All precipitation occurs from clouds, and by far the most important cause of clouds is the adiabatic cooling resulting from the upward movement of air.
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Therefore precipitation is classified on the basis of the conditions under which large masses of moist air are actually induced to rise to higher elevations.
There are three possible ways in which an air mass may be forced to rise, and each of these produces its own characteristic type of precipitation. Thus, the following three types of precipitation are based on the types of ascent and the precipitation characteristics:-
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(1) Convectional precipitation
(2) Orographic precipitation
(3) Cyclonic or Frontal precipitation
(1) Convectional precipitation:
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In this type of precipitation, the actuating force is the thermal convection of warm and moist air masses.
Therefore in order to cause precipitation two conditions are necessary: (1) the intense heating of the surface so as to expand and raise the lower layer of the atmosphere, and (2) abundant supply of moisture in the air to provide it with a high relative humidity. Solar radiation is the main source of heat to produce convection currents in the air.
Since convectional precipitation is a warm weather phenomenon, it is generally accompanied by thunder, lightning and local winds. Convectional precipitation is entirely in the form of rain. There may be occasional hail associated with this type of precipitation.
Under favourable conditions it occurs in the low-latitudes and in the temperate zones. The doldrums invariably gets this type of precipitation.
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In this belt of calms lying between the north and south trade winds, the mid-day witnesses the formation of clouds, followed in the afternoon or evening by the occurrence of showery rainfall.
The clouds dissolve away late in the night, and the morning sky is clear. According to W.M. Davis, the large amount of equatorial precipitation is due not only to the activity of the convectional processes on which it depends, but also and largely to the rapid decrease of the capacity for water vapour when air cools at high temperatures prevailing round the equator.
Since convectional precipitation is largely due to the heating of the earth’s surface, the most favourable conditions for its occurrence are always found in the summer months and in the warmer parts of the day.
Though intense heating of land surfaces is of great importance, it should not be taken to be the only factor. Vertical air currents and turbulence as well as surface obstructions such as hills, mountains, etc. may provide the initial upward push for the air that already tends to be unstable. This type of precipitation is of a very short duration and consists of heavy showers.
Convective precipitation is less effective for crop growth than the steady rain. This is so because much of it is drained off in the form of surface drainage, and little remains for entering the soil. Slope wash and gulling are a menace to the loose soil.
However, in the temperate regions, it is most effective in promoting the growth of plants. The main reason is that in the mid-latitudes this type of precipitation occurs in warm seasons when the vegetation is very active.
This type of precipitation is peculiar in that it gives the maximum rainfall with the minimum cloudiness. Clouds involved in this type of precipitation are generally cumulo-nimbus or clouds with great vertical development.
(2) Orographic precipitation:
When mountains or highlands acting as barriers to the flow of air force it to rise, the air cools adiabatically and clouds and precipitation may result. The precipitation thus obtained is referred to as orographic (from Greek: oros = a mountain).
According to Foster, orographic precipitation is that which results from the cooling of moisture laden air masses which are lifted by contact with the elevated land masses.
This type of precipitation is commonly found on the windward sides of mountain ranges lying across the path of prevailing terrestrial winds where those winds pass from the relatively warmer ocean to the land.
After striking the high land, the air is forced to rise and thereby cooled. The moisture, therefore, is condensed and precipitated as rain or snow. However, the process of orographic precipitation is not that simple.
Once the air has been initially pushed upward and condensation starts, the stage has been set for the origin of convection currents. Beyond the condensation level, the latent heat of condensation reduces the adiabatic lapse rate and the ascending air becomes unstable and continues its ascent until its temperature equals that of the surrounding air. The mountain barriers produce only the trigger effect.
Orographic precipitation occurs far inland also where the elevated land masses rise above the surrounding country in the path of moisture-bearing air masses. It usually takes the form of either rain or snow.
Wherever the mountain ranges obstruct the path of moisture-bearing winds and force them to ascend, the maximum precipitation always occurs on the windward slope.
On the other side of these physical barriers, the amount of precipitation abruptly decreases. Thus, on the leeward slopes of these mountain ranges, there always exists a relatively dry area, which is known as the rain shadow.
There are many extensive regions that are found in rain shadows. The cause of these rain shadows may easily be found out. The moist air ascends on the windward side and its moisture is precipitated, but on crossing the peak of the range, no lifting occurs.
Hence there is only a little rainfall, residual of the previous condensation. Another reason for the existence of rain- shadow Pareas is that the descending wind is heated by compression and becomes more unfavourable for precipitation.
In India the south-west monsoon gives copious rainfall on the windward slope of the Western Ghats, whereas on the leeward side there are extensive rain shadow areas.
Another salient feature of orographic precipitation is the inversion of rainfall. An air stream approaching the mountain ranges is given uplift by the air masses lying close to them. Therefore the amount of precipitation starts increasing some distance away from the mountains.
There is a continuous increase in precipitation on the windward slope up to a certain height beyond which it starts diminishing. This is called the ‘inversion of rainfall’. The cause can be easily discovered.
A larger fraction of the moisture of the ascending air mass is precipitated up to a certain altitude, so that by the time the air currents reach the peak, the moisture content is completely depleted.
In mountainous regions, precipitation is not entirely due to the direct effect of uplift, but there are indirect effects as well. In day time, there are convectional currents set up in the air because of the heating of mountain slopes and valleys.
Besides, the belt of heaviest precipitation is determined by .the latitude, season and exposure. In the Himalayan ranges the elevation at which maximum condensation takes place is estimated to be about 1200 meters.
Because of their location in the higher latitudes, the maximum condensation in the Alps occurs at about 2000 meters. It may also be noted that the effect of orographic uplift is felt some distance away from the physical barrier, such as a mountain range or a steep escarpment of a plateau.
This is so because the mass of stagnant air in front of the barrier has a blocking effect, and the rain-bearing wind has to ascend the wind block.
(3) Cyclonic or frontal precipitation:
Cyclonic or frontal precipitation occurs when deep and extensive air masses are made to converge and move upward so that their adiabatic cooling results. Whenever there is lifting of the air masses with entirely different physical properties, the atmosphere becomes unstable.
When this happens, the stage for large-scale condensation and precipitation has been set. If an additional process is in operation so that the rain drops of the required size are formed, the precipitation results.