Effects of Forests on Rainfall or Precipitation
What is Precipitation?
Precipitation is the total supply of all forms of moisture emanating from the clouds and falling to the ground. It is the deposition of atmospheric moisture. It is the most important phase in the hydrological cycle. It is the immediate source of stream run-off, hence its occurrence, distribution and intensity determine the hydrologic behaviour of streams, namely total discharge, discharge regime, and quality of the discharge.
Classification of Precipitation
Precipitation can be classified as Convective precipitation in which Energy form the source ‘sun’ reaches the earth by passing through different zones in the form of rays. On reaching the atmosphere, these reduce the bulk of air and increase its temperature. With less bulk, light air tends to rise in a cooler, denser, surrounding. For every 200 ft, 1 C temp is reduced. By vertical convection, the ascending air expands and in consequence cooling dynamically this leads to convective precipitation.
Cyclonic precipitation is the main type of ppt occurring in the plains and in fact, much of the monsoon ppt in the lowlands of Pakistan is of this type (80 to 90%). As solids heat and cool more quickly than liquids A land mass is, therefore, heated more quickly in summer and cooled more easily in winter than the oceans which surround it and so does the air above the land mass. In summer over the hot land mass, the air heats and rises and a low-pressure zone or cyclone is created. High pressure, anticyclones, persists over the neighboring cooler oceans, and air currents begin to converge from the high-pressure zones to low-pressure cyclones. As these currents meet, they ascend. The summer monsoon in Pakistan originates from this unequal heating b/w the oceans (the Indian Ocean and the Arabian Sea) in winters, the mechanism is reversed, and winters blow off-shores (NW monsoon).
Frontal precipitation in conjugation with the convergence of air masses of different temp in low-pressure cyclones, fronts (a line along which one mass of air meets another that is different in temperature or density) may be formed and air can be forced to ascend mechanically in that warmer, lighter air glides over denser, cooler air. Occasionally, also, cold air gets wedged under warm air. In this way, frontal storms, originate out of the conflict of warm and cold air masses. Frontal storms are caused by the force of a layer of warm air over a layer of colder as a longer period. In cold front, warm moist air produces intense rains over a small area. In a warm front, cold air produces rain falls over a large area.
Orographic precipitation results from the mechanical lifting of warm moist air over mountain barriers. It is causal by lifting of air when traversing mountain range laying across their paths. Much of the rain in the outer Himalaya hills both during the summer monsoon and in winter.
Role of Forests in Forming Precipitation / Rainfall
Forests play a vital role in forming local precipitation and rainfall.
According to some recent calculations, forests increase local precipitation by about 5 t0 10%. One of the main reasons is due to their Orographic and microclimate effect. These effects create conditions favourable for the condensation of clouds which results in the form of precipitation which includes rainfall, snow, hail etc depending upon the rate of condensation and lifting up of the air.
Forests reduce temperature and increase humidity. The temperature in forests is 3 to 8 C less than in adjoining open areas. Reduced temperature results in fewer evaporation losses and increased humidity. This increase in humidity favors rainfall.
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