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Old Wednesday, November 14, 2007
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Default Geography One - PRECIPITATION and RAINFALL

PRECIPITATION


INTRODUCTION
Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form. It can be seen as a small cloud of minute water particles in one’s breath on a cold morning. It forms on the inside of a window when the temperature outside the room is very much colder. It may condense on to the ground as dew or frost. It forms mist and fog in the lower atmosphere, and it gives rise to clouds, rain, hail, snow and sleet, throughout the troposphere.

COOLING OF AIR – A necessary requirement for precipitation formation
The air must be cooled before any of these types of precipitation can form and this can happen in several ways. An air mass may be forced to rise to cooler levels, for example, when it blows across a range of hills. It may blow over a land or sea at a lower temperature or it may meet the air of different temperatures and relative humidity, and the warmer and lighter of the two is forced to rise over the cooler, thereby becoming cooler itself. Air may blow from warmer into cooler latitudes, and the earth surface may itself radiate heat, so the air in contact with it is cooled. Each of these processes may lead to some form of precipitation.

IMPORTANCE OF PRECIPITATION
While clouds may be the major adjusters of global energy imbalances, precipitation can be considered as a short-term intense form of energy exchange. Precipitation, moreover, is one of the global climatic elements important to man, since it yields his major source of fresh water, a basic requirement for nearly all terrestrial plant and animal life. The total precipitation that falls on the surface is not accurately known, but estimates range from 25 to 40 inches/year. One of the most accepted figures is 34 inches. However, amount differs widely from place to place.

FORMS OF PRECIPITATION
Precipitation may take several forms and each has its own process of development and distribution:

1. Drizzle or Drizzly Rain
This is a fine settling of small rain drops, usually ranging from 0.1 to 0.5 mm (0.0025 to 0.02 in) and intensity less than 0.01 mm/hour. Although small, the drops are numerous. The direction of movement is highly variable and tends to follow air currents easily. The drops descend to earth vertically only in unusually calm air.

2. Rain
Rain is formed when cloud drops in large numbers are caused to coalesce (come together) into drops too large to remain suspended in the air. The drops, from 0.5 mm, may grow by colliding with other drops and joining with them to become as large as 7 mm (0.25 in.) in diameter but above this size they are unstable and break into smaller drops. The typical form taken by most rain drops is kidney-shaped. Small drops are possible, but if so, they are much more widely spaced then in drizzle.

3. Sleet
Sleet, as the term is used in the United States, consists of pellets of ice produced from freezing of rain. The rain drops form in an upper, warmer layer, but fall into an underlying cold air layer. Elsewhere in the English speaking countries, sleet means a mixture of rain and snow.

4. Snow

Snow is the precipitation of crystalline water, mainly in the hexagonal, star like forms. At times, however, the flakes may be in the form of hexagonal prisms, or platelets. They display infinite variations in their beautiful symmetrical patterns. Larger flakes are formed when crystallization takes place just below freezing. Snow formed in extremely cold air, is likely to be of much smaller, prismatic forms.
Snow varies considerably in its liquid water equivalency per volume, but a rough figure used is 10 inches of snow per inch of melt water.

5. Hail
Hail consists of rounded lumps of ice, having an internal structure of concentric layers, much like an onion. Hailstones range from 0.2 to 2 inches (0.5 to 5 cm) in diameter and may be extremely destructive to crops and light buildings. Hails occur only from the cumulonimbus cloud type, inside of which are extremely strong updrafts of air. Suspended in the powerful updrafts, hailstones grow by the attachment and freezing of droplets. Eventually, hailstones escape from the updrafts and fall to earth.
In warmer countries (tropical and sub-tropical), the clouds attain great heights leading to the largest hail stones. In temperate lands, the hail stones attain a much larger size in summer. Soft hails fall in winter consisting of small pellets.

6. Glaze
When rain or drizzle falls upon a ground surface that is covered by an air layer of below-freezing temperature, the water freezes into ice after striking the ground or other solid surfaces such as trees, houses or wires, which destroys the surface tension of the raindrops. The cooling of ice that results is called a “Glaze” and as icing storm is said to have occurred. Icing storm causes great damage, especially to telephone and power wires and to tree limbs. Roads and side walks are make extremely hazardous.

7. Grapple or Granular Snow
This is composed of small opaque soft, snow like pellets that are the equivalent of frozen drizzle.

PRECIPITATION PRODUCING CONDITIONS

Precipitation results when air rises and is adiabatically cooled below the dew point so rapidly that not only do clouds form, but rain, snow or hail is produced. There are three ways due to which large masses or air are induced to rise to higher elevations. They are:

1. Convection Lifting
Convectional precipitation results from a “convection cell”, which is simply an updraft of warmer air, seeking higher altitude because it is lighter than surrounding air.
Suppose that on a clear, warm summer morning the sun is shinning upon a landscape consisting of patches of open fields and woodlands. Certain of these types of surfaces, such as the bare ground, heat more rapidly and transmit radiant heat to the overlying air. Air over a warmer patch is thus warmed more than adjacent air and begins to rise as a bubble. As the air rises, it is cooled adiabatically so that eventually it will reach the same temperature as the surrounding air and come to rest. Before this happens, however, it may be cooled below the dew point. At once condensation begins and the rising air column appears as a cumulus cloud. Usually the small cumulus cloud dissolves after drifting some distance down wind. However, should this convection continue to develop, the cloud may grow to a cumulonimbus mass or thunder storm, from which heavy rain will fall.
The air rising during condensation is unstable in properties. In such air the updraft tends to increase in intensity as times goes on. Of course, at very high altitudes, bulk of the water vapor having condensed and fallen as precipitation, the energy source is gone; the convection column then weakens and air rise finally ceases.
Unstable air, given to spontaneous convection in the form of heavy showers and thunderstorms, is most likely to be found in warm, humid areas, such as the Equatorial and tropical oceans and their bordering lands throughout the year, and the middle latitude regions during the summer season.

2. Orographic Lifting
The second precipitation producing mechanism is described as “Orographic”, which means “related to mountains”. Prevailing winds or other moving masses of air may be forced to flow over mountain ranges. As the air rises on the windward side of the mountain range, it is cooled at the adiabatic rate. If cooling is sufficient, precipitation will result. After passing over the summit of the mountain, the air will begin to descend to the leeward side of the range. Now it will under go warming through the same adiabatic process, and having no source from which to draw up moisture, will become very dry. A belt of dry climate, often called a “Rain Shadow” may exist on the leeward side of the rain. Several of the important dry deserts of the earth are of this type.
An excellent illustration of the orographic precipitation and rain shadow occurs in the far west of the United States. Prevailing westerly winds bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada range, whose summits rise to 14,000 ft. (4,000 m) above sea level. Heavy rainfall is experienced on the windward slopes of these ranges, nourishing great forests. Passing down the steep eastern face of the Sierras, air must descend nearly to sea level, even blow sea level in Death Valley. The adiabatic heating thus caused, and the consequent drop in humidity, produces part of the America’s great desert zone, covering a strip of eastern California and all of Nevada.
Much orographic rainfall is actually of the convectional type, in that it takes the form of heavy convectional showers and thunder storms. The storms are induced, however, by force ascent of unstable air as it passes over the mountain barrier.

3. Convergent / Cyclonic / Frontal Lifting
This occurs when surface air streams (winds) converge towards the same air space. When this happens, part of the converging air is displaced upwards. It would be exceedingly uncommon for such air streams, coming from different directions, to have exactly the same temperatures. Invariably, one is warmer than the other, and being warmer, is lighter and hence displaced upwards.
The line of contact along the separation plane between air masses and the surface is termed as a “Front” and hence such lifting of the warmer air is termed as “Frontal Lifting” or “Frontal Displacement”. The amount and steepness of frontal lifting vary greatly; hence, precipitation are likely to vary accordingly.
Convergence may take place at times, even within the same air stream. If some external factor, such as a slight pressure change, forces a huge air stream hundreds of miles wide and thousands of miles long to kink (twist) or meander (twist) slightly in its course, more air converges in one portion than in another, and a broad but shallow upward displacement may take place.
Given a slight angular deflection and sufficient water vapor pressure, the large convergence, if lifted into the cloud zone may be developed into a huge diffusion eddy (whirlpool) hundreds of miles in diameter and capable of releasing large amounts of precipitation for protracted (prolonged) periods. If such giant eddies shrink in size, they become more concentrated and violent.
Some convergent precipitation may also be produced as a result of orographic effects. The heavy rainfall in the Assam region of north-eastern India is partially caused by the convergence of air in the pocket formed by the sharp north-east curve of the Himalayan mountain front. The heavy rainfall, although, heaviest on the foothills, continues well out in front of the mountains, and undoubtedly is the result of convergence.

