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Weather Doctor - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 05/10/2008, USA Words: 198

Mesotortuous Segments
'Straight' segments between five and 100 metres long separated by sharp bends which comprise the lightning channel. (Segments shorter than five metres are called microtortuous and those longer than 100 m, macrotortuous.)

Mesoscale
The middle of three scales used to describe the size range of atmospheric processes. The mesoscale covers the range from a few kilometres to a few tens of kilometres. Scales smaller than the mesoscale are considered microscale processes and those larger, macroscale processes. Often loosely called the local scale.

Meridional Flow
Large-scale atmospheric flow in which the north-south component, i.e., flow parallel to the longitude lines or meridians, is much greater than the zonal east-west)component. Compare with zonal flow.

Mercury Barometer
The most common form of liquid barometer designed on the principle that the downward pressure of the air's weight will support a column of liquid mercury in an inverted, evacuated glass tube. The height of the column is the measure of atmospheric pressure and was originally reported as millimetres or inches of mercury (Hg). Today, the preferred unit is the Pascal, usually expressed as the kilopas...

Mediterranean Climate
Climate type characterized by mild, wet winters and warm to hot, dry summers. Originally describing the climate of the area around the Mediterranean Sea, it also describes the climate of the North American Pacific Coast, ranging from hot Mediterranean in Southern California, to warm Mediterranean on Washington and British Columbia coasts.

Lightning Stroke
One of a series of repeated lightning discharges over a common channel.

Low Level Jet
A relatively narrow band of high-speed winds found in the lower troposphere, well below the upper troposphere where the jet stream is found.

Lightning Flash
The complete visible flash of lightning accompanying a lightning discharge during one or a series of lightning strokes. A flash comprised of a number of strokes in rapid succession is also termed a composite flash.

Lightning Discharge
A transfer of electrical charge along a narrow channel (the lightning channel), the result of a buildup of electrical pressure (voltage) between two distant points to levels to levels greater than the channel can resist. When the resistance breaks down, charge jumps across the space in a lightning discharge.

Lightning Channel
The irregular path taken by a lighting discharge from its point of origin to end. Channel is formed of many small segments known as mesotorturous segments. A number of lightning strokes may follow a single lightning channel or the major portions of it. The channel may be highly ionized.

Lightning Bolt
Alternate, popular name for lightning stroke, usually when seen in streak or forked form.

Lightning
All forms of visible electrical discharges moving through the atmosphere, usually eminating from tall cumulus and cumulonimbus clouds during thunderstorms. Lightning is often categorized for the manner in which it is visible to the observer: streak lightning, forked lightning, sheet lightning, heat lightning.

Leeward
The direction or side of an object (building, mountain, etc.) that faces away from the wind.

Latent Heat
The energy released or absorbed during a change of state. For example, the change of water vapour to liquid water releases the latent heat of condensation.

Lake Effect Snowfall
Snowfalls along the lee shore of a lake or downwind some distance from the shore caused by the modification of cold, subfreezing air by the relatively warmer lake water. The intensity of the lake-effect snowfall depends upon several factors: the temperature contrast between the lake surface and the air passing over it, the over-water distance the air has traversed (the fetch), and the regional wea...

Lake Effect
The effect of any lake in modifying the weather and climate along its shore and a distance inland, which depends on the size of the lake. In some areas such as the North American Great Lakes, lake effect refers to snowfall generated by the lake on the downwind shore. See lake-effect snowfall.

Khamsin
A dry, dusty and often hot wind blowing from the Egyptian desert over Red Sea, generally from the south or southeast. In the spring, their onset is usually preceded by a heat wave lasting about three days and followed by a dust storm.

Jet Stream
A relatively narrow band of high-speed winds, generally greater than 50 knots (57 mph or 93 km/h), found in the upper troposphere above regions of strong horizontal temperature contrasts such as fronts. The major jet streams are the subtropical jet and the polar jet.

Isobar
A line that connects points of equal pressure on a surface weather map.

January Thaw
A period of mild weather, usually following a cold spell, which often occurs in late January in the eastern regions of North America, particularly around the Great Lakes region, New England and the Canadian Maritime Provinces. The thaw is associated with a strong flow of warm air northward from the Gulf of Mexico on southerly winds on the back of a high pressure system.

