Here is a compilation of essays on ‘Rainfall’ for class 6, 7, 8, 9 and 10. Find paragraphs, long and short essays on ‘Rainfall’ especially written for school students.
Essay on Rainfall
- Essay on the Origin of Rainfall
- Essay on the Types of Rainfall
- Essay on the Theories of Rainfall
- Essay on the Distribution of Rainfall
- Essay on the Regime of Rainfall
- Essay on the Measurement of Rainfall
Essay # 1. Origin of Rainfall:
The presence of warm, moist and unstable air and sufficient number of hygroscopic nuclei are prerequisite conditions for rainfall. The warm and moist air after being lifted upward becomes saturated and clouds are formed after condensation of water vapour around hygroscopic nuclei (salt and dust particles) but still there may not be rainfall unless the air is supersaturated.
The process of condensation begins only when the relative humidity of ascending air becomes 100 per cent and air is further cooled through dry adiabatic lapse rate but first condensation occurs around larger hygroscopic nuclei only. Such droplets are called cloud droplets.
The aggregation of large number of cloud droplets forms clouds. These cloud droplets are so microscopic in size that they remain suspended in the air. Rainfall does not occur unless these cloud droplets become so large due to coalescence that the air becomes unable to hold them.
This is why, sometimes the sky is overcast by thick clouds but there is no rainfall. If by chance these cloud droplets fall downward they are evaporated before they reach the ground surface. Rainfall occurs only when cloud droplets change to raindrops.
There are two possible processes of change of cloud droplets into raindrops:
(1) If warm and moist air ascends to such a height that condensation begins below freezing point, then both, water droplets and ice droplets, are formed. The water droplets are evaporated because of difference of vapour pressure between them and ice droplets and there is condensation of evaporated water around ice crystals which go on increasing in size. If they become sufficiently large in size, they cannot be held in suspension by the air and consequently they begin to fall down. If the temperature above the ground is high they fall in the form of raindrops.
(2) The suspended cloud droplets in the clouds are of different sizes. These cloud droplets collide among themselves at varying rates due to difference in their sizes and thus form large droplets. In the process several cloud droplets are coalesced to form raindrops. When they become so large in size that ascending air becomes unable to hold them, they fall down as rainfall.
The diameter of a raindrop is upto 5mm and one raindrop contains about 8,000,000 cloud droplets. Rain drops fall down at the velocity 200 times greater than cloud droplets. When raindrops become very large and fall down at greater speed (more than 30 kilometres per hour), they are split in the transit but give heavy downpour.
When the air ascends slowly, the process of condensation is also very slow and hence small raindrops are formed and the resultant rainfall is drizzle but if the air ascends hurriedly with greater speed. Very large raindrops are formed and resultant rainfall is heavy downpour. When condensation occurs below freezing point, the resultant precipitation is in solid form and is called snowfall.
Essay # 2. Types of Rainfall:
Rain is the most common form of precipitation. For rainfall, it is necessary that moist air must ascend, saturate (relative humidity 100 per cent) and condense. Adiabatic cooling due to upward movement of air is by far the most important mechanism of condensation and related precipitation including rainfall.
It is apparent that upward movement is a prerequisite condition for cloud formation and rainfall. Thus, precipitation and rainfall are classified on the basis of conditions and mechanisms of upward movement of air.
There are three ways in which air is forced to move upward and thus cools according to adiabatic lapse rate e.g.:
(1) Due to heating of ground surface the air being heated expands and rises upward in the form of convection currents, the mechanism is known as thermal convection,
(2) Ascent of air over an orographic barrier, and
(3) Uplift of air associated with low pressure system, known as cyclonic or frontal ascent.
It may be pointed out that it is not necessary that all these three factors work independently in relation to the ascent of air. Sometimes, more than one factor are operative. In such situation, the form of precipitation is determined on the basis of dominant factor.
