Humidity of the air can be measured in two ways. A device known as a hygrometer indicates relative humidity on a calibrated dial. A continuous record of humidity obtained by means of a hygrograph.
A different principle is applied in the sling psychrometer is fully saturated (relative humidity 100 percent), there will be no evaporation from the wet cloth and both thermometers will read the same. If, however, the air is not fully saturated, evaporation will occur, cooling the cloth-covered thermometer below the temperature shown on the ordinary thermometer. Because the rate of evaporation depends on dryness of the air, the difference in temperature shown by the two thermometers will increase as relative humidity decreases.
How condensation occurs - Falling rain, snow, sleet, or hail, referred to collectively as precipitation, can result only where large masses of air are experiencing a steady drop in temperature below the dew point. This condition cannot be brought about by the simple process of chilling of the air through loss of heat by long wave radiation during the night. Instead, it is necessary that the large mass of air be rising to higher elevations.
One of the most important laws of meteorology is that rising air experiences a drop in temperature, even though no heat energy is lost to the outside. The drop of temperature is a result of the decease in air pressure at higher elevations, permitting the rising air to expand. Because individual molecules of the gas are more widely diffused and do not strike one another so frequently, the sensible temperature of the gas is lowered. When no condensation is occurring, the rate of drop of temperature, termed the dry adiabatic rate, is about 5 l/2F° per 1000 feet of vertical rise of air. In metric units the rate is 1C° per 100 meters. The dew point also declines with rise of air; the rate is 1 F° per 1000 ft (0.2 C° per 100 in).
If water vapour in the air is condensing, the adiabatic rate is less, about 3.2 F° per 1000 ft (0.6 C° per l00m), owing to the partial counteraction of temperature loss through the liberation of latent heating during the condensation process. This modified rate is referred to as the wet adiabatic, or saturation adiabatic rate. Adiabatic cooling rate should not be confused with the environmental lapse rate. The environmental lapse rate applies only to still air whose temperature is measured at successively higher levels.
Where condensation is occurring directly in the form of snow (ice crystals), the adiabatic rate is intermediate in value between dry and saturated rates.
Because the actual fall of rain, snow, or other forms of precipitation is preceded by the formation of clouds, we must first consider the various types of clouds and their significance.
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