Inversion
A layer of air in which the temperature increases with height. Meteorological convention considers temperature decreasing with height as the norm, thus when temperature decreases with height, it is inverted. There are four common causes of a temperature inversion: radiational cooling, advection of warm air over cold air as in frontal situations, advection of warm air over a cold surface such as sn...

Instability
The tendency for air parcels to accelerate their motion when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted by either topographical features, convergence, or the condensation of water vapour within the air parcel which warms the air.

Infrared Radiation
The long wave, (between 0.8 and 100 micrometres in wavelength) electromagnetic radiation emitted by all objects. Often also referred to as heat radiation. Terrestrial radiation, the radiant heat from the Earth's surface, is a form of infrared radiation. The so-called greenhouse gases readily absorb infrared radiation.

Ice Stars
Ice crystals forming six-armed dendritic (branching) patterns looking like stars. These is the most common image of a snowflake. These crystals form when air temperatures are between -12° C to -16°C (10° F to 3° F).

Indian Summer
A period in mid to late Autumn in the eastern United States and eastern Canada characterized by light winds, clear skies and temperatures which are unseasonably warm during the day and refreshingly chill at night. The period usually begins after the first major frost of the season.

Ice Plates
Ice crystals that resemble dinner plates with a hexagonal pattern in their long dimension and are thin relative to their width. Plate formation is favoured at air temperatures from 0° C to -4° C (32° F to 25° F) and from -10° C to -20°C (14° F to -4° F).

Ice Pellets
A form of precipitation consisting of transparent or translucent pellets of ice, 5 mm (2 in) or less in diameter. Ice pellets may be spheres or irregularly shaped. Ice pellets will usually bounce on impacting a hard surface, often with an audible sound. Ice pellets may be classed as either sleet and small hail.

Ice Needles
Ice crystals which are long and thin with a hexagonally-shaped cross section, thinner than ice columns. Ice needle formation is favoured at air temperatures from -4° C to -6° C (25° F to 21° F)

Ice Fog
Fog composed of minute ice particles that occurs in very low temperatures (typically minus 30 °C/ minus 22°F or below) under clear, calm conditions in the polar latitudes.

Ice Crystals
1) Hexagonal (6-sided) crystals that form upon the freezing of water, may be in one of several shapes: stars, needles, plates, columns or combinations of these forms. 2) Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates, may be called diamond dust.

Ice Caps
A perennial ice/snow cover over an extensive area of land or sea, today found only around the two geographic poles.

Ice Columns
Ice crystals resembling stubby pencils rather than the delicate branched snowflake shape.Columns typically form in the temperature ranges -5° C to -8° C (23° F to 18° F) and below -25° C ( -13° F). They are long in comparison to their hexagonal cross-section

Hydrological Cycle
The vertical and horizontal transport of water through the global environment in all its states between the earth, the atmosphere and seas, lakes, rivers and other water bodies. Often also called the water cycle

Hygrometer
An instrument that measures the water vapour content of the atmosphere

Humidex
A discomfort index used in Canada derived to combine the temperature and humidity into one number and is intended to reflect perceived temperature.

Hurricane-force Winds
Force 12 on the Beaufort Wind Scale denoting winds exceeding 118 km/h (74 mph). Hurricane-force winds may occur during a non-hurricane storm.

Hook-shaped Echo
A radar reflectivity pattern observed in a thunderstorm appearing like a hook. It often indicates favourable conditions for tornado development.

Humidity
Generally, some measure of the water vapour content of the air. See also, absolute humidity, specific humidity, relative humidity.

Hoar Frost
Formation of interlocking ice crystals directly from the water vapour in the atmosphere on objects which usually are of small size and exposed freely to the air such as plant leaves and branches. Hoar frost, more commonly know as frost, is fluffier and more feathery in appearance than rime ice.

Heat Lightning
A popular term for lightning that is visible but for which no thunder is heard. It usually occurs in scattered thunderstorms/showers on hot nights when storm are at a distance from the observer.