Thus, precipitation and rainfall are classified into the following three types:
(i) Convectional rainfall, occurring due to thermal convection currents caused due to insolational heating of ground surface.
(ii) Orographic rainfall, occurring due to ascent of air forced by mountain barrier, and
(iii) Cyclonic or frontal rainfall, occurring due to upward movement of air caused by convergence of extensive air masses.
(i) Convectional Rainfall:
The principal motivating force behind the ascent of warm and moist air is thermal convection caused by heating of the ground surface through insolation.
Two conditions are necessary to cause convectional precipitation and rainfall e.g.:
(i) Abundant supply of moisture through evaporation to the air so that relative humidity becomes high, and
(ii) Intense heating of ground surface through incoming shortwave electromagnetic solar radiation (say, insolation heating). The mechanism of convectional rainfall may be explained in the following manner.
The ground surface is intensely heated due to enormous amount of heat received through solar radiation during daytime, with the result the air coming in contact with warm ground surface also gets heated, becomes warm, expands, and ultimately rises upward.
The ascending warm and moist air cools according to dry adiabatic lapse rate (decrease of temperature at the rate of 10°C per 1000 metres). The cooling of ascending air increases its relative humidity. The moist air becomes saturated soon (relative humidity becomes 100 per cent) and further ascent of air beyond saturation level causes condensation and cloud formation (cumulo-nimbus clouds) and thus rainfall starts.
The air still continues to rise and in the process further cools but at moist adiabatic lapse rate or retarded adiabatic rate (decrease of temperature at the rate of 5°C per 1000 metres) due to addition of latent heat of condensation to the ascending air released after condensation of atmospheric vapour.
When the ascending air reaches such height where its temperature matches with the temperature of surrounding air, the process of condensation is more activated and hence cumulo-nimbus clouds are formed and there begins instantaneous heavy rainfall (fig. 36.3).
Since the ascending moist air (convectively motivated) cools soon after rising to very little height, causing immediate saturation and condensation, the convectional rainfall occurs in the form of heavy downpour. It is also apparent from the above description that convectional rainfall is a warm weather phenomenon and is associated with lightning and cloud thunder.
Convectional rainfall mainly occurs in equatorial regions of low latitudes where daily heating of ground surface up to noon causes convection currents. Consequently, the sky becomes overcast by 2-3 P.M. daily causing pitch darkness and heavy rains and the sky becomes clear by 4 P.M. Thus, the convectional rainfall in the equatorial region is a daily regular feature.
Convectional rainfall also occurs in the tropical, subtropical and temperate regions in summer months and in the warmer parts of the day.
The following are characteristic features of convectional rainfall:
(i) It occurs daily in the afternoon in the equatorial regions.
(ii) It is of very short duration but occurs in the form of heavy showers (heavy downpour).
(iii) It occurs through thick dark and extensive cumulo-nimbus clouds.
(iv) It is accompanied by cloud thunder and lightning.
(v) Though much of the rainfall becomes runoff and is drained off in the form of overland flow to the streams but still there is sufficient moisture in the soils due to daily rainfall in the equatorial regions. Out-side equatorial regions convective rainfall is of little significance to crop growth because most of rainwater is drained to the streams through surface runoff which causes severe rill and gully erosion resulting into enormous loss of loose soils.
(vi) Convective rainfall supports luxurious evergreen rainforests in the equatorial regions.
(vii) Convective rainfall in the temperate regions is not in the form of heavy showers rather it is slow and of longer duration so that most of rainwater infiltrates into soils. Here rains are always in summers.
(viii) Convective rainfall in hot deserts is not regular but is irregular and sudden.
(ii) Orographic Rainfall:
Orographic rainfall occurs due to ascent of air forced by mountain barriers. The mountain barriers lying across the direction of air flow force the moisture laden air to rise along the mountain slope and thus lifted air mass cools according to dry adiabatic lapse rate (decrease of temperature at the rate of 10°C per 1000 metres) which increases the relative humidity of the air.
