Glossary of Weather Terms
-----
- Air Mass A very large body of air, covering thousands of square miles, that contains similar temperature and humidity conditions throughout.
- Air Pressure Or Atmospheric Pressure, is the weight exerted by the air. It is commonly measured by a barometer and expressed as inches of mercury. At sea level, a column of air that is one square inch in area will normally weigh 14.7 pounds and be able to hold a column of mercury in a vacuum tube to 29.92 inches high.
- Anemometer An instrument used to measure wind speed. It is typically composed of hemispherical cups mounted on cross arms atop an axle. As wind blows into the cups, it causes them to spin. The rate of spinning is measured and converted to wind speed.
- Backing Wind Wind that is changing direction in a counterclockwise manner. A backing wind is usually a sign of deteriorating weather conditions.
- Barometer An instrument used to measure atmospheric pressure. There are two main types: aneroid and mercury. The aneroid barometer makes us of a small sealed chamber connected to a pointer mechanism. Mercury barometers use a glass tube filled with mercury and inverted over a mercury-filled chamber. Normally, the atmosphere at sea level can support the column of mercury to 29.92 inches. An aneroid barometer is calibrated so that its pointer reads the pressure as inches of mercury, too.
- Blizzard A severe winter storm characterized by very cold temperatures, high winds, and blowing snow.
- Climate The average weather conditions characteristic of an area as determined over a long period of time.
- Cloud The visible conglomeration of water droplets that occurs above ground level. They typically form as a result of moisture-laden air that is rising and cooling, causing its moisture to condense around solid airborne nuclei. There are many types of clouds, depending upon how they are formed and their appearance.
- Condensation The change in state of water from a gas to a liquid, usually as a result of decreasing temperature or increasing humidity.
- Coriolis Effect The twisting motion given to moving fluids as a result of the rotation of the earth.
- Cyclone A low pressure cell, or a large storm system with counterclockwise air circulation.
- Deposition The change in state of water from a gas directly into a solid without first becoming a liquid. Frost is formed in this manner.
- Dew Water droplets that form on grass or other objects as a result of condensation of atmospheric moisture as the nighttime temperature falls, resulting in an increase in relative humidity.
- Dew Point The temperature at which the relative humidity is at 100 percent. In summer, most people are uncomfortable when the dew point goes above 65. Dew points above 70 cause very muggy conditions.
- Drizzle Rain falling as very tiny droplets, small enough not to cause splashes in a puddle.
- Evaporation The change in state of water from a liquid to a gas.
- Fog Water droplets that form around condensation nuclei in the air at ground level.
- Freezing Rain Rain that freezes upon contact with a surface that is below the freezing point of 32 degrees Fahrenheit (0 degrees Celsius).
- Front The boundary between two differing air masses. Cold fronts separate an advancing cold air mass from a retreating warm air mass. Warm fronts separate an advancing warm air mass from a retreating cold air mass. Stationary fronts separate air masses that are not moving relative to each other.
- Frost The ice crystals that form on cold surfaces as a result of deposition of moisture in the air on objects that are below the freezing point.
- Hail A type of frozen precipitation formed when rain droplets are lofted high into the atmosphere by strong updrafts repeatedly, adding new layers of ice with each up-and-down trip, until it is finally heavy enough to fall to the ground. Hail accompanies strong thunderstorms and is usually a summertime phenomenon.
- High Pressure Cell A large area of high atmospheric pressure. Winds circulate a high pressure cell, or high, in a clockwise direction. Highs typically bring fair weather.
- Humidity The amount of water vapor in the air at any given time. See also Relative Humidity.
- Hurricane A powerful tropical cyclone. Called a typhoon in the western Pacific. See Cyclone.
- Hygrometer An instrument used to measure relative humidity.
- Isobar The lines drawn on a weather map that connect points of equal air pressure.
- Jet Stream The narrow ribbons of fast-moving wind in the upper atmosphere that guide weather systems.
- Lightning An enormous discharge of electricity in the atmosphere that results when a thunderstorm cloud becomes polarized.
- Low Pressure Cell An area of lower than normal atmospheric pressure, usually associated with frontal systems and cloudy or rainy weather. Wind circulates around low pressure cells, or lows, in a counterclockwise direction.
- Meteorology The scientific study of the weather.
- Outflow Cool downdrafts from a thunderstorm that hit the ground and flow outward. An "outflow boundary" is the leading edge of this cool air and sometimes behaves much as a cold front would.
- Precipitation Any form of liquid or solid moisture falling from the clouds. Precipitation can be in the form of rain, freezing rain, snow, sleet, or hail.
- Psychrometer An instrument used to measure relative humidity by comparison between a wet-bulb thermometer and a dry-bulb thermometer.
- Rain Precipitation in the form of liquid drops falling from a cloud.
- Rainbow An arc of color in the clouds opposite the sun, resulting from light being refracted (bent) as it passes through water droplets in the cloud and separated into its constituent colors.
- Relative Humidity The amount of moisture in the air measured as a percent of the maximum amount the air could hold at any given temperature.
- Sleet Frozen precipitation that results when rain drops freeze while falling, before hitting the ground. This occurs when rain falls through a layer of colder air underlying a layer of warmer air. See Temperature Inversion.
- Snow Frozen precipitation that forms when water vapor is deposited as ice crystals around nuclei in the clouds.
- Sublimation The process whereby ice or snow change directly to water vapor without first going through the liquid state.
- Temperature A measure of the amount of heat in the atmosphere. Technically, the average amount of kinetic energy of the molecules in a substance. Measured in degrees Fahrenheit (in the U.S.), or degrees Celsius.
- Temperature Inversion The state of the atmosphere resulting when a warm layer of air overrides a cooler layer of air, the opposite of the normal condition in which the temperature decreases with altitude.
- Thermometer An instrument used to measure air temperature.
- Thunder The sound produced as a result of the rapid heating and expansion of air during a lightning strike.
- Tornado A column or funnel of very rapidly rotating wind that forms at the base of a thunderstorm cloud.
- Updraft Air currents moving vertically upward. Updrafts typically produce cumulus clouds.
- Vapor Water in the gaseous state.
- Veering Wind Wind that is changing direction in a clockwise manner.
- Weather The state of the atmosphere, including temperature, wind, air pressure, relative humidity, and sky conditions, at any given time and place.
- Wind The horizontal motion of air caused by the uneven heating of the atmosphere combined with the rotation of the earth.
- Wind Vane An instrument used to determine the direction from which the wind is blowing.
Copyright © 2001 by Jerry Wilson. Get permission to reprint this article.
Return to SkyWatch Those who travel regularly know better than to stay at hotels. They instead go for apartments with rental cars. The deal becomes cheaper than cruises.