Height Contours
Isolines which denote the distribution of equal heights of a particular atmospheric pressure on a constant-pressure map

Heat Index Apparent Temperature
More commonly known as the Heat Index, the Heat Index Apparent Temperature is the accepted measure of thermal discomfort in the United States. The Heat Index is calculated from temperature and relative humidity only. It is a simplification of an index developed by R.I. Steadman in 1979.

Halo
A ring or arc of coloured or white light that encircles the sun or moon when seen through a cloud of ice crystals. Halos are produced by the refraction of light. The most commonly observed halo forms at a 22 degrees radius from the sun/moon. One at 46 degrees radius may also be seen.

Gust Front
The leading boundary of relatively cold air flowing out of a thunderstorm, usually producing gusty winds, a noticeable wind shift and temperature drop when the gust-front passes (similar to a cold front). A shelf cloud may be seen above its surface position.

Glory
Coloured circular bands of light 2 to 10 degrees across appearing on clouds opposite the sun. Produced by diffraction of light by water droplets or ice crystals.

Greenhouse Effect
A popular term used to describe the effects of gases such as water vapour, carbon dioxide and other trace gases (collectively known as the greenhouse gases) in keeping the Earth's temperature warmer than it would be otherwise. These gases absorb long-wave (heat) radiation and reradiate some of that energy back to the surface and the atmosphere. In the process, less of the heat from the sun is los...

Glaze
Transparent and homogeneous ice forming on vertical and horizontal surfaces by the freezing of supercooled water. The amorphous, dense structure of glaze helps it to cling tenaciously to any surface. Density of glaze can be as high as 0.8 to 0.9 grams per cubic centimetre.

Glacier
A large mass of freshwater ice originally of atmospheric origin that forms on land over many years. There are two main types of glacier: mountain glaciers which form at altitude among mountain peaks and valleys; and continental glaciers, which form over continents at high latitude. Glaciers formed over much of the high latitudes of continents during Ice Ages. Today only Greenland and Antarctica ha...

Frost Point
The temperature to which air must be cooled at constant pressure and humidity to achieve a condition of saturation with respect to ice at or below 0oC (32oF).

Fujita Tornado Intensity Scale
The Fujita Scale (generally F0 to F5) for Tornado Intensity is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a human-made structure. The scale was developed by the late Dr T. Theodore Fujita of the University of Chicago.

Frost
The deposition of ice crystals on a surface directly from the water vapour in the atmosphere. The process is similar to dew formation except that the temperature of the object must be below freezing, the frost point.

Freezing Rain
Rain that falls as liquid drops but freezes upon impact with horizontal or vertical surfaces. Freezing rain is characterized as either glaze or rime depending on the nature of the ice. Rain may freeze on the surface because its drops are supercooled and freeze on contact or that the surface is well below freezing. Technically, only the former is considered freezing rain, but the public usually con...

Frontal fog
A form of fog associated with weather fronts caused by the cooling of moist air to its dew point in the zone along and ahead of the front.

Flurries
Popular term for a usually light and brief snow shower. Accumulations of snow are very light, such as a dusting, or none at all.

Foehn Winds
Winds descending downslope from mountains which are characteristically warm and dry due to adiabatic compression. The term originally referred to such winds flowing down the Alpine valleys of Germany and Austria but is now used as the generic term for such airflows. The Chinook of western Canada is a foehn wind.

Flash Flood
A flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. A flash flood rises rapidly, often with little or no warning.

Fetch
The distance over which a wind of nearly constant direction has blown, usually over a consistent surface such as water or a forest or a field. Most often the fetch is applied to a distance over water and used to determine the height of wind-generated waves. However, fetch can be applied to many studies of energy and water vapour transfers from the surface to the air. For example, the intensity of ...

Electronic Barometer
A barometer that uses a pressure transducer as a pressure sensor. The sensor has electrical properties (resistance or capacitance) that change when the atmospheric pressure changes. Electronic circuitry connected to the sensor then converts its output into a visual display.

Equinox
Either of two occasions during the year when the apparent sun's path crosses the plane of the Earth's equator. Also the date when the sun is directly overhead at noon on the equator, occurring on or around both March 21 and September 22, the former is the vernal equinox and the latter the autumnal equinox in the Northern Hemisphere and the reverse on the Southern Hemisphere. On the date of the e...