The ascending air becomes saturated after reaching certain height and condensation begins around hygroscopic nuclei. The addition of latent heat of condensation to the air causes it to move further upward and cool at moist adiabatic lapse rate (decrease of temperature with increasing height at the rate of 5°C per 1000 metres).
Thus, ascending air continues to yield precipitation with increasing height. It is apparent that mountain barriers produce trigger effect for the moist air to ascend, cool and become unstable. The slope of the mountain facing the wind is called windward slope or onward slope and receives maximum precipitation while the opposite slope is called leeward slope or rain shadow region because the ascending air after crossing over the mountain barrier descends along the leeward slope and thus is warmed at dry adiabatic lapse rate (increase in temperature with decreasing height at the rate of 10°C per 1000 metres).
Consequently, the humidity capacity of the descending air increases resulting into substantial decrease in relative humidity. Secondly, the moisture present in the air is already precipitated on the windward slope and thus there is very little precipitation on the leeward slope. Most of the world precipitation occurs through orographic rainfall.
The following conditions are necessary for the occurrence of orographic rainfall:
(i) There should be mountain barrier across the wind direction, so that the moist air is forced on obstruction to move upward. If the mountain barriers are parallel to the wind direction, the air is not obstructed and no rainfall occurs. For example, Aravallis ranges running in southwest-northeast direction are parallel to the Arabian Sea Branch of south-west Indian monsoon and hence Rajasthan receives very low amount of rainfall.
(ii) If the mountains are very close and parallel to the sea coasts, they become effective barriers because the moisture laden winds coming from over the oceans are obstructed and forced to ascend and soon become saturated. For example, Coast Range mountains situated on the western margins of North America are parallel to the Pacific coast. Similarly, the situation of the Western Ghats in India presents ideal conditions for orographic rainfall.
(iii) The height of mountains also affects the form and amount of orographic rainfall. If the mountains are very close to the sea coast, even low height can be effective barrier and can yield sufficient rainfall because the moist air becomes saturated at very low height. On the other hand, the inland mountains should be of higher height because the air after covering long distances loses much of its moisture content.
(iv) There should be sufficient amount of moisture content in the air.
The following are the characteristic features of orographic rainfall:
(a) The windward slope, also called as rain slope, receives maximum amount of rainfall whereas leeward side of the mountain gets very low rainfall. For example, Mangalore located on the western slope (windward slope) of the Western Ghats receives mean annual rainfall of above 2000 mm whereas Bangalore situated in the rain shadow region gets only 500 mm of mean annual rainfall.
The southern slopes of the Himalayas receive mean annual rainfall of more than 2000 mm whereas the northern slope receives only 50 mm of mean annual rainfall. Similarly, the western slopes of the Coast Ranges of North America receive more than 2000mm of mean annual rainfall while the eastern slopes fall in rain-shadow region.
(b) There IS maximum rainfall near the mountain slopes and it decreases away from the foothills. For example, the cities and towns located at the southern slopes of the Himalayas receive more rainfall e.g. Simla 1520mm, Nainital 2000mm and Drazeeling 3150 mm whereas the places away from the Himalayan foothills receive relatively low rainfall e.g. Patna 1000 mm, Allahabad 1050 mm and Delhi 650 mm.
(c) If the mountains are of moderate height, the maximum rainfall does not occur at their tops rather it occurs on the other side.
(d) The windward slopes of the mountains at the time of rainfall are characterized by cumulus clouds while leeward slopes have stratus clouds.
(e) The amount of rainfall increases with increasing height along the windward slopes of the mountains upto a certain height beyond which the amount of rainfall decreases with increasing height because or marked decrease in the moisture content of the air. This situation is called inversion of rainfall.
