- The water circulation in the atmosphere primarily occurs through the processes of evaporation, transpiration, condensation and precipitation.
Condensation and Precipitation in Tropical areas
- The capacity of air to hold water vapour decreases with decrease in temperature and at one point ceases.
- The excess water vapour the air cannot hold anymore condenses into liquid form or solid form depending on temperature, in the process called condensation.
- At a height where there is free air, condensation happens around small particles called hygroscopic condensation nuclei.
- Dust, smoke, salts etc. act as condensation nuclei.
- These minute water droplets or ice crystal formed around a condensation nuclei as a result of condensation form what we know as clouds.
- Precipitation in tropical areas
- In tropical areas, convectional rainfall occurs.
- Air when heated becomes light and rises up in convection currents.
- As it rises, it expands and loses heat and consequently, condensation takes place and clouds are formed.
- Continuous condensation in free air helps the condensed particles to grow in size.
- When the condensed particles are so big that they are not able to hold against gravity, they fall on to the earth’s surface as rainfall.
- Thus, precipitation is the process of release of moisture after condensation.
In Focus: Artificial Rain by Cloud Seeding
- Cloud seeding is the process of augmenting the process of precipitation by manipulating the size of condensation nuclei.
- In this process, substances capable of absorbing water vapour such as silver iodide, sodium chloride, potassium chloride, dry ice (solid carbon dioxide) are sprayed in the clouds using an aircraft or an artillery gun.
- Silver Iodide or other artificial salts, which are hygroscopic, are capable of absorbing more moisture.
- This catalyzes the growth of condensation nuclei which in turn results in enhanced precipitation.
- India, being a tropical area, does not have ice crystals as condensation nuclei in its clouds. (This is due to higher temperature and moisture content in the air)
- In rain-bearing clouds of India, water droplets forming the clouds are smaller in size.
- Being small in size, the water droplets formed in the clouds are blown away in the wind before they reach earth.
- Salts such as silver iodide, sodium chloride, potassium chloride, dry ice ensure that the size of the droplets is large enough to fall as rain.
- Further, once the condensation occurs, it releases latent heat which in turn increases the capacity of the air to hold more moisture and also draws more moist air from the ground, thereby enhancing the process of condensation and in turn precipitation
Cloud Seeding in India
- Could Seeding is done in various parts of India in drought-affected areas, including rain shadow region of Maharashtra, Northern Karnataka, rainfall deficient regions of Central India, Gujarat, Rajasthan etc.
- Urban areas like Delhi and Mumbai have contemplated the use of cloud seeding for combating air pollution.
Section : Environment & Ecology