Humidity

Humidityis the amount of water vapour in the air. Water vapour is the gaseous state of water and is invisible.Simply it is the concentration of water vapour within the air. This concentration is expressed in four ways: maximum humidity, absolute humidity, specific humidity, and relative humidity. It is a highly variable climatic factor which forms only a small portion (from 0-4 % with the average of around 2% in the atmosphere). Humidity is measured by the instrument called Hygrometer.Humidityindicates the likelihood of precipitation, dew, or fog. Higherhumidityreduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. It is of the following types:

Maximum humidity: Itrefers to the maximum amount of water vapour that a definable body of air can hold. The maximum humidity of an air parcel is completely governed by the air temperature. Warmer air is capable of holding much more water vapour than colder air, because air expands in the process of warming, which leaves room for more water vapour. For instance, at a temperature of 100c, one cubic meter of air can hold 11.4 grams of water vapour. The same volume of air can hold 22.2 grams of water vapour, once the temperature rises to 21oC. The relationship between air temperature and maximum humidity is described by a saturation curve, as presented in Fig. 2.2.

Absolute humidity: This term is used to describe the actual amount of water vapour that is saturating the air. Absolute humidity is calculated by finding the mass of water vapour in an area and dividing it by the mass of air in the same area. It is usually expressed as grams per cubic meter of air. It changes from place to place and from time to time.

Relative humidity: This is the type of humidity that meteorologists are typically referring to on their weather reports because it is a more practical measure of atmospheric moisture. It is the ratio of the air’s actual water vapour content to its water vapour capacity at a given temperature. Since the relative humidity is based on the air’s water vapour content as well as on its capacity, it can be changed in either of the two ways.

  1. If moisture is added by evaporation, the relative humidity would increase
  2. A decrease in temperature will cause an increase in relative humidity.

Figure 2.2: The saturation curve Maximum humidity increases as temperature increases. In other words, warmer air can hold more water vapour than cooler air.

Specific Humidity: This term is used as a ratio of the amount of water vapour in the air to the amount of dry air in the area. Since it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by change in temperature or pressure.

Significance of Humidity

The humidity is of paramount importance because of the following:

  1. The water vapour present in rain bearing clouds is responsible for all kinds of precipitation, and the amount of water vapour present in a given volume of air indicates the potential of the atmosphere for precipitation.
  2. Water vapour absorbs radiation – both incoming and terrestrial. It thus plays a crucial role in the earth’s heat budget.
  3. The amount of water vapour present in the atmosphere decides the quantity of the latent energy stored up in the atmosphere for the development of storms and cyclones.
  4. The atmospheric moisture affects the human body’s rate of cooling by influencing the sensible temperature.

Teacher in-charge: Irshad Ahmad Bhat