The height of the mountains beyond which the amount of rainfall decreases upward is called maximum rainfall line which varies spatially depending on the location of mountains, their distance from the sea, moisture content in the air, mountain slope, season etc. Maximum rainfall line is at 24,000 feet (7,000 m) at the equator, at 12,000 feet (3,600 m) in the Himalayas, at 21,000 feet (6.300m) in the Alps, at 18,000 feet (5,400 m) during summer and at 12,000 feet (3,600m) during winter in the Pyrenees mountains etc.
(f) Orographic rainfall may occur in any season. Unlike other types of rainfall it is more widespread and of long duration.
(g) It may be pointed out that orographic rainfall is induced not only because of lifting of moist air due to mountain barrier but convective and cyclonic mechanisms also help in the process of orographic rainfall.
For example, in warm regions valleys are heated during daytime and hence winds are also heated and ascend along the hillslopes in the form of convection currents and yield rainfall after being saturated. Some limes, forward moving cyclones are also forced to ascend along the hillslope due to obstructions offered by mountain barriers.
(iii) Cyclonic or Frontal Rainfall:
Cyclonic or frontal rainfall occurs due to ascending of moist air and adiabatic cooling caused by convergence of two extensive air masses. The mechanism of cyclonic precipitation is of two types on the basis of two types of cyclones viz. temperate cyclones and tropical cyclones. Rainfall associated with temperate cyclones occurs when two extensive air masses of entirely different physical properties (warm and cold air masses) converge.
When two contrasting air masses (cold polar air mass and warm westerly air mass) coming from opposite directions converge along a line, a front is formed. The warm wind is lifted upward along this front where as cold air being heavier settles downward.
Such cyclonic fronts are created in temperate regions where cold polar winds and warm westerlies converge. The warm air lying over cold air is cooled and gets saturated and condensation begins around hygroscopic nuclei. It may be pointed out that lifting of warm air along cyclonic front is not vertical like convective currents rather it is oblique.
Since the lifting of warm air along the warm front of temperate cyclone is slow and gradual and hence the process of condensation is also slow and gradual, with the result precipitation occurs in the form of drizzles but continues for longer duration. Thus, the precipitation associated with warm front is widespread and of long duration.
On the other hand, the precipitation associated with cold fronts is always in the form of thunder showers but is of very short duration. Sometimes, the precipitation occurs in the form of snowfall and hailstorms. This is because of the fact that lifting of warm air along cold front occurs quickly as cold air pushes warm air upward with great force. Most of the rains of temperate regions are received through cyclones.
In tropical regions two extensive air masses of similar physical properties converge to form tropical cyclones wherein lifting of air is almost vertical and is very often associated with convection.
It may be pointed out that convergence mechanism provides initial trigger effect to the upward movement of convectively unstable air which if full of moisture becomes saturated and yields heavy showers characterized by lightning and thunder. Tropical cyclones, regionally called as typhoons, hurricanes, tornadoes etc., yield heavy downpour in China, Japan, South-East Asia, Bangladesh, India, USA etc.
Essay # 3. Theories of Rainfall:
Various theories of precipitation and rainfall have been put forth from time to time but the riddle of raindrop formation still remains unresolved. Very generalized two processes and mechanisms of raindrop formation have been outlined above.
The early theories related to the formation of raindrops and their growth may be briefly summarized in the following manner:
(i) The raindrops are differently electrically charged and thus they are coalesced by electrical attraction and grow in large size. This theory is opposed on the ground that the distances between raindrops are so large and difference between electrical charges is so small that coalescence of raindrops due to this mechanism is not possible. In other words, the coalescence and growth of raindrops due to differential electrical charges are not possible.
(ii) The large rain drops capture small rain drops and thus become further large in size but the observations have revealed that there is regular pattern in the size and distribution of drops in the clouds. In other words, generally most of the drops are more or less uniform in size as their diameters range between 20 to 30 micrometres and only a few drops are larger than 80 micrometres.
