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Glossary of Weather Terms

Anabatic wind A local wind which blows up a slope heated by sunshine. Advection The horizontal transportation of air or atmospheric properties. Air density The mass density of air expressed as weight per unit volume. Air mass An extensive body of air with uniform conditions of moisture and temperature in the horizontal. Albedo The ratio of the amount of solar radiation reflected by a body to the amount incident on it, commonly expressed as a percentage. Anticyclone An area of high atmospheric pressure which has a closed circulation that is anticyclonic (clockwise) in the Northern Hemisphere. Blizzard A winter storm with winds exceeding 40 km/h, with visibility reduced by falling or blowing snow to less than one kilometre, with high windchill values and lasting for at least three hours. All regional definitions contain the same wind speed and visibility criteria but differ in the required duration and temperature criterion. Cat's paw A cat paw-like, ripple signature on water given by strong downdrafts or outflow winds. A good indication of turbulence and wind shear. Ceiling Either (a) the height above the surface of the base of the lowest layer of clouds or obscuring phenomena (i.e. smoke) that hides more than half of the sky; (b) the vertical visibility into an obstruction to vision (i.e. fog). Chinook A warm dry wind blowing down the slopes of the Rocky Mountains and over the adjacent plains. Clear air turbulence (CAT) Turbulence in the free atmosphere not related to convective activity. It can occur in cloud and is caused by wind shear. Clear icing The formation of a layer or mass of ice which is relatively transparent because of its homogeneous structure and smaller number and size of air spaces; synonymous with glaze. Climate The statistical collection of long-term (usually decades) weather conditions at a point; may be expressed in a variety of ways. Cold front The leading edge of an advancing cold air mass. Convection Atmospheric motions that are predominately vertical, resulting in the vertical transport and mixing of atmospheric properties. Convergence A condition that exists when the distribution of winds in a given area is such that there is a net horizontal inflow of air into the area; the effect is to create lift. Cumuliform A term descriptive of all convective clouds exhibiting vertical development. Cyclone An area of low atmospheric pressure which has a circulation that is cyclonic (counterclockwise) in the Northern Hemisphere. Deepening A decrease in the central pressure of a pressure system; usually applied to a low. Indicates a development of the low. Deformation zone An area in the atmosphere where winds converge along one axis and diverge along another.Where the winds converge, the air is forced upward and it is in these areas where deformation zones (or axes of deformation as they are sometimes referred to) can produce clouds and precipitation. Disturbance Applied loosely: (a) any small-sized low pressure system; (b) an area where the weather, wind, and air pressure show signs of cyclonic development; (c) any deviation in flow or pressure that is associated with a disturbed state in the weather; and (d) any individual circulatory system within the primary circulation of the atmosphere. Divergence A condition that exists when the distribution of winds in a given area is such that there is a net horizontal outflow of air from the area. Downdraft A small scale downward current of air; observed on the lee side of large objects that restrict the smooth flow of air or in or near precipitation areas associated with cumuliform clouds. Downburst An exceptionally strong downdraft beneath a thunderstorm usually accompanied by a deluge of precipitation. Filling An increase in the central pressure of a pressure system; applied to a low. Föhn wind (foehn wind) A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Front A surface, interface or transition zone of discontinuity between two adjacent air masses of different densities. Fujita Scale A scale used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure. Funnel cloud A tornado cloud or vortex cloud extending downward from the parent cloud but not reaching the ground. Gust A sudden, rapid and brief increase in wind speed. In Canada, gusts are reported when the highest peak speed is at least 5 knots higher than the average wind and the highest peak speed is at least 15 knots. Gust front The leading edge of the downdraft outflow ahead of a thunderstorm. High An area of high barometric pressure; a high pressure system. Hurricane An intense tropical weather system with a well defined circulation and maximum sustained winds of 64 knots or higher. In the western Pacific, hurricanes are called 'typhoons,' and similar storms in the Indian Ocean are called 'cyclones' Icing Any deposit of ice forming on an object. Instability A state of the atmosphere where the vertical distribution of temperature is such that a parcel displaced from its initial position will continue to ascend. Inversion An increase of temperature with height - a reversal of the normal decrease of temperature with height. Isothermal layer Equal or constant temperature