# Humidity and Precipitation [NCERT Geography Notes for UPSC, SSC, Railways Exam]

## Presence of vapor in the air

• The amount of water vapor present in the atmosphere is called 'humidity'. The amount of humidity of water vapor varies with time and place. It is a variable component of the atmosphere.
• Its unequal distribution can be estimated from the fact that it is 0 (zero quantity) in the warm arid region, 3 percent in the middle latitudes, and 4 percent in the humid low latitudes.
• Although water vapor represents two percent of the total amount of the atmosphere and only four percent of the volume, it controls the weather and climate. This is very important for several reasons-

## Global distribution of water vapor

• The atmosphere receives water vapor from the earth's surface, whether it is from reservoirs or plants, or living beings.
• It is contained only from the lower layers of the atmosphere. About half of all water vapor is available from the ground up to an altitude of 2,000 meters. The amount of humidity of water vapor decreases with increasing altitude. There is a lack of humidity above the troposphere.
• The highest humidity in January is at the equator, especially over the oceans. This pattern of distribution is also found in June, but the humidity is higher in the northern hemisphere above the middle latitudes than in the middle latitudes of the southern hemisphere. The reason for this is that in those days it is winter in the Southern Hemisphere. With the increase in temperature, the capacity of holding water vapor in the atmosphere also increases.

## Measurement of the amount of water vapor (humidity)

• The amount of water vapor present in the air is expressed by the following methods:

### absolute humidity

• The amount of water vapor present per unit volume of air is expressed in grams per cubic meter. For this, a hygrometer is used.

### specific humidity

• Which is the weight of water vapor per unit weight of air (grams per kilogram). The specific humidity is not affected by the change in temperature, air pressure.

### relative humidity

• Which expresses the ratio of the water vapor holding capacity of the air at the same temperature to the water vapor present in the air at any given temperature.
• In other words, if the actual humidity is expressed as the ratio of the vapor holding capacity of that air, then it will be the relative humidity of that air.
• The relative humidity is expressed in percentage. Vapor pressure is the pressure or weight of water vapor in the atmosphere expressed in millibars.
• Dew point temperature, that is, the temperature at which the air attains saturation. In this, the vapor is not removed, but the air is cooled (cooled). Let the actual humidity of the air at a certain point of time at 600 °C be 5 g. If this temperature has the capacity to hold 10 grams of vapor, then the relative humidity of the air is said to be 50%. This is the formula for finding relative humidity.
• The higher the relative humidity, the more moisture in the weather, even if the actual humidity of that air is low. The highest relative humidity occurs in the equatorial regions (due to the abundance of water vapor).
• From the Tropics of Cancer and Capricorn, it rises again towards the poles (due to lack of temperature). If there is so much water vapor in the air that it is unable to absorb any more water vapor, then its relative humidity will be called 100. Such air is called saturated air. The probability of rain increases as the relative humidity increases.
• The relative humidity is the most famous of the above methods, as it can be easily understood. The instrument that has to be used to determine this is called 'dry and wet-bulb thermometers'. Humidity is greatly affected by temperature, hence the importance of this instrument.
• Relative humidity does not measure the amount of moisture in the air, but rather, how close the air is to saturation. The temperature at which the air is saturated or saturated is called the dew point.

## Evaporation

• Wherever there is water or any wet object on the surface, it becomes a gas and gets absorbed in the atmosphere. This change is called evaporation, evaporation, or evaporation, the evaporation from plants is called transpiration.
• Some vapor also enters the air by exhalation by animals and people. However, most of the evaporation comes from the oceans. This is why the humidity is higher in the air near the sea.
• 16 km in the atmosphere. There is no steam or humidity at higher altitudes. The steam obtained from evaporation is transferred by wind and by other atmospheric transmissions.
• Evaporation mainly refers to the process of moving water from a liquid state to a gaseous state. In this process, 607 calories of heat are required to bring one gram of water to the vapor state.
• Due to this the temperature decreases by 25 °C in that air. This heat loss (in evaporation) is secretly absorbed into the water vapor (what we call latent heat/latent temperature) and when the process reverses.

