Humidity and Rainfall PDF

Humidity (08 7 Rainfall L79 Communication, Real-Hife connect; 21C:...

Humidity (08 7 Rainfall L79 Communication, Real-Hife connect; 21C: integration(Science), Epistemic valuesIL(InculSubjcatecte s Dispositions(Developing curiosity LEAD IN>» that temper), rainbows are mere and wonde sienitfri know rainbows rainbow? Did you sky? We see a the Haveyyou ever seen never actuallyformin Nowthink back(to t your optical illusions that ofrain. happens. throughdrops when this when sunlight passes seeseven colours rainbows is the occurrence of and say why we witness Science lessons for us to is precipitation? though illusory, whatis needed and sunlight. But whatunderstand So even precipitation) rainfall (or some other form of ofprecipitation? Let us study this chapter to these natural How is rainfall aform phenomena better. its gaseous form--water EO Learning oufcomes theWater isfound atmosphere. in Water vapour is formed when water vapour-in By the end of this chapter, you will is heated intensely. known as This process evaporation, ofFtransformation is Water vapour or moissture in be able to: the atmosphere plays a veryimportant role in our lIves. causes the describe humidity and differentiate between absolute Moisture inthe atmosphere and so on. of formation and relative humidity clouds,rain, snow, hail, sleet define condensation and discuss the formation of HUMIDITY different types of clouds Humidityisthe amount of water vapour or the moisture air %. there is in in the air. The more water vapour define precipitation and from greater is its humidity. Humidityvaries greatly discuss its different types " describe the different kinds of amount of rainfall place to place. In deserts, for instance, the zero; while in water vapour in the air can be almost coastal areas and rainforests, water vapour can make of air. up almost 4% of the total volume our On a day when humidity is high, the sweat from fel body does not evaporate easily, and we tendair to sticky and uneasy. On the other hand, if the is very dry, our skin feels dry and our lips get chapped. common measures are: Humidity can be measured jn several ways. Some of the more volume ofaj. Absolute Humidity is the amount of water vapour present in a unit and iausually expressed in kilograms per cubic metre.) Relative Humidity: (Thisis the ratio of the amount of water vapour in the air to the total amount of water vapour the air can carry at that temperature and pressure In other words, it is a measure of how dry or moist the air is.(Relative humidity Is expressed as a percentage) 58  water amOunt of B4 The less watervapour becomes, the that air can hold the air can depends on its temperature. The colder it vapourin theair is at hold{The its maximum limit is temperature at which the falls below the dew called the dew point of air. Ifamount of water ofair point, the water vapour starts the temperature cONDENSATION condensing Condensationis the change of water vapour to its liquid moisture-laden air warm, comes in (water) or solid (ice) forms when contact with a cold surface. Usually, place,the atmosphere must be fully saturated. In for condensation totake the maximum other holding amount of moisture it can at that words, the air must be In addition to the air being saturated, temperature. there should bea surface condense. This surface might be a blade of grass, a upon which water can Condensation usually occurs around small particles window, or of dust in the any other cool surface. Water vapour condenses to form dew, mist or clouds. When atmosphere. contact with a cold surface, dew or frost is saturated air comes into formed. Dew is formed if the temperature is above 0°C, and frost is formed if it is below 0°C. theground may produce mist or fog, When Cooling of alarger layer of air near clouds are formed. condensation occurs high up in the air, clouds Couds are floating masses of super cooled wåter droplets and ice crystals Each of these droplets of water is smaller tbove 6.000m than a grain of flour, and they are so light that they can float on air. Cumurrits There are many types of clouds. Some of 20004.000m them are cirrus clouds, stratus clouds and cumulus clouds. Cirrus clouds are thin,wispy clouds 02000% that streak across the sky. They are found high up in the atmosphere and are made up of ice crystals) Stratus clouds are low-lying clouds and appear Do and learn EL; IL: (A: as large, grey sheets covering the entire sky. Use cottonwool to make sample " Cumulus clouds are massive, of the different types of clouds. that often extend to great billowing clouds Paste them on a chart. altitudes. 59  PRECIPITATION becomes more thanthe A When the amount of water vapourin the air come down to tarth ass old it, the condensed cloud droplets andrain. capacity precipitation, airt ot the is of different kinds, such asSsnow, hail, condenses around an ice crystal, Tieppens as water snowflakes. As snow fals tothe ground, water vapour turns. B Snow forms whenttemperatures are solowthat Snow talls tothe it often melts on the warm Earth is cold enough, snow begins to pile up Surface ot forn Preciptain diEartrectlyh iinntohe ice, Thi If the surface of the Hailstones are ice pellets that are too large andwithin creating snowthe Eal heavy tor winds to cumulus carry and drifs htha drop to the ground lce crystals form high upthe cumulus clouds. towards the Earth, theystart melting Within.cloudsthereAs they fall ther fore up the are updrafts of air that carry these ice crystals back towards cloud, The crystals freeze again and begintofall several strong the Earth's surface This process may be repeated mfOre times, until the hailstones become too large and heavy to be heldin the cloud and falltowards the Earth) The most important form of precipitation is rain. Rain takes place when drops of water fall from the clouds to the surface of the Earth. Rain can occur as showers or drizzles, Ashower lasts for a short period of time and is usually made up of large heavy drops. A drizzle usually lasts for longer and is made up of smal, fine droplets of water. Rain mixed with snow causes sleet. Rