Meteorology

C

Air Pressure

Air is held to the earth by gravity. This strong invisible force pulls the air downward, giving air molecules weight. The weight of the air molecules exerts a force upon the earth and everything on it. The amount of force exerted on a unit surface area (a surface that is one unit in length and one unit in width) is called atmospheric pressure or air pressure. The air pressure at any level in the atmosphere can be expressed as the total weight of air above a unit surface area at that level in the atmosphere. Higher in the atmosphere, there are fewer air molecules pressing down from above. Consequently, air pressure always decreases with increasing height above the ground. Because air can be compressed, the density of the air (the mass of the air molecules in a given volume) normally is greatest at the ground and decreases at higher altitudes.

A column of air 1 sq cm (.16 sq in) in area, extending from the ocean surface (sea level) up to the top of the atmosphere would contain slightly more than 1 kg (about 2.35 lb) of air. If more air molecules are packed into the column, the total weight of air at the bottom of the column would increase, and the air pressure there would increase. If air is removed from the column, the total weight of the air at the bottom of the column would decrease, and the air pressure would decrease. The most common unit of pressure found on surface weather maps is the millibar (1 millibar equals 100 newtons/sq m, where newtons are the metric unit of force). Inches of mercury is a pressure unit commonly used in television and radio weather broadcasts. On average, at sea level, the standard value of the atmospheric pressure is 1013.25 millibars, 29.92 inches of mercury, and 14.7 lbs/sq in. Barometers are instruments that measure air pressure.

D

Wind

Wind is air in motion. It is caused by horizontal variations in air pressure. The greater the difference in air pressure between any two places at the same altitude, the stronger the wind will be. The wind direction is the direction from which the wind is blowing. A north wind blows from the north and a south wind blows from the south. The prevailing wind is the wind direction most often observed during a given time period. Wind speed is the rate at which the air moves past a stationary object.

A variety of instruments measure wind. A wind vane measures wind direction. Most wind vanes consist of a long arrow with a tail that moves freely on a vertical shaft. The arrow points into the wind and gives the wind direction. Anemometers measure wind speed. Most anemometers consist of three or more cups that spin horizontally on a vertical post. The rate at which the cups rotate is related to the speed of the wind.

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E

Precipitation

Precipitation is any form of water (either liquid or solid) that falls from the atmosphere and reaches the ground, such as rain, snow, or hail. Rain gauges are instruments that measure rainfall. The standard rain gauge consists of a funnel-shaped collector that is attached to a long measuring tube.

F

Humidity

Humidity refers to the air’s water vapor content. Hygrometers are instruments that measures humidity. The maximum amount of water vapor that the air can hold depends on the air temperature; warm air is capable of holding more water vapor than cold air. Relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor that the air could hold at that particular temperature. When the air is holding all of the moisture possible at a particular temperature, the air is said to be saturated. Relative humidity and dew-point temperature (the temperature to which air would have to be cooled for saturation to occur) are often obtained with a device called a psychrometer. The most common type of psychrometer is a sling psychrometer. This instrument consists of two thermometers mounted side by side and attached to a handle that allows the thermometers to be whirled. A cloth wick covers one thermometer bulb. The wick-covered thermometer bulb (called the wet bulb) is dipped in water, while the other thermometer bulb (the dry bulb) is kept dry. Whirling both thermometers allows water to evaporate from the wick, which cools the wet bulb. By looking up the dry and wet bulb temperatures in a set of tables, known as humidity tables, it is possible to find the corresponding relative humidity and dew-point temperature.

III

Special Meteorological Instruments

Meteorologists have developed several sophisticated instruments that measure multiple physical characteristics of the air simultaneously and at more than one location. The most important of these special instruments are radiosondes, Doppler radar, and weather satellites.

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