Weather Words

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 equinox (literally, meaning "equal nights") nights are of equal length ( just under 12 hours) all across the globe from pole to pole.

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 lake-effect snow depends in part on the distance the wind blows over open water.

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.

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.

Fog - Advection: 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.

Fog - Frontal or Pre-frontal: 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.

Fog - Radiation: A form of fog caused by the cooling of moist air to below its dew point by the process of heat radiation. Radiation fog usually forms near the surface at night under clear skies and light wind conditions. Also known as ground fog.

Fog - Steam: A form of fog caused by the advection of cold air over a warm water surface or a very saturated soil surface, giving the impression of rising steam. Common over large lakes and near-shore ocean areas during the cold season.

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 considers any rain that falls and is observed frozen as freezing rain.

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.

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 0^oC (32^oF).

Fujita Scale or 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.

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 have continental glaciers.

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.

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 lost to space.

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.

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.

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.

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

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.

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.

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.

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

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

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 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 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 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 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 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.

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.

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.

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 increases 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 snow or ice, and subsidence, the sinking of air which is then warmed by compression.

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

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