Humidity and Precipitation
Humidity of the air refers to the content of water vapour present in the air at a particular time and place. The moisture retaining capacity or humidity capacity refers to the capacity of an air of certain volume at certain temperature to retain maximum amount of moisture content.
Saturated air: The air having moisture content equal to its humidity capacity is called saturated air. Humidity capacity of air is directly proportional to the temperature, i.e. higher the temperature higher the humidity capacity of "the air.
Dew Point: The temperature at which the air becomes "saturated is called Dew Point. Humidity is expressed in three forms:
- Absolute Humidity : The total weight of. moisture content per volume of air at a definite temperature is called absolute humidity.
- Specific Humidity : It represents the actual quantity of moisture present in a definite air. It is expressed In gm/kg3.
- Relative Humidity : Relative humidity is defined as a ratio of the amount of water vapour actually present in the air having definite volume and temperature (absolute humidity) to the maximum amount the air can hold (humidity capacity). It is generally expressed in percentage.
Forms of Condensation
The transformation of gaseous form of water into solid or liquid form is called condensation. The process of condensation depends upon two factors:
- Temperature deficiency and
- Relative humidity of air
Due to fall in temperature, condensation starts around the dust particles present in the atmosphere. These tiny dust particles are called 'hygroscopic nuclei'.
Dust, smoke and salt particles are considered good hygroscopic nuclei. The process of condensation results into the formation of dew, fog, clouds and mist. These are known as different forms of condensation.
- Dew : When the temperature of the. air falls below the dew point, the water vapour present in it starts condensing and gets accumulated on the leaves of plants and trees in the form of small water droplets. It is called dew.
- Frost : When the process of condensation takes place at such temperature that the water vapour gets converted into solid form without condensing into liquid form, it is called frost. In other words we can say that frozen dew is called frost.
- Fog : Fog consists of small microscopic water droplets which are kept in suspension in the air near the ground surface. It is formed when the moist air (with relative humidity above 97%) becomes saturated, reaches its dew point and further cools.
- Mist : Mist is a type of fog. In case of mist, the visibility is more than 1 km but less than 2 km, whereas in the case of fog it is difficult to see the objects beyond 200 m of distance.
Clouds are defined as aggregates of innumerable tiny water droplets, ice particles or mixture of both in the air generally much above the ground surface.
Classification of Clouds
- High Clouds (Height 6000-12000m)
- Cirrus clouds : The high altitude detacher clouds having fibrous (chain-like) or silk; appearance are called cirrus clouds. These clouds are indicative of cyclones.
- Cirro-cumulus : These are white coloured clouds having patches of small white flakes or small globules which are arranged it distinct groups.
- Cirro-stratus clouds : These clouds are generally white in colour and spread in the sky like milky thin sheets. They indicate the arrival of a cyclone in the near future.
- Middle clouds (Height 2000-6000m)
- Alto-stratus clouds: These are thin sheets of grey or blue clouds having fibrous or uniform appearance.
- Alto-cumulus clouds: These clouds are characterised by white and gray wavy layers or globular forms are called as 'Sheep clouds', or 'wool pack clouds'.
- Low clouds (Height upto 2000m)
- Stratus clouds: These are dense, low lying fog-like clouds of dark gray colour, but are seldom close to the ground surface.
- Nimbo-stratus clouds: These are low clouds of dark colour, very close to the ground surface.
When the moist air rises upwards, it starts condensing due to fall in temperature. Clouds are formed after condensation of water vapour around hygroscopic nuclei. Rainfall occurs only when cloud droplets become so large due to that the air becomes unable to hold them. Based on the mode of origin, rainfall is classified into these types:
- Convectional Rainfall: It occurs due to thermal convection currents caused by the insolation heating of ground surface. It occurs when the warm and moist air rises due to convection. When this reaches at certain height it becomes saturated which cause heavy rainfall.
- Orographic Rainfall: When warm and moist air is obstructed by any hill or plateau, it starts ascending along the slope of the hill or plateau and gets cooled. As a result, it gets saturated and the process of condensation starts. But, as the wind starts descending along the slope of opposite side, it becomes warm and dry and and due to decreased humidity a little rainfall occur. This region is called 'Rain shadow region' or 'Leeward slope'.
- Cyclonic or Frontal Rainfall: Cyclonic rainfall is caused due to ascending of moist air and adiabatic cooling caused by convergence of two extensive air masses of entirely different physical, properties.