(iii) There is variation in saturation vapour pressure with varying temperature. In such condition the atmospheric turbulence brings warm and cold cloud droplets in close conjunction, with the result there is super-saturation of air with reference to the surface of the cold cloud droplets and under-saturation of air with reference to the surface of warm cloud droplets and growth of cold droplets at the expense of warm droplets.
This situation causes evaporation of warm droplets. This theory is opposed on the ground that the difference of temperature of cloud droplets is not so great that this differential mechanism may operate.
(iv) Raindrops grow around very large condensation (hygroscopic) Nuclei but it is argued that no doubt the process of condensation around exceptionally large hygroscopic nuclei is very rapid but their further growth cannot be explained on this mere ground of size of cloud droplets.
The theories of precipitation and rainfall fall in two main categories e.g.:
(1) Rapid growth of raindrops due to growth of ice crystals at the cost of water droplets and
(2) Rapid growth of raindrops due to coalescence of small water droplets by the sweeping actions of falling drops.
(i) Cloud Instability Theory of Bergeron Findeisen:
Tor Bergeron, an eminent Norwagian meteorologist, postulated his theory of precipitation, known as ‘cloud instability theory’ or ‘ice crystal theory’ in 1933. The core of the theory is based on the concept of mechanism of the growth of raindrops. According to him water droplets and ice crystals are found together in unstable clouds when temperature is below freezing point.
The theory is based on commonly accepted fact that “the relative humidity of air is greater with respect to an ice surface than with respect to water surface”. With the fall of air temperature below 0°C the atmospheric vapour pressure decreases more rapidly over ice surface than over water surface with the result saturation vapour pressure becomes greater over water surface than ice surface when the air temperature ranges between -5°C and -25°C and the difference between saturation vapour pressure over water and ice surfaces exceeds 0.2 mb.
In such condition, when air temperature ranges between -5°C and -25°C, water droplets become super-saturated. If ice crystals and super-cooled water droplets exist together in a cloud, then the water droplets are evaporated and resultant vapour is deposited on to the ice crystals.
It may be pointed out that the formation of ice particles requires freezing nuclei (e.g., fine soil particles, meteoric dust etc.) in the same manner as the formation of water droplets requires the presence of hygroscopic nuclei. Slowly and slowly ice crystals grow in size as the deposition of vapour derived through evaporation of super-cooled water droplets on their surfaces continues. Ice crystals then aggregate due to their mutual collision and thus they form large snow-flakes.
The aggregation of ice crystals is more prevalent when air temperature ranges between 0°C and – 5°C. When the ice crystals become large (snow-flakes) and their falling velocity exceeds the velocity of rising air currents, they fall downward. When the falling ice crystals pass through a thick layer of air with temperature more than 0°C, they are changed into raindrops and thus begins rainfall.
(ii) Collision Theory:
Though the Bergeron process of the origin of precipitation and rainfall satisfied most of the observed facts but it could not explain the mechanism of rainfall in the tropical areas where cumulus clouds over the oceans give copious rains when they are only 2000m thick and the air temperature at their top is 5°C or even more. It is thus evident that ice crystals do not help in the formation and growth of raindrops of large size in warm clouds.
Thus, collision theory involving collision, coalescence and sweeping for the formation and growth of raindrops was postulated by several meteorologists. According to some meteorologists atmospheric turbulence causes collision of cloud droplets. Due to collision they coalesce and grow in size.
This concept suffers from two shortcomings e.g.:
(i) Collision, may cause splitting and scattering of cloud droplets rather than their aggregation due to coalescence, and
(ii) There is little and often no precipitation from highly turbulent clouds.
Longmuir suggested modifications in the ‘general coalescence theory’ in order to plug its loopholes as mentioned above.
According to him the terminal velocities of falling drops are directly related to their diameters. In other words, larger drops fall with greater velocity than smaller drops. Thus, large drops absorb smaller droplets. Smaller droplets are also swept by larger droplets. All these lead to increase in the size of larger droplets which become raindrops which fall as rains because they cannot be held in suspension by rising air currents.