Divergence
The flow of the wind resulting in a horizontal outflow of air from a region. The opposite of divergence is convergence.

Downbursts
Surface winds in excess of 62 km/hr (39 mph) caused by a small-scale downdraft from the base of a convective cloud. Downbursts occur in regions of a severe thunderstorm where the air is accelerated downward by exceptionally strong evaporative cooling occurs (a dry downburst) or by very heavy rain which drags dry air down with it (a wet downburst). When the rapidly descending air strikes the ground...

Diamond Dust
Ice crystals glittered like diamonds in sun, moon or artificial light. Individual crystals appear to float in the air. Term often used to describe fall of ice crystals from apparently clear sky.

Dew Cell
A hygrometer used to determine the dew point temperature directly.

Dew Point Temperature
The temperature to which a parcel of air must be cooled at constant water vapour content (and constant pressure) for saturation to occur.

Cyclonic
Air flow around the centre of a surface Low. In the Northern Hemisphere, this flow is counter-clockwise as seen from above. (In the Southern Hemisphere, it is clockwise.) Also, any rotation in the direction of the Earth's rotation.

Cumulus
A principal cloud type of vertical elements having a flat base and dense, bulging upper portion resembling a heap or pile. The base of cumulus clouds is generally found from 500 to 3000 metres. Large cumulonimbus clouds may extend to over 18,000 metres and be topped with anvil-shaped ice clouds. Cumulus is derived from the Latin for 'heap.'

Coordinated Universal Time
The currently accepted name of the twenty-four hour time scale used throughout the scientific and military communities to coordinate and standardize time using an atomic clock rather than solar observations. Coordinated Universal Time is denoted by the acronym UTC or by the letter Z UTC is the modern successor of Greenwich Mean Time (GMT) which was used when the unit of time was the mean solar day...

Convergence
The flow of the wind resulting in a horizontal inflow of air into a region. Convergence of winds near the surface are associated with upward motion known in meteorology as convection. The opposite of convergence is divergence.

Convective Clouds
Clouds formed atop rising air columns. Cumulus clouds are convective clouds.

Convection
In physics, convection is the transport and mixing of properties (energy, heat, moisture, etc.) of a fluid by mass motion of that fluid. In meteorology, convection generally refers to such transport and mixing in the vertical direction, and advection refers to processes in the horizontal plane.

Condensation Nuclei
Liquid or solid particles, such as those in smoke or dust, that provide a surface upon which water vapour can condense into cloud droplets or form ice crystals.

Constant Pressure Maps
A weather map of a particular constant pressure surface, such as the 50 kPa or 500 mb surface, in which atmospheric pressure is uniform everywhere. Plotted elements may include the of height above sea level of the particular pressure, wind, temperature, and humidity or dew point temperature.

Condensation Level
The altitude at which a rising air parcel reaches saturation, usually the cloud base height.

Cloud Base
For a given cloud or cloud layer, it is the level in the atmosphere cloud particles (droplets or ice crystals) become visible.

Cloud Droplets
Small drops of liquid water, approximately 4 to 100 micrometres in diameter, that remain suspended in the air. They are smaller in size than either drizzle or rain drops. An aggregate of cloud droplets forms a visible cloud.

Blowing Snow
Snow lifted off the surface by the wind to a height of 6 or more feet and blown about in sufficient density to restrict visibility.

Cirrus
A principal cloud type present at high altitudes, generally above 7000 metres, and composed of ice crystals. Cirrus comes from the Latin term for 'curl or wisp of hair.'

Bluebird day
The most gorgeous day imaginable. A bluebird day is a bright, sunny day after a fresh snowfall the night before, a phrase common to the skiing and snowboarding community but common in many outdoor sport lingo. To duck hunters it is a still, calm day, giving difficult conditions in which to hunt ducks.

Biogeochemical Cycle
The chemical interactionsand transport within and among the atmosphere, biosphere, hydrosphere and lithosphere (rocks).

Blizzard
Although blizzard is often used to describe any major snow storm with strong winds, the technical definition for a blizzard requires: at least 3 hours in duration; low temperatures (usually less than minus 7C or 20F), strong winds (greater than 55 km/h or 35 mph), blowing snow which reduces visibility to less that 1 kilometre (0.6 miles). Surprisingly, snowfall need not be falling as long as the a...