## Conducive to evaporation

Following are the major favorable conditions for evaporation:

### Dryness

• It means a lack of humidity in the air. If there is enough humidity in the air (the amount of water vapor is very high, as in the rainy season), evaporation will be less.
• But if there is a lack of humidity, that is, if the air is dry, then evaporation can be high. As a general rule, the drier the air, the greater the evaporation from the reservoirs.

### Temperature

• Evaporation requires a supply of heat to the air. If the temperature of the air is high and the water adjacent to it continues to receive heat, then the evaporation intensifies. As the temperature increases, the vapor holding capacity of the air also increases. Due to higher temperatures in the summer season than in winter, evaporation is more.
• Air circulation- This means the acceleration of wind speed. In comparison to the calm air, the moving wind helps in removing the wet air and conveying the dry air there to become humid. Therefore, the higher the speed of the wind, the more effective is the evaporation (the more the amount of water vapor increases).

### Reservoir

• This means the presence of huge reservoirs. Evaporation is moreover water than on land. Therefore, in the area where ponds, rivers, or oceans meet, evaporation accelerates.
• The general rule is that the oceans have more evaporation than the continents. Yes, the position of these reservoirs must be in the tropical or temperate zone, where the sun's rays mainly fall.

## Condensation

• Condensation is the process by which water vapor attains a state of change from a gaseous state to a liquid or solid form. The transformation of vapor into water or ice is the result of this process. The most favorable condition for this is the cooling of the air.
• The main factor of evaporation is the rise in temperature and the main factor of condensation is the decrease in temperature.
• Only condensed air can change into a liquid or a solid (in the form of water droplets or in the form of snowflakes).

## Condensation factor

• The favorable conditions for condensation are as follows:

### moisture content in the air

1. Due to the intensity of evaporation, if the amount of water vapor in the air becomes so high that the air becomes saturated, then the process of condensation can start. Such a situation comes very rarely.
2. The temperature at which the air becomes saturated is called the dew point or dew point. Lowering the air temperature to the dew point is favorable for condensation.

### phenomenon of heat

1. If the temperature of the air decreases so much that its proportional humidity becomes 100%, then the air starts becoming saturated without increasing the amount of water vapor and the condensation process starts.
2. As the temperature decreases, the vapor holding capacity decreases, and even though there is less water vapor, the vapors start condensing and the air becomes condensed.

### increase in the volume of air

1. Increases the volume of air only when it becomes hot, but without increasing the heat, if the air expands and increases in volume, then automatically its temperature will decrease and there will be an opportunity for condensation.
2. This type of temperature change is called 'adiabatic'.
3. The energy that is spent in the expansion of air is received in the form of heat from that air, hence the temperature decreases.
• When the unsaturated air rises up to cool down and becomes saturated, it will be a process of 'Adiabatic Cooling'.
• Similarly, when the air increases the temperature by descending compression from top to bottom, then this process will be of 'Adiabatic Heating', in which there is no transfer of heat to the surroundings.

This happens when,

1. Let the air continuously lift up,
2. The wind is forced to rise up through the hill slopes,
3. disturbance in the flow of air, or
4. The air should go up in the upward movement of the front.
5. The pressure of the rising air decreases and it expands.