Essay # 4. Distribution of Rainfall:
Global Distribution of Rainfall:
Rainfall is highly correlated with air temperature and atmospheric humidity while humidity is closely related with temperature through the process of evaporation. The regions having high temperature and abundance of surface water for evaporation receive higher amount of annual rainfall. Equatorial regions are typical example of such situation.
Subtropical regions are also characterized by above conditions but the western parts of the continents receive least rainfall because there are anticyclonic conditions due to descent of air. Middle latitudes also have favourable conditions for sufficient rainfall but polar areas receive their precipitation in the form of snowfall instead of rainfall. Before attempting to describe world distribution of rainfall it is necessary to discuss certain facts related to rainfall distribution e.g. total amount of annual rainfall, seasonal distribution, and variability of rainfall.
Mean annual rainfall for the whole globe is 970 mm but this mean annual amount is unevenly distributed on the earth’s surface. Some places receive less than 100 mm of mean annual rainfall (for example, tropical hot deserts like Sahara, Thar, Acatama, Kalahari etc.) while some places receive more than 12000 mm of annual rainfall (e.g. Cherrapunji of India). Not only this, there is much temporal variation of annual rainfall in a particular area.
Most of the annual amount of rainfall is received during a few months of the year while most of the months either remain dry or receive little rainfall. For example, 12000 mm of rainfall at Cherrapunji is received only in 159 days. The equatorial regions receive rainfall throughout the year but other areas are characterized by seasonal rainfall.
For example, more than 80 per cent of annual rainfall in India is received during 3 wet summer monsoon months (July, August and September). On the other hand, the Mediterranean regions receive most of their annual rainfall during winter months while summer season remains dry.
Zonal Distribution of Rainfall:
It is true that the cooling of ascending air is a prerequisite condition for the occurrence of rainfall. The air is lifted through thermal convective mechanism, convergence of two extensive air masses and obstruction of mountain barriers. If the relative importance of these three factors in different areas of the world is taken into account, it appears that air is generally lifted due to convergence of two extensive air masses in most parts of the world. Convergence of air masses is directly related with air temperature and air pressure.
There are two major convergence zones of air masses i.e., trade winds converge along the equatorial low pressure belt and westerlies and polar winds converge along high latitude low pressure (60°-65° latitudes in both the hemispheres). On the other hand, winds descend near subtropical high pressure belt (30°-35° latitudes in both the hemispheres) and diverge in opposite directions and form anticyclones which introduce dry weather.
Since the convergence of air masses is in zonal form and hence rainfall distribution is also found in zonal pattern. Besides, mountain barriers and land and water (continents and oceans) also influence world distribution of rainfall. Since air moisture depends upon temperature and horizontal distribution of temperature is found in zonal patterns and hence rainfall distribution is also characterized by zonal pattern.
Based on above considerations, 6 major zones of rainfall distribution are identified on the earth’s surface:
(1) Equatorial Zone of Maximum Rainfall:
This zone extends upto 10° latitudes on either side of the equator and falls within intertropical convergence characterized by warm and moist air masses. The mean annual rainfall ranges between 1750 mm and 2000 mm. Most of the rains are received through convectional rainfall accompanied by lightning and cloud thunder.
There is daily rainfall in the afternoon. The rainfall intensity is very high as it occurs in the form of heavy showers. The clouds are cleared within short period and sky becomes cloudless in the late evening.
(2) Trade Wind Rainfall Zone:
Trade wind rainfall zone extends between 10°-20° latitudes in both the hemispheres and is characterized by north-east and south-east trade winds. These winds yield rainfall in the eastern parts of the continents because they come from over the oceans and hence pick up sufficient moisture but as they move westward in the continents they become dry and thus the western parts of the continents become extremely dry and deserts. The monsoon regions located in this zone receive much rainfall. Summers receive most of the mean annual rainfall.
