. The larger the wind chill factor, the faster the rate of cooling. Note, however, that an object will not be cooled below the actual air temperature, it will just get there faster.

Wind Shear -

See shear.

Windward -

Upwind, or the direction from which the wind is blowing; the opposite of leeward.

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-Z-

Zonal Flow -

Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.

Bibliography

• A Comprehensive Glossary of Weather Terms for Storm Spotters: by Michael Branick NOAA/NWSFO Norman OK
• more Weather Terms from Environment Canada
• American Meteorology Society, 1990: Glossary of Meteorology. American Meteorological Society Press, Boston.

Weather Terms Assembled by Pat Wong
meteorologist @ Pacific Weather Centre
~ Environment Canada

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