with height. Jet stream A quasi-horizontal stream of wind concentrated within a narrow band; generally located just below the tropopause. Katabatic wind Downslope gravitational flow of colder, denser air beneath the warmer, lighter air. Also known as 'drainage wind' or 'mountain breeze'. Strength can vary from gentle to extremely violent winds. Knot A unit of speed equal to one nautical mile per hour. Lapse rate The rate of change of an atmospheric variable (usually temperature) with height. Lee wave Any stationary wave disturbance caused by a barrier in a fluid flow; also called mountain wave or standing wave. Lightning Any and all forms of visible electrical discharge produced by a thunderstorm. Low An area of low barometric pressure; a low pressure system. Meridional flow Airflow in the direction of the geographic meridians, i.e. southnorth or north-south flow. Meteorology The science of the atmosphere. Mixed icing The formation of a white or milky and opaque layer of ice that demonstrates an appearance that is a composite of rime and clear icing. Occluded front A front that is no longer in contact with the surface. Orographic Of, pertaining to, or caused by forced uplift of air over high ground. Outflow A condition where air is flowing from the interior land area through mountain passes, valleys and inlets onto the coastal areas; used most commonly in winter when cold Arctic air spreads onto the coastal area and adjoining sea. Overrunning A condition when warm air overtakes or is lifted by colder denser air. Parcel A small volume of air, small enough to contain uniform distribution of meteorological properties, and large enough to remain relatively self-contained and respond to all meteorological processes. Plow wind Usually associated with the spreading out of a downburst from a thunderstorm; a strong, straight-line wind in advance of a thunderstorm that often results in severe damage. Precipitation Any and all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the surface. Quasi-stationary front A front that is stationary or nearly so; commonly called stationary front. Ridge An elongated area of relatively high atmospheric pressure; also called ridge line. Rime icing The formation of a white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water droplets. Saturation The condition in the atmosphere where actual water vapour present is the maximum possible at the existing temperature. Shower Precipitation from cumuliform cloud; characterized by suddenness of beginning and ending, by rapid changes in intensity, and usually by rapid changes in the appearance of the sky. Squall Essentially gusts of longer duration. In Canada, a squall is reported when the wind increases by at least 15 knots over the average speed for a duration of at least 2 minutes and the wind reaches a speed of at least 20 knots. Squall line A non-frontal line or narrow band of active thunderstorms. Stability A state of the atmosphere where the vertical distribution of temperature is such that a parcel will resist displacement from its initial position. Stratiform Term descriptive of clouds of extensive horizontal development; flat, lacking definition. Stratosphere The atmospheric layer above the tropopause; characterized by slight increase in temperature from base to top, very stable, low moisture content and absence of cloud. Subsidence The downward motion of air over a large area resulting in dynamic heating. Supercooled water Liquid water at temperatures below freezing. Thunderstorm A local storm invariably produced by a cumulonimbus cloud, and always accompanied by lightning and thunder. Tornado A violently rotating column of air, shaped from a cumulonimbus cloud, and nearly always observed as 'funnel-shaped;' other names are cyclone and twister. Tropopause The transition zone between the troposphere and the stratosphere; characterized by an abrupt change in lapse rate. Troposphere The portion of the earth's atmosphere from the surface to the tropopause; characterized by decreasing temperature with height and appreciable water vapour. Often referred to as the weather layer. Trough An elongated area of relatively low atmospheric pressure; also called trough line. Trowal A trough of warm air aloft; related to occluded front. Turbulence Any irregular or disturbed flow in the atmosphere. Updraft A localized upward current of air. Upper front Any frontal zone which is not manifested at the surface. Virga Water or ice particles falling from a cloud, usually in wisps or streaks, and evaporating completely before reaching the ground. Warm front The trailing edge of retreating cold air. Weather The instantaneous conditions or short term changes of atmospheric conditions at a point; as opposed to climate. Wind Air in motion relative to the earth's surface; normally horizontal motion. Wind direction The direction from which the wind is blowing. Wind speed Rate of wind movement expressed as distance per unit time. Wind shear The rate of change of wind direction and/or speed per unit distance; conventionally expressed as vertical and horizontal wind shear. Zonal wind A west wind; conventionally used to describe large-scale flow that is neither cyclonic or anticyclonic; also called zonal flow.