(3) Subtropical Zone of Minimum Rainfall:
Subtropical zone of minimum rainfall extends between 20° and 30° latitudes in both the hemispheres, where descending air from above induces high pressure and winds diverge in opposite directions at the ground surface, with the result anti-cyclones are formed. This condition is not conducive for rainfall and hence dry conditions prevail over large areas. Mean annual rainfall is 900mm.
It may be pointed out that all the tropical hot deserts are located in this zone where mean annual rainfall is below 250mm. The average annual rainfall becomes, for the whole zone, higher (900mm) than the average value for the deserts because the eastern parts of the continents receive more rainfall from relatively moist trade winds which come from over the oceans. Most of annual rainfall occurs during summer months while winter season is dry.
(4) Mediterranean Rainfall Zone:
Mediterranean rainfall zone extends between 30°-40° latitudes in both the hemispheres where rainfall occurs through westerlies and cyclones during winter season while summers remain dry because this zone comes under the influence of trade winds due to northward shifting of wind and pressure belt during northern summer (summer solstice). Mean annual rainfall is 1000mm.
(5) Mid-latitudinal Zone:
Mid-latitudinal zone of high rainfall extends between 40°-50° latitudes in both the hemispheres where rainfall occurs through westerlies and temperate cyclones. Mean annual rainfall ranges between 1000m and 1250 mm. The western parts of the continents receive more rainfall. It decreases from the western coastal areas inland.
Southern hemisphere records more rainfall than northern hemisphere because of dominance of oceans in the former. Winter season receives maximum precipitation through temperate cyclones. The precipitation is of long duration but occurs in the form of light showers.
(6) Polar Zone of Low Precipitation:
Precipitation decreases from 60° latitude pole-ward in both the hemispheres. Mean annual precipitation becomes only 250mm beyond 75° latitude. Most of the precipitation occurs in the form of snowfall.
Patterson has divided the globe into 15 rainfall zones wherein the northern and southern hemispheres account for 7 zones each and the remaining zone is on either side of the equator.
These rainfall zones have been identified mainly on the basis of seasonal behaviour of rainfall:
Essay # 5. Regime of Rainfall:
Precipitation or rainfall regime refers to seasonal behaviour and variation of rainfall.
Haurwitz and Austin have identified 6 rainfall regimes:
(i) Equatorial Rainfall Regime:
Equatorial rainfall regime is characterized by rainfall in all seasons but there are two maxima in March (vernal equinox) and September (autumnal equinox). Thermal convection air currents generated by intense insolational heating of the ground surface account for most of the rains. Besides, convergence of trade winds also causes cyclonic rains.
This zone extends between 10°N and 10°S latitudes. At the outer limit of this zone there is only one rainfall maximum. The rainfall is accompanied by lightning and thunder and occurs in the form of heavy showers but is of short duration.
(ii) Tropical Rainfall Regime:
Tropical rainfall regime has one rainfall maximum and one minimum in a year. In the northern hemisphere the eastern parts of the continents receive maximum and minimum rainfall in the months of July and December respectively whereas the western parts of the continents get maximum rainfall in December and minimum in July.
(iii) Monsoon Rainfall Regime:
Monsoon rainfall regime is characterized by maximum rainfall in July and August (northern hemisphere). Thus, there is summer maximum and winter minimum. Summer rainfall is received through southwest and south wet monsoon winds associated with tropical atmospheric disturbance (cyclones). Most of the rains are orographic and cyclonic in origin.
(iv) Mediterranean Rainfall Regime:
Mediterranean rainfall regime receives maximum rainfall during winter season because the zone of this regime comes under the domain of prevailing westerlies during winters due to southward shifting of pressure and wind belts. Summer is a dry season.