## In the form of condensation

• Due to the condensation of vapor, many forms are visible in the atmosphere - such as dew, frost, fog, mist, frost, hail, cloud, and rain.
• When condensed vapor falls on the surface in the form of frost, hail, or rain, then the scientific term for it is used as precipitation.
• Dew or frost does not fall from above, but forms where we see it.
• Precipitation or downfall means falling down. Here the fall (precipitation) of condensed water vapor in solid or liquid form from the atmosphere to the surface will be taken. Snowflakes are often found on the high peaks of clouds.
• They grow up taking in the water-drops around them and start falling. Snowfall or rain also depends on the temperature between the clouds and the surface.
• If the temperature falls below the freezing point, snow will fall, otherwise, the snowflakes will melt and become raindrops.
• Today's scientists do artificial rain on this principle. They go up in the plane and drop snow powder or other chemicals in the clouds. This is done so that the precipitation is quick.
• As a result, cloud droplets fall on the earth and agricultural work takes place. But, to make rain we need such vast and vast powers that it is beyond our power to produce such powers.
• Water vapor can remain in the atmosphere in a limited amount, not much. There may also come a stage when it cannot contain much water vapor. This state is of saturation and is called the dew point.
• Dew point or dew point is not a fixed temperature. The temperature at which the air reaches the state of saturation will be the dew point.
• To reach the state of saturation, the temperature of the air decreases or the amount of water vapor increases, these are the two necessary conditions. The saturation of the air is to reach the dew point. The relative humidity at the dew point is 100.
• It is only after reaching the dew point that the water vapor turns into water, that is, precipitation occurs only when the saturated air cools. As the temperature of the atmosphere increases, the amount of water vapor in it becomes more capable of storing it.
• If the temperature falls below the freezing point, it is possible that the water vapor will turn into snowflakes. These forms of condensation are formed on or near the surface - dew, frost, fog, and mist.
• Some forms of condensation are formed at higher altitudes (in the sky), such as clouds (clouds). These different forms of condensation depend on the amount of water vapor, the thickness of the cold layer of air, and the dew point.

### Dew

• Earth receives insolation during the day and emits or radiates it at night. On dissipation of heat, the earth's surface cools down and the air in contact with it also starts cooling.
• When the air cools, the water vapor that remains in it condenses and starts sitting on the objects lying on the earth in the form of water points. It is necessary for the dew to form
1. There is enough water vapor in the air and at the same time, the temperature of the surface should be low enough that it can cool the air and condense the vapor. However, the temperature should not fall below the freezing point.
2. After the rains, the amount of water vapor in our air increases, and after that comes the winter season in which the temperature drops very much, so there is enough dew in October-November.
3. On a night when, due to the presence of clouds, heat is not dissipated from the earth's surface, the air near the surface does not get too cold, as a result, the dew is found less in that morning.
4. Clear and calm long nights are more conducive to dew formation.

### Fog and mist

• Dew is formed when the vapor present in the air coming in contact with the surface condenses. But when there is a condensation of water vapor above the surface, it is called mist or mist. In this, water droplets are seen floating in the air.
• The water particles are smaller and denser than the mist in the mist. Dust and smoke are mixed in the air in big cities and near factories. So the fog is denser there. There the mixture of fog and smoke is called 'Dhuansa' (smoke + fog = smog).
• The fog is also called the 'cloud of the earth'. Surface objects are not clearly visible through 'fog'. Visibility is half a kilometer. It is less than a kilometer, whereas in 'fog (mist)' one km. Visibility is maintained till Both of these indicate the blurring of surface air.
• Following are the types of fog-

• It is formed on a calm night on the ground when the nearby moist air is cooled by terrestrial radiation, its thickness is 10 to 30 meters.

#### innovative fog

• It is formed when moist warm air comes into contact with a cold surface. The decrease in temperature at the time of horizontal distribution of air accelerates its formation. Hence it is called transit fog. Its thickness varies from 300 to 600 meters.

#### mountain fog

• It is built on hill slopes. There the moist air has to be forced to rise.

#### front fog

• Which is formed near the fronts separating two different air masses.

### Cloud

• Due to the change in air pressure, when the air rises vertically and expands there and cools down, then a fog-like situation occurs at a higher altitude than the surface.
• The presence of water droplets in the sky in this way is called giving birth to a cloud or cloud. At higher altitudes the water vapor reaches the dew point sooner, the dew point attained is a large cloud of air.
• The water particles in a cloud are so fine that 'at least 5,000,000 particles of them would be needed to fill a teaspoon of tea.' These water particles form clouds when they freeze on the hygroscopic particles.
• Clouds can be classified on two bases. they are-
1. their size, structure, and shape
2. Their height in the atmosphere, where they meet.
• There are four major types of clouds on the first basis-

#### side cloud

• They are found at a very high altitude like the wings of a white bird. They are made of snowflakes. Side clouds are indicative of the arrival of cyclones.

