Weather Glossary

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A

Absolute Humidity - A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter. Top

Air Mass - An extensive body of air throughtout which the horizontal temperature and moisture characteristics are similiar. Top

Air Pollution - The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant or animal life, or prevent the use and enjoyment of the outdoors. Top

Alberta Clipper - A fast moving, snow producting weather system that originates in the lee of the Canadian Rockies. It moves quickely across the northern United States, often bring gusty winds and cold Artic air. Top

Altimeter - An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure. Top

Altitude - In meteorology, the measure of a height of an airborne object in respect to a constant pressur surface or above mean sea level. Top

Anemometer - An Instrument that measures the speed or force of the wind. Top

Aneroid Barometer- An instrument for measuring the atmospheric pressure. It Registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thinwalled metal capsule or cell, usually made of phosphor bronze or berylliam copper. The scales on the glass cover measure pressure in both inches and millibars. Top

Anticyclone - An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposute of an area of low pressure or a cyclone. Top

B

Barometer - An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer. Top

Barometric Pressure - The pressure exerted by the attmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury. Top

Beauford Wind Scale - A system of estimating and reporting wind speeds. It is based on the Beuford force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upton land objects and/or sea surfaces. The scale was devised by sir Francis Beuford(1777-1857), hydrographer to the British Royal Navy. Top

Black Ice - Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces. Top

C

Chinook - A type of foehn wind. Refers to warm downslope wind in the Rocky Mountains that may occur after an intense cold spell when temperatures could rise by 20° F to 40°F in a matter of minutes. Top

Climate - The historical record and description of average daily and in seasonal weather events that help describe a region. Statistics are generally drawn over several decades. The word is devived from the Greek klima, meaning inclination, and reflects the importance early scholars attributed to the sun's influence. Top

Cold Front - The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the presure rises, and the wind shifts(usually from the southwest to the northwest in the Northern Hemisphere). Precipitaion is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. Top

Coriolis Effect - A force per unit mass thar arises solely from the earth's rotation, acting as a deflecting force. It is dependent on the latitude and the speed of the moving object. In the Northern Hemisphere, air is deflected to the right of its path, while in the Southern Hemisphere, air is deflected to the left of its path. It is greatest at the poles, North and South, and almost nonexistant at the equator. Top

Cut-off Low - A closed cold core low completely removed from the primary westerly flow. Cutoff lows may remain detached from the westerlies fo days while exhibiting very little forward(eastward) progress. In some instances, a cutoff low may move to the west, or retrograde, opposite to the prevailing flow. Top

Cyclone - An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for a tropical cyclone in the Indian Ocean. The opposite of anticycle or a high pressure system. Top

D

Dawn - The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twight. The visual display is created by the scattered of light reaching the upper atmosphere prior to the sun's rise to the oberver's horizon. Top

Dew Point - The temperature to which air must be cooled at a constant pressure to become saturated. Top

Dry Bulb Thermometer - A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer. Top

Dusk - The period of waning light from the time of sunset to dark. Top

G

Greenhouse Effect - The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's ray's to heat up the earth, but then restrict some heat-entergy from escaping back into space. Top

Gust Front - The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary. Top

H

Haboob - Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought. Top

Heat Index - The combination of air temperature and humidity that gives a descrition of how the temperature feels. This is not the actual temperature. Top

High Pressure System - An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposute of an area of low pressure or a cyclone. Top

L

Low Pressure System - An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a cyclone and the term used for a tropical cyclone in the Indian Ocean. The opposite of anticycle or a high pressure system. Top

M

Monsoon - The seasonal shift of winds created by the great annual temperature variation that occur over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the souhtern United States. Top

O

Occluded Front - Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet. Top

R

Relative Humidity - a type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in a percentage. Top

T

Thermometer - An instrument used for measuring temperature. The different scales used in meteorolgy are Celcius, Fahrenheight, and Kelvin or Absolute . Top

W

Warm Front - The leading edge of an advancing warm air mass that is replaceing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts(usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitaion, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thundertstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produce fog in the warm air. Top

Weather Vane - Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses. Top

Wet Bulb Thermometer - A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

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