(v) Continental Rainfall Regime:
Continental rainfall regime is characterized by maximum precipitation in summers when convective mechanism due to insolational heating of the ground surface is maximum. Winters are dry because of the prevalence of anticyclonic conditions. This regime is found in the interior of the continents.
(vi) Maritime Rainfall Regime:
Temperate areas record maximum precipitation in winter over the oceans and adjoining coastal areas due to maximum cyclonic activity. This regime is found along the western margins of the continents in middle latitudes.
Essay # 6. Measurement of Rainfall:
Rain is the precipitation of water in liquid form. Droplets with more than 0.5 mm diameter are, generally, considered as rain. Widely scattered smaller drops are also called rain. All forms of precipitation are measured as vertical depth of water on a level surface, if the entire precipitation remained where it fell.
The total amount of precipitation falling on earth’s surface in a given period of time is expressed as the depth to which it would accumulate on horizontal projection of earth’s surface if there were no losses by evaporation or runoff and if any part of precipitation falling as snow or ice were also melt.
Any open receptacle with vertical sides can be used as a gauge for measuring rainfall. The refined receptacles for measuring rainfall are called rain gauges. Non recording rain gauges (ordinary rain gauges) and self-recording rain gauges (automatic recording rain gauges) are the two commonly use instruments for measuring rainfall (Fig. 2.11).
Ordinary Rain Gauge:
The non-recording rain gauge, as the name indicates, does not record the rain, but only collect the rainfall.
The collected rain can be measured as indicated below:
However, there is no need for using any equation since the collected rain is measured by means of a graduated cylinder to directly represent the amount of rainfall volume receives in cm of water depth.
Simon type ordinary rain gauge used to be widely used in India till 1969. Since then, the IMD has adapted another model called Standard gauge at all the observatories in the country. It consists of a collector with gun metal rim, a base and a polythene bottle for collecting rainwater (Fig. 2.11).
The 20 cm capacity rain gauge with 200 cm2 collector and 4 litres bottle is widely used and is sufficient to measure 24 hours rainfall of most of Indian observatories. The rain gauge is installed at a standard height of 30 cm above the ground level. Rainfall collected in the bottle will be measured daily at 8:30 IST and 14:00 hours LMT, if rain occurs.
The process of measurement at 8:30 AM for the past 24 hrs is most common throughout the country. However, at times of heavy rainfall, two or three intermediate readings may be taken and their sum reported as rainfall for the past 24 hours. Rainfall exceeding 2.5 mm in a day is called a rainy day.
Self-Recording Rain Gauge:
This type of rain gauges, with mechanical arrangement for recording rainfall in a graph paper, can give us automatic record of rainfall without any bottle reading.
Various models have been designed with different gauges such as:
1. Tipping bucket type.
2. Weighing type.
3. Floating type.
A float type gauge, provided with a self-starting siphoning arrangement, is most widely used in India. In India, it is popularly called as natural siphon recording rain gauge (Fig. 2.11).
Rainwater entering the gauge at the top of the cover is lead via funnel to the receiver consisting of a float. As the water level rises in the receiver, the float rises and the pen records on the chart, wrapped round a clockwise rotating drum, the amount of water in the receiver at any instant.
The rotating drum completes on revolution in 24 hours or sometimes in 7 days and the chart will have to be replaced accordingly. Siphoning occurs automatically when the pen reaches top of the chart, after siphoning pen arm comes down to zero.
As the rain continues, the pen rises again from the zero line of the chart. The rain gauge should be installed on a level ground and fixed on a masonry foundation of 60 x 60 x 60 cm sunk into ground. Base of the gauge is cemented into the foundation so that the rim of the gauge is exactly 75 cm above the case of self-recording rain gauge.
Source of all water is precipitation.
A number of factors, including the following, affect the amount of rainfall at a given location:
1. Ocean currents.
2. Trade winds.
3. Air-mass movement.
4. Orographic effects.
5. Location of place with respect to physical barriers.