Weather Words

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.

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: 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.

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%.

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.

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.

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.

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 barometers for many years and are still found on many walls. Aneroid means "without liquid."

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.

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.

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.

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, where gases are found in differentiated layers.

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.

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.

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.

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 amount of snow in the air (falling or blowing) reduces visibility to less than 400m (0.25 miles).

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.

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.

Cape Verde tropical storm: an Atlantic tropical storm/hurricane that develops near the Cape Verde islands off the west coast of Africa.

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."

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.

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

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.

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.

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

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.

Coordinated Universal Time or UTC: 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.

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."

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.

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

Dew Point or 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.

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.

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, it spreads outward in all directions in a circle, like a fast-running faucet hitting the sink bottom.

The Weather Doctor's Weather Glossary ©2006, Keith C. Heidorn, PhD. All Rights Reserved.
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