Atmospheric humidity Water is always present in the atmosphere in the form of invisible water vapour, normally known as humidity of the air. When the atmosphere contains the maximum possible amount of water vapour it is said to be saturated at the particular temperature and pressure. Any increase in temperature, water remaining constant, will make the air unsaturated. In unsaturated condition, the water vapour content of air is usually expressed as relative humidity, which is the ratio between the actual humidity present and the saturation humidity possible at that temperature.
The relative humidity of a place, is being affected by temperature and pressure. It is also affected by wind, exposure to radiation, vegetation and water content of the soil. The evaporation of water from plants or a body of water is directly dependent on the relative humidity of the atmosphere.
Humidity: The terminology related to humidity and concerned with gaseous form of water i.e., water vapour, several expression of the amount of water vapour in the air is used.
Absolute humidity: It denotes the actual mass of water vapour in given volume of air. It may be expressed as the number of grams of water vapour in a cubic meter of moist air or mass of water vapour per unit volume of air.
Specific humidity: It is defined as the moisture content of moist air as determined by the ratio of the mass of water vapour to the mass of moist air in which the mass of water vapour is contained.
Relative humidity: Relative humidity is a common parameter for expressing water vapour content of the air. It is the percentage of water vapour present in the air in comparison with saturated condition at a given temperature and pressure.
Mixing ratio: The mass of water vapour per unit mass of dry air is a convenient parameter to express the relative composition of the mixture. It is defined as the ratio of the mass of water vapour to the mass of dry air with which the water vapour is associated.
Dew point: The temperature at which saturation occurs in a given mass of air. The dew point temperature is often compared with the temperature of free air and also used to predict the occurrence of fog, dew, frost or precipitation.
Vapour pressure: This is the amount of partial pressure created by water vapour in the air expressed in the units of millibar (or) inches of mercury. Evapotranspiration of crop plants increases with temperature but decreases with high relative humidity affecting the quantity of irrigation water.
Moist air favours the growth of many fungi and bacteria and these affect seriously the crop. The blight diseases of potato and tea are common examples of diseases spread under moist weather. Similarly many kinds of insect parasites such as aphids and jassids thrive well moist conditions.
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