PRECIPITATION
PRECIPITATION :
The process of continuous condensation in free air helps the condensed particles to grow in size. When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth's surface. So after the condensation of water vapor, the release of moisture is known as precipitation. This may take place in liquid or solid form. The precipitation in the form of water is, called -rainfall, when the temperature is lower than the 0°c, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals. These crystals form flakes of snow. Besides rain and snow, other forms of precipitation are sleet and hail, though the latter are limited in occurrence and are sporadic In both time and space.
Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freeing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet. Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.
Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones. These are formed by the rainwater passing through the colder layers. Hailstones have several concentric layers of ice one over the other.
Type of Rainfall: On the basis of origin, rainfall may be classified into three main types- the convectional, orographic or relief· and the cyclonic or frontal.
Conventional rain: The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and losses heat and consequently, condensation takes place and cumulous clouds are formed. With thunder and lightening, heavy rainfall takes place but this does not last long. Such rain is common in the summer or in the hotter part of the day. It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.
Orographic rain: When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed. The chief characteristic of this sort of rain is that the windward slopes a receive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. It is also known as
the relief rain.
Cyclonic rain: Cyclonic rain is associated with cyclones and depressions. Warm air and cold air are of different densities, and they do not mix well. The warm air, being less dense, rises gradually over the mass of cold air, expands further and cools and rain falls.
Rainbow :-
A rainbow is an isolated optical effect caused by the Sun's ray being refracted (bent) and reflected as they pass through millions of raindrops. For a rainbow to occur there needs to be bright sunshine and rain occurring at the same time.
Cloud deeding :- Cloud seeding, a form of weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practiced in airports.
Process:-
1. Aircraft or artillery spray chemicals (often silver iodide or dry ice) into clouds to encourage tiny vapour droplets to coalesce
2. Droplets of super cooled water (liquid below freezing) coalesce into snow and melt as they fall
3. Heat released as the droplets freeze boosts updrafts, which pull more moist air into the cloud
WORLD DISTRIBUTION OF RAINFALL :-
Different places on the earth's surface receive different amounts of rainfall in a year and that too in different seasons.
In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily. the coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water. Between the latitudes 35° and 40° N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45° and 65° N and S of equator, due to the westerlies, the rainfall is first received on the western margins
of the continents and it goes on decreasing towards the east. Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decrease towards the leeward side.
On the basis of the total amount of annual precipitation, major precipitation regimes of the world are identified as follows:
The equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone and the coastal areas of the monsoon land receive heavy rainfall of over 200 cm per annum. Interior continental areas receive moderate rainfall varying from 100-200 cm per annum. The coastal areas of the continents receive moderate amount of rainfall. The central parts of the tropical land and the eastern and interior parts of the temperate lands receive rainfall varying between 50100 cm per annum. Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive. very low rainfall-less than 50 cm per annum. Seasonal distribution of rainfall provides an important aspect to judge its effectiveness. in some regions rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of the cool temperate regions.
OTHER FORMS OF PRECIPITATION :-
Precipitation means deposits of water, in either liquid or solid form which reach the earth from the atmosphere. It includes rain dew, sleet, hail and snow. Rainfall and precipitation are the same things and both words can be used to denote the total amount. of water in all forms which is deposited on a given area.
Snow : In this case precipitation takes the form of ice crystal of a delicate, feathery structure. Snow is formed from water vapour in the atmosphere at temperatures below freezing point. In hot countries this snow is melted into rain as it falls through the lower warmer air, but in cold countries and on the tops of high mountains and plateaus it is not melted but falls in flakes like tiny feathers, covering everything with a soft white mantle. On the tops of high mountains such as the Himalayas practically all rainfall is in the form of snow. On the average it requires 10-12 cm of snow to equal 1
cm of rain.
Hail : consists of the hard pellets of ice which fall from cumulonimbus clouds and are often associated with thunderstorms. They are of various shapes and sometimes they have been known to weigh nearly 1 kg. A severe hair-storm can cause great damage to growing crops.
Dew is the moisture deposited on the earth's surface, or on objects near to the earth's surface such as blades of grass and small bushes. It occurs at night under calm clear conditions when radiation from the ground has cooled the lower layers of the atmosphere below the Dew Point and the water vapour has condensed into drops. Cairn weather and a clear sky provide the best conditions for production of dew.
Frost forms when air temperatures have fallen below the freezing point. Conditions favorable for frost are a mass of dry cool polar air followed by clear, calm nights during which the surface air may be reduced to below freezing. Frost is common in temperature lands. It is very harmful to plants. Most crops are sensitive to frost.
Fog is the dense mass of small water drops on smoke or dust particles in the lower layers of the atmosphere. Fog is essentially a cloud at the surface of the earth. A Fog will arise also when a warm damp current of air passes over a cold surface.
Mist is the result of condensation of water droplets on particles of smoke and dust. Mist is said to prevail as long as the visibility exceeds between 1 and 2 km i.e., when visibility is better than in a fog.