#### cotton cloud

• They are spread like a pile of cotton. The domed on the top but the bottom (base) are flat.

#### layered cloud

• They are spread in layers and cover the sky.

#### rain cloud

• They are found at low altitudes and are black and dense. It rains more than them. On the second basis, that is, according to the height, there are three main types of clouds-

### Low-level clouds

• They are found from the ground up to an altitude of 2,000 meters. Kapasi, Strati, Strati Kapasi, Rain Strati and Rain Cloud come under it.

### Mid-level cloud

• They are situated between 2,000 meters to 6,000 meters. The middle cotton and middle-tier come under it.

### High clouds

• They are located at an altitude of 6,000 meters to 12,000 meters. Pakshabh, Pakshabh-strata, and Pakshabh-Kapasi clouds come under this.
• On the basis of height, the names of ten types of clouds have also come. The World's Department of Meteorological Information has also recognized these names. Their photos taken from satellites have proved to be very helpful in forecasting the weather. Some of their features are given below-

#### side cloud

• These clouds, found at the highest altitude (6000 to 12,000 meters), neither shadow nor rain occurs on the earth. When they appear in the sky, then understand that they are giving information about the arrival of the cyclone.

#### sided-cotton cloud

• These are also clouds found at higher altitudes, which have the same color-like appearance but also have clouds like a pile of cotton. These look like wave signs.

#### sided cloud

• They are also high, which cover the sky like milk with a veil of the white or fine veil. The layer of clouds in these is so thin that the sun or the moon cannot hide from them. Due to these, the halo around the Sun and the Moon is covered. These are also indicators of cyclones' approach.

#### mid-cotton cloud

• These clouds live in the middle altitudes (2,000 to 6,000 meters above) and have some blackness in the lower part, but the color of white cotton in the upper part.
• They present beautiful views in the sky. Despite being dense, they cannot cover the sun. It does not rain from them.

#### mid-level cloud

• These clouds are also found at normal height, but the gray and blue color is visible in their layers. These are denser than the mid-cotton cloud. For some time they can cover the Sun or the Moon, but their light does not get much dim.

#### layer-cotton cloud

• These clouds are of low altitude (height up to 2,000 m). Their shape is large and the color is brown and sometimes black. Cloud levels can be seen in these.

#### layered cloud

• These clouds, which are found at lower altitudes, are covered in the form of a layer whose color is gray. From their presence, it appears as if there is a thick fog shadow. It rains in the form of showers from them.

#### rain cloud

• These clouds found at low altitudes do not have a definite shape. They are dense, uniform, and dark gray in appearance. It would not be inappropriate to call them 'Sajal Jalad'.

#### cotton cloud

• These clouds, found at some height (500 m) above the earth's level, look like a pile of cotton. A blue sky is visible between the clouds. The form of these clouds is likened to a cauliflower.
• The upper part is really raised like a cabbage, which is believed to be caused by the convection action of air. For this reason, they are visible in the third half of the summer. Sometimes it rains from them.

#### cotton rain cloud

• These are only a kind of cotton clouds, but taking a more massive form, they seem like covering the sky. Their color looks black when viewed from below, but the edges of the upper part remain white.
• The shape of the upper part becomes anvil-shaped. They cause torrential rains and sometimes even hail.

## Classification of clouds

### Normal (Height in Meters)

high clouds 1. Partisanship horizontal extension white 6000 to 12,000
2. Partisan level like a thin sheet, horizontal expansion white and transparent white 6 to 10,000
3. sided cotton like parallel ridges of sand, horizontal expansion white 7 to 10,000
medium cloud 1. Intermediate like a continuous uniform sheet, extended horizontally brown, blue 2 to 5,000
2. Middle cotton Spherical and partially fibrous with thick folds, horizontal extension white and brown 3 to 8,000
clouds of low vertical expansion 1. Layered Cotton continuous thin even-folded, horizontally expanded, spherical or cylindrical brown 1600 to 3000
2. Level horizontally and vertically extended cloud The upper part is bright and the lower part is dark brown or black below 3000
3. Cotton Cabbage flower-like vertical cloud The upper part is bright and the lower part is dark brown or black 300 to 1600
4. Cotton year or thunder cloud Huge size like mountains, vertical extension The upper part is bright and the lower part is dark brown or black Base 300 to 1500 and Head 14 to 1500 in the tropics