Bermuda High
The semi-permanent, subtropical anticyclone located over the western North Atlantic Ocean at about 30°N. When this center of high pressure is found over the island of Bermuda, it has a profound influence on weather over the eastern United States and Canada, usually characterized by very warm temperatures.

Barometer
An instrument for measuring atmospheric pressure. There are three common types of barometers used widely today: mercury barometer; aneroid barometer; electronic barometer. See glossary entries for more on these types.

Autumnal
Pertaining to the Autumn season. The Autumnal Equinox occurs on or about September 22 in the Northern Hemisphere and on or about March 21 in the Southern Hemisphere.

Arctic Screamer
Strong, cold winds from North or Northwest, often following cold front.

Atmosphere
The envelope of gases that surround a planet's surface held by the planet's gravity. Over the Earth, the atmosphere is divided into several layers based on their properties. The most common layer designations are: the troposphere, the stratosphere, the mesosphere and the exosphere. Atmospheric chemists divide the atmosphere into the heterosphere, where gases are well mixed, and the homosphere, w...

Anticyclonic
The wind circulation pattern in Highs, or anticyclones, that has a sense of rotation opposite to that of cyclones and the Earth's rotation. Also, any rotational pattern that is opposite to that of cyclones and the Earth's rotation.

Anticyclone
An area of high pressure, also called a High, around which the winds circulate in a clockwise direction in the Northern Hemisphere (and counter-clockwise in the Southern Hemisphere). It is usually responsible for fair, dry weather. When the area of highest pressure is elongated, it is called a high pressure ridge, or simply a ridge.

Antarctic Ozone Hole
An extended area of extreme depletion of the ozone layerwhich forms over the southern polar region each austral spring (September through November).

Anomaly
Difference between a prevailing weather condition and long-term average for that conditions.

Aneroid Barometer
A barometer whose pressure sensor consists of an aneroid capsule, a thin, hollow disk partially evacuated and sealed. The difference between the exterior air pressure and the interior pressure causes the disk to expand or contract slightly. This movement is amplified by a spring or gear mechanism to indicate changes in air pressure on a scale or recording chart. These were the most common home bar...

Alto
A prefix to cloud-type names for clouds generally found between 3000 and 7000 metres. Alto comes from the Latin word meaning 'middle.' E.g., Altostratus.

Aleutian Low
A semi-permanent, atmospheric low pressure centre located near the Aleutian Islands on charts of mean sea-level pressure. It is most intense in the winter. From this zone, many storm systems form in the winter half of the year and move toward the North American coast.

Alberta Clipper
A fast moving winter storm originating in the Alberta, Canada region. Often these snow storms have high winds reaching 100 km/h which reduce visibility through blowing and drifting snow. Blizzard conditions are common in a well-developed Alberta Clipper.

Air
The mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are: Nitrogen (N2) 78.09%; Oxygen (O2) 20.946%; Argon (A) at 0.93%; and Carbon Dioxide (CO2) 0.033%. One other important constituents of air is water vapor (H2O) which varies from 0% to about 4%.

Aeroplankton
The atmospheric equivalent to oceanic plankton, the term literally means 'air wanderer' because aeroplankton, tiny plants and animals and bacteria that live -- eating, excreting, even reproducing -- on the air are ever at the mercy of the wandering wind.

Advection Fog
A form of fog caused by the movement (advection) of moist air over a colder surface which cools the air to its dew point. The resulting condensation of water vapour causes a ground-level cloud to form. Moist air moving over cold lake or ocean waters or moving over an ice or snow surface is the most common cause of advection fog.

Advection
Refers to processes of transport and mixing of properties (energy, heat, moisture, etc.) of a fluid by mass motion of that fluid in the horizontal plane. In the atmosphere, the horizontal transfer of anything by the movement of air, i.e. wind. Common examples of advection include heat and moisture. Contrast with convection.

Absolute Humidity
The ratio of the mass of water vapor present in the air to the volume occupied by the gas; the density of water vapour in the air, usually expressed as grams of water vapour per cubic metre of air.