RAINFALL


INTRODUCTION
Air, which rises higher in the atmosphere, is normally cooled in the process. Condensation takes place if the temperature falls below the dew point and billions of tiny water droplets or ice crystals gather to form a cloud. Initially the droplets are too small to fall against the resistance of the surrounding air. Eventually a disturbance in the atmosphere may cause the small droplets to be thrown together and form drops, which are large enough to overcome the atmospheric resistance, and to fall as rain. The minute drops, which occur as drizzle, are only just large enough to fall, whereas those, which form in thunder storms, are often very large.

STAGES OF RAINFALL
According to Humphreys, there are four stages in the rain process: (see Gupta)
1. Vertical convection and the formation of particles of cloud around dust particles after the condensation has taken place. Vertical convection is caused by the following three factors:-
a. due to surface heating, a temperature gradient is formed which cause vertical convection. This is common in thunder storms and in the tropics or everywhere generally in summer;
b. due to the convergence of winds at one place, vertical convection is also caused. This is true for temperate regions, specially in the front of the cyclones; and
c. due to forced rise of air mass either by its flow over elevation of land and barriers of cold air; or due to its being under run by cooler winds.
2. The saturated air continues to rise and on its way abandons more and more cloud particles.
3. Condensation on the droplets present in the rising saturated air mass and various droplets go on joining together.
4. Through coalescence and electric charge, the droplets grow and fall through the cloud to the earth.

NECESSARY REQUIREMENTS OF RAIN
In order that there may be rain:
1. there must be sufficiently great evaporation;
2. there must be winds to carry the saturated air mass from one place to another;
3. there should be some way to reduce the temperature of the air. This condition may be called the “cooling of the air mass”.

WAYS IN WHICH AN AIR MASS IS COOLED
Generally, an air mass cools or looses heat in one of the following ways:-
1. The warm air rising from the earth in the form of convection currents expands on reaching higher altitudes and on expansion its temperature falls.
2. The warm saturated air mass may have in its way some elevation and it may be forced to ascend to a higher altitude so that its temperature will fall.
3. The air may flow towards colder latitudes and in the process of so moving, its temperature may fall.
4. The warm air may meet the cold air, and its temperature may fall. In this, there can be two positions. Either the warm air converging on the cold air may rise above the latter and condense its moisture, or the cold current may under run the warm air masses.

TYPES OF RAINFALL
When minute droplets of water are condensed from water vapor in the atmosphere on to the nuclei; they may float as clouds. If, however, the droplets coalesce, they form larger drops, which when heavy enough to overcome by gravity ascending air currents within a cloud, fall as rain. For condensation and precipitation to occur naturally, the appreciable ascent of an air mass is essential. This ascent is brought about in three main ways, hence there are three main types of rainfall.

1. Convectional Rainfall
When the earth surface is heated by conduction, moisture laden vapors rise because heated air always expands, and becomes lighter. Air rises in a convection current after a prolonged period of intense heating as shown in figure. In ascending, its water vapors condense into “Cumulonimbus Clouds” with a great vertical extent. This probably reaches its maximum in the afternoon when the convectional system is well developed. Hot rising air has great capacity for holding moisture, which is abundant in regions of high relative humidity. As air rises it cools and when saturation point is reached, torrential downpours occur, often accompanied by thunder and lightening. The summer showers in temperate regions are equally heavy with occasional thunder storms. (See notes for figure)
Convectional rainfall occurs throughout the year near the equator, where constant high temperature and humidity produces this type of rainfall daily in the afternoon. With increasing distance from the equator, the rainfall is associated more markedly with summer, and both the total amount and duration of rainy season decrease as the hot deserts are approached. In middle latitudes, convectional rainfall occurs in summer, when the upper atmosphere is still cool following winter, but when heating of the earth surface is becoming active. The rainfall in the continental interiors in early summer is mainly of this type.