## Precipitation and rain

• Hail, snow, and water can be included under precipitation. After condensation, if the water droplets in the path meet the freezing point or less than the falling temperature, then the precipitation will be in the form of hail.
• Strong vertical wind currents sometimes carry raindrops above the snowline, where they solidify and enlarge as snowflakes. Then they start falling on the earth, but the convection currents of the air bring them up again, due to which the layers of snow climb on them. In this way, their size becomes bigger and they start falling on the earth. This is called a rainstorm.
• If the temperature of the entire atmosphere condenses at a lower freezing point, then it will be in the form of frost or snowflakes and we will get frost or snowfall (snow). If there is normal condensation in the atmosphere, then rain will be of water. The simple meaning of rain is taken as water rain.

## Types of precipitation

• Like snowfall or snowfall, 'sleet rain' is also found everywhere, which is called sleet in English. On getting the temperature below freezing point, some of the lava remains melted in the rain that occurs when the rain freezes or re-freezes when it melts. Thus, it is a mixed form of rain and snow.

## Favorable conditions for rainfall

 forms of precipitation precipitation Precipitation details precipitation clouds Rain When 0.5 mm from the clouds If the big drops more than the diameter fall on the ground, then it is named rain. Na, As, Sc, Ac Castellanus, Cu congestus snow The rain of white-colored crystalline snowflakes is called snowfall. The diameter of the snowflakes in the snow is 4 or 5 mm. and the nature of the snowflakes is like stars. St, Sc Spray It is a rain of fine droplets. In this, the diameter of the water droplets is 0.5 mm. is less than. It usually falls in a calm or very slow wind current. St, Sc hail When big balls of ice from the clouds, whose diameter is from 5 to 50 mm. or sometimes 4 cm. 5 cm from If it falls, then this form of precipitation is called hail. Cb drizzle Under this, small balls of ice or frozen water droplets and unfrozen water droplets fall on the ground. In the freezing rain, very cold water droplets fall on the ground, due to which it is called Sahim i.e. rain with snow. You It is a combination of snow and mist. Snow in it is opaque. It is formed by the sudden freezing of very cold water droplets. freezing rain It is that form of rain in which the rain reaches the land in the form of water, but after reaching the land, it freezes. St, Sc snow bullet It is the precipitation of white-colored opaque ice spherical grains. The diameter of the snow grains is 2 to 5 mm. of can be. Generally, while falling on the ground, these snow grains break and become disintegrated by hitting the ground. Ice pill and ice list These are also ice balls whose diameter is less than 5 mm. These are transparent and round-shaped or irregular-shaped tablets. Sometimes some ravines of snow fall 2 mm. become elongated. Which have been named ice charts. Cb
• ### The following two conditions are essential for rainfall:

1. There should be enough water vapor in the air.
2. Having a means by which the vaporized air can cool down and condense (condensate).
• ### Evaporated air can be cooled in the following ways:

1. warm air
2. warm air coming into contact with high mountains and rising up and cooling down by contact with glaciated areas at higher altitudes
3. warm air moving to colder latitudes
4. Cooling of hot air by contact with cold air or cold current.

## Types of rain

### convective (vehicular) rain

• It mainly occurs in the equatorial regions, because due to the excessive heat there, the air gets heated up and at the same time the water of the sea rapidly turns into vapor and starts rising.
• Their vertical air currents arise. This action is called 'convection'. Cotton rain clouds are formed due to the rise of the vapor and there is often heavy rain in the third half of the day.
• When air rises and expands, it cools down and becomes liquefied or condensed. A convective current moves in the air. Thus, it starts raining. This process releases latent heat and reheats the air there, forcing it to rise further.
• The rain is heavy and torrential. In addition to the equatorial regions, warm and temperate regions also receive convective rainfall at the beginning of summer, while the surface begins to warm, but the upper atmosphere remains cool.

### mountain rain

• It can also be called surface rainfall. This rain is more in the world, in which the vapor-laden air has to cool down by rising up on the support of the mountain.
• When the air cools, it condenses and it starts raining. The summer rainfall here (by the monsoon blowing from the Arabian Sea and the Bay of Bengal) is similar.
• In mountain rain, there is a lot of rain towards the wind direction on the mountains, but on the other hand, there is a lot of rain because the raised air after raining starts crossing the mountains and starts descending due to pressure and contact with the warm land. Because of coming in, it becomes hot and it does not rain or it is very little.
• These parts of the mountain are called rain shadow areas. For example, Tibet and Central Asia fall under the rain shadow of the Himalayas and the Deccan plateau of India falls under the rain shadow of the Western Ghats.
• The most favorable conditions for mountain rainfall are found there. Where any ranges meet near the coast and the position of the ranges is such that the moist air from the sea hits it at almost right angles.

### cyclonic rain

• It occurs mainly in cold-pressed countries (in the temperate zone where westerly winds blow). It receives rainfall from cyclones. In cyclones, the air moves faster towards the center and is stronger to rise up. It is filled with water vapor as it passes through the ocean.
• When the cold polar air comes in contact with it, a kind of front area is formed in the middle and the hot air containing vapor cools down and gives rain.
• This is cyclonic rain. This rain is not torrential but occurs throughout the year in the form of light showers. In tropical regions, cyclonic rain is accompanied by storms in summer, as this is the season favorable for the occurrence of cyclones.
• This type of rainfall is widely seen in temperate regions. When the warm air mass is mounted on the cold air mass, then there is cyclonic rain on the fronts, which is also called front rain.

## Factors affecting rainfall

### distance or latitude from the equator

• There is more rainfall at low latitudes and less rainfall at higher latitudes. The reason for this is the rapid evaporation due to high heat in the lower latitudes.

### height above sea level

• The mountains receive more rainfall than the plains. This is because the steamy air on the sides of the mountains gets a chance to condense. However, there is a lack of rainfall in the rain-shadow area.

### prevailing wind

• If they come from the sea, they are more likely to rain due to being humid. Conversely, if the prevailing wind comes from the land, it will not be able to rain due to being dry or it will rainless.

### ocean currents

• The air flowing through a cold stream does not have the power to rain. Conversely, the wind blowing from the warm current brings more rainfall.

### distance from sea

• The farther away from the sea, the less rainfall there is. In addition to these, water and land conditions, the different temperatures of both, the direction of mountain ranges, the north-south shift of air pressure belts, etc. also affect the water rainfall.

## Global distribution of rainfall

• The distribution of average rainfall throughout the year over the globe or in the world is shown. Seeing it clearly shows that most of the parts are where 50 cm. (centimeter) less than
• Areas of rainfall above 200 cm are limited. The areas of high rainfall are the equatorial region, the east coast in the zone of trade winds, and the western coast of the continents in the temperate zone where there is cyclonic and partly mountainous rainfall. Rainfall is directly related to air temperature.
• Higher air temperature has more ability to hold moisture in the air, so there is a possibility of more rainfall. Due to the high temperature in the tropics, more water vapor is formed.
• Due to low air pressure there, more rainfall occurs through the process of convection. There is relatively little rainfall in the temperate zone and the least in the polar region.
• On the way, there is a loss of temperature and there is no more humidity in the air, as a result, there is a decrease in rainfall. Rain is found to follow the sun.
• Many geographical conditions affect the distribution of rainfall - such as the location of water and land, the direction of the prevailing wind, the direction of mountain ranges, water currents, shifting of air pressure belts, etc. Areas of low rainfall are more on the surface. Undoubtedly, the distribution of rainfall is uneven.

On the basis of uneven distribution of rainfall, the world can be divided into four rain regions-

### areas of high rainfall

• Here the average is 200 cm. It rains above. The equatorial region and some parts of the monsoon region receive heavy rainfall.

### areas of normal rainfall

• Here 100 cm. 200 cm from Till it rains. Such areas are found mainly in the tropics and sub-tropics. The western coastal parts of the temperate zone are also areas of normal rainfall.

