Water exists in the atmosphere in all of the three states of matter- solid, as snow, hail and ice particles; liquid, as rain or cloud droplets; and gas, as invisible water vapor. Water may change from one state to another. The change from solid ice to liquid water is called melting; the reverse process is freezing. The change from liquid water to water vapor is called evaporation. The change from water vapor to liquid water is called condensation.
Most water vapor in the atmosphere comes from oceans, lakes, marshes, and glaciers. Some water vapor also comes from moist ground, from the leaves of plants, and from erupting volcanoes. Water vapor is spread throughout the troposphere by convection currents and winds. Since rising air currents stop at the tropopause, there is little water vapor above the troposphere.
The molecules of liquid water are always in motion. Molecules with sufficient energy to escape the water’s surface in to the atmosphere are said to evaporate. At ordinary temperatures, evaporation is slow because few molecules have enough energy to escape the liquid’s surface. As water molecules absorb heat energy, they speed up. Then more molecules have sufficient energy to escape the water’s surface. With increasing temperatures the evaporation rate increases. When water evaporates, it enters the atmosphere in the form of water vapor.
Water and other liquids absorb heat energy from their surroundings when they evaporate. Since the high-energy molecules leave the liquid’s surface, the remaining liquid molecules have less average energy. This makes evaporation a cooling process.
The capacity of air for holding water vapor depends on the temperature of the air. The warmer the air, the more water vapor it can hold. The amount of water vapor actually present in the air is called specific humidity. It is the number of grams of water vapor in one kilogram of air. On a hot, humid summer day, the specific humidity is about 20 grams per kilogram. On a cold winter day, the specific humidity is about 5 grams per kilogram.
When the specific humidity equals the air’s capacity for holding water vapor, the air is saturated. For example, the air between the water’s surface and the glass cover of a fish tank is likely to be saturated. Drops of water hang from the underside of the cover. The drops show that although water continues to evaporate from the warm tank, an equal amount of water condenses from the saturated air. The air’s capacity for holding water vapor roughly doubles for every rise in temperature of about 11 degrees Celsius. For example, a kilogram of air at 15.5 degrees Celsius can hold about 11 grams of water vapor. A kilogram of air at 26.5 degrees Celsius can hold about 22 grams. A regression equation for finding water vapor capacity of a kilogram of air in terms of its temperature is roughly .02066*X2 + .1322*X + 3.9865
Meteorologists need to know how near the air is to its capacity for holding water vapor. This information is the relative humidity. Relative humidity compares the actual amount of water vapor in the air (its specific humidity) with the maximum amount of water vapor the air can hold a that temperature (its capacity). Relative humidity is usually stated as a percent. It can be calculated by dividing the specific humidity by the capacity. The result is multiplied by 100 to express the answer as a percent. For example, air at a temperature of 26.5 degrees Celsius has a capacity of 22 grams. If its specific humidity at a particular time is 11 grams, its relative humidity is 50 percent. Saturated air has a relative humidity of 100%.
Instruments used to measure relative humidity are called hygrometers. One simple type of hygrometer, the hair hygrometer, is based on the principle that human hair stretches when it is humid. One end of a bundle of hairs is fixed. The other end is attached to a pointer. When the air is humid the hair stretches and changes the humidity reading indicated by the pointer. When the air becomes dry again, the hair shrinks back to a shorter length, once again changing the position of the pointer.
Another form of hygrometer is the psychrometer. It works on the principle that evaporation causes cooling. A psychrometer consists of two identical thermometers. One, the wet-bulb thermometer, has a water-soaked wick wrapped around its bulb. The other is the dry bulb thermometer. The thermometers are whirled or fanned so air circulates past the two bulbs. The dry-bulb thermometer shows the air temperature. The wet-bulb thermometer usually shows a lower temperature. As the water evaporates from the wick of the wet-bulb thermometer, heat is taken from its bulb. The drier the air, the faster the evaporation, and the lower the reading. The psychrometer itself only shows two readings. These readings give signs of how dry the air is but do not show the relative humidity. With the thermometer readings, however, the relative humidity can be found by using a table. Tables have been worked out by actual experiments.
