Some Weather Definitions

These are a few terms used to describe the weather. Just a few ... to make your weather station more fun. These definitions are general and condensed. We don't recommend that you use them as answers to questions on your meteorology final. If you want to really get into it, the bookstores and libraries have thousands of books on weather, climate, the atmosphere, and other related topics.

Anemometer

The instrument that measures wind speed. In mechanical systems a rotating member, such as a propellor or a multiple cup device, spins at a rate proportional to the wind speed. In solid state systems, a thermal device cools at a rate proportional to wind speed.

Atmosphere

The blanket of gaseous chemicals surrounding the earth. It is broken into layers according to temperature. A given volume of pure, dry air contains about 78% nitrogen, 21% oxygen, and a 1% mixture of 9 other gases, including argon, carbon dioxide, neon, helium, krypton, hydrogen, xenon, ozone, and radon.

Barometric pressure

The pressure of the atmosphere.

Barometer

The instrument that measures the pressure of the atmosphere.

Cloud types

Clouds are divided into four families: high, middle, low, and clouds with vertical development. The first three types have two main sub-types: clouds formed vertically (cumulus), and clouds formed horizontally (stratus).

Cumulus. Starting from the highest altitude, clouds are named cirrocumulus, altocumulus, cumulus and stratocumulus. A cumulonimbus cloud starts low and can rise to 50,000 feet. This is a thunderhead or anvil cloud, and it is avoided by smart pilots like the plague.

Stratus. Again starting from the highest, we have cirrostratus, altostratus, nimbostratus and stratus (scud).

Another very high cloud is the cirrus, thin, featherlike clouds made of ice crystals. These are mares tail clouds.

Dew

Atmospheric moisture condensed to liquid on a surface. Cold dew is frost.

Dew Point

The temperature to which the air must be cooled at a constant pressure to become saturated. Pilots watch temperature/dew point spread for formation of fog.

Fog

Fog forms when the temperature is within 4 degrees F of the dew point. The air condenses and water vapor forms. Two basic varieties of fog are available: advection and convection.

Advection fog (toolie fog) usually forms close to the ground or water when warm air travels over a cool surface, or cool air travels over warm surface.

Convection fog forms higher in the air when rising warmer air meets cooler air.

Front

The boundry of two air masses. Generally considered warm fronts and cold fronts, but includes many others - occluded, upper, surface, and stationary to name a few.

A cold front is the leading edge of a cold air mass that is displacing warmer air in its path.

A warm front, conversely, is the leading edge of a warm air mass that is displacing a retreating colder air mass.

Humidity

Absolute humidity is the amount of water vapor in a given volume of air. The amount of vapor that the air will hold increases with temperature. Relative humidity is the amount of moisture in the air at any temperature compared to the maximum amount it could hold at that temperature.

Hygrometer

The generic term for instrument that measures humidity. A specific type of hygrometer is a psychrometer. It consists of two thermometers, one wet bulb, one dry bulb. The wet bulb cools in relation to the moisture in the air.

Lapse rate

Temperature decreases with altitude. The rate of decrease is the lapse rate. The average rate is 3-1/2 degrees F per 1000 feet of increased height. The rate is different in moist air and dry air.

Rain gage

The instrument that measures the amount of rainfall.

Spherics

Also called sferics and atmospherics. The electromagnetic disturbance that accompanys lightning. It is a crackling sound that can be heard in regular AM radios. Spherics are a convenient method of tracking thunderstorms.

Temperature

Measured in Fahrenheit or Centigrade (and other scales). To convert degrees F into degrees C, first subtract 32 from the F temperature then multiply by 5/9. To convert degrees C into degrees F, first add 32 to the C then multiply by 9/5.

Temperature inversion

Air temperature normally lowers with increasing height. When the temperature of the air increases with height, an inversion is present.

Vane

Also called a weather vane or wind vane. The instrument that measures wind direction.

Wind

General circulation of air moving from an area of high barometric pressure to an area of lower pressure.

Wind chill

Relates the cooling effect of the wind to the outside air temperature. It is less a weather term than it is a useful warning of potential freezing of exposed skin.