2. Orographic Rainfall

Unlike convectional rain, which is caused by convection currents, orographic rain is formed where ever moist air is forced to ascend a mountain barrier. It is best developed on the windward slopes of the mountains where the prevailing moisture-laden winds come from the sea and thereby cooled by expansion in the higher altitudes, condensation takes place forming clouds and eventually rain. Since it causes relief of the land, it is known as “Relief Rain”.
Much of precipitation experienced on the windward slopes of the north-east of peninsular Malaysia, western New Zealand, western Scotland and Wales and the Assam hills of India and Bangladesh is Relief Rain. (notes)

3. Cyclonic or Frontal Rain
This type of rainfall is independent of relief or convection. It is purely associated with cyclonic activity, whether in temperate regions (depression or a low) or in tropical regions (cyclones). Basically, it is due to convergence of two different air masses with different temperatures and other physical properties. As cold air is denser, it tends to remain closer to the ground. The warm air is lighter and tends to rise over the cold air.
In ascent, pressure decreases, air expands and cools, condensation takes place and light showers called “Cyclonic” or “Frontal Rain” occur. The rainfall is mostly in the front and back area of the cyclone. It is never heavy and comes in gentle showers – once when the front is passing and the other when the tail is passing.
Cyclonic rain occurs when cyclonic activity is greatest. It also occurs when there is marked temperature difference between two different types of air masses in temperate regions e.g., between the Arctic and the Continental air masses, Inter-tropical Convergence Zones (ITCZ) and Polar Fronts. The cyclonic rain is most common in the belt of Westerlies especially during the winter.

4. Border Rain
It is caused by a warm oceanic air mass passing over a cold land in winter. Due to contact, it cools and condensation takes place. It is restricted to the coastal areas; only running a few miles inside the shore. It is common in Australia.

5. Lightening and Thunder
One often wonders what causes lightening and thunder during a thunderstorm. A thunderstorm occurs when the ground is greatly heated on a hot afternoon causing strong currents of air to expand and contract between the flat bottom and the anvil top of the Cumulonimbus thunder clouds.
As rain droplets fall through such Cumulonimbus clouds with vertical extent of 6,100 m (20,000 ft.) they are charged with electrically producing flashes of lightening when the positive top of the cloud is attracted to the negative bottom. Such flashes of electric currents set giant air waves in motion, which reach us a few moments later as roars of thunder.

GENERAL DISTRIBUTION OF RAINFALL
Isohyts are imaginary lines passing through places having the same amount rainfall.
The earth can be divided into a series of alternate dry and wet belts, corresponding with the general distribution of pressure and permanent winds. Such a division is purely diagrammatic and there are modifications due to various factors.
The low and high pressure belts of the earth are permanent. Towards the middle of the low pressure area the air must be rising and towards the middle of the high pressure area, the air must be sinking. P. Lake writes that where the air is rising and where the winds are blowing towards the poles, a climate will generally tend to be wet; where the air is sinking and where the winds are blowing towards the equator, the climate will be dry.
The greatest rainfall will be in the middle of the equatorial low pressure, for there the rising air must be heavily charged with vapor.
A study of the map of the world showing the mean annual rainfall shows that:
1. Rainfall decreases towards the poles.
2. The coastal areas receive greater amount of rainfall.
3. Between 35o and 40o north and south of the equator, the rain is heavier on the eastern coast and goes on decreasing as we move west.
4. Between 45o and 65o north and south of the equator, the westerlies prevail and hence the rainfall is first received on the western coasts and goes on decreasing towards the east.
5. In the case of mountains parallel to a coast, the rain is more on the coastal plane and the windward slope but decreases towards the leeward side.
6. Rainfall is more on the oceans.

Causes of Rainfall in Different Regions
Equatorial Region: Convection, relief features of the land.
Extra Tropical: Cyclones, convection
Temperate Zone: Convection, cyclones, and relief of land winds
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