### areas of low rainfall

• Such an area is 25 cm. 100 cm from While the interior of the continents and the east coast of the temperate zone receive less rainfall.

### Rainfed area without rain

• Such an area is 25 cm. Receive less rainfall, hence remain dry or desert. The Polar Regions, Greenland, Sahara, Arabia, Thar, Gobi, Mongolia, Atacama, Kalahari, and Colorado are rainless regions and the rain shadow regions also receive negligible rainfall.
• The average rainfall for the whole world is 100 cm. Of which 19% is received by the continents and 81% by the oceans.
• Rainfall occurs throughout the year on the western coasts of the equatorial regions and temperate zones. In monsoon regions and some parts of the tropics, rainfall occurs especially in summer.
• Around the Mediterranean Sea and other Mediterranean regions, it rains only in the winter, because at that time the wind travels over the water to reach there.
• There is little rainfall in the deserts. They are dry regions. In higher latitudes, the rainfall is very less (due to low temperature) and it is often in the form of snowflakes.

## World distribution of precipitation

• The average annual year in the world is 100 cm. Is. The distribution of precipitation in the world is highly uneven. The distribution of rainfall is influenced by latitude, altitude, mountain slope, relative humidity, and air masses. The maximum amount of rainfall is decreased in the equatorial regions by more than 200 cm.
• Rainfall is the lowest in the sub-tropical regions, where the average annual rainfall is around 25 cm, in the temperate zone, 100 to 125 cm of annual rainfall is received by temperate cyclones. The annual average of precipitation in the polar regions is 25 cm. is near.

## El Nino

• Coldwater current flows along the west coast of South America. Sometimes the temperature of a cold water stream rises and it turns into a warm stream.
• The change in temperature of the current is called El-Nino. Due to the high temperature of the current on the Peruvian coast, there is less wind load generated there. As a result of which the trade winds weaken and the equatorial warm current is drained from west to east.
• The Peruvian coast receives heavy rainfall during an El Nio year and droughts occur in Australia and Indonesia. The Indian monsoon is more likely to fail in an El-Nino year.
• In 1982-83, there was a situation of heavy El Nino, which had the following consequences:-
1. Monsoon failed over the whole of South-East Asia.
2. Drought occurred in the Sahel of Africa and the countries of South-East Asia.
3. Drought occurred in northern Australia and northern New Zealand.
4. In South America, the countries of Peru, Colombia, and Ecuador were flooded due to excess rainfall.
5. Due to the increase in temperature in Alaska and Canada, more ice has melted in the glaciers.
6. The highest temperature recorded in the United States in the last 25 years.
7. Flooding occurred in Cuba and the southern United States.
8. The disease spread to the coral reefs of the Pacific Ocean.
9. The number of fish on the Peruvian coast decreased.
10. Many birds of the Pacific Ocean have declined.
11. Malaria disease spread in the countries of South-East Asia.

### El Nino prediction

• There is no reliable prediction of El Nino. According to the data, El Nino occurs after about seven years.
• Some scholars are of the opinion that the main cause of El Nino is Sea Floor Spreading. Due to this more heat from the asthenosphere flows into the water of the ocean.

## La-Nina

• La Nina literally means little girl. This is the opposite of El Nino. Under this condition, the temperature in the central and western Pacific oceans drops too much below normal. The main reason for this is the stronger than normal sub-tropical high pressure in the South Pacific Ocean during the summer season.
• In such a situation the trade winds become very strong. In the year of La Nia, the temperature of the Peruvian stream drops significantly. In such a situation, there is more rainfall on the coast of Australia, Indonesia, and the Indian sub-continent.

## Iso-lines

• The lines drawn on the map of the world joining the places of equal rainfall are called iso-rain lines or lines of iso-rain. But the change of rainfall occurs gradually from one place to another, so these lines are not being used to show rainfall.

## Measurement of rainfall

• Rainfall is measured by a special type of instrument called a rain gauge. Rainfall is measured in inches, in millimeters, or in centimeters. These days the use of automatic rain gauges is increasing. Which is called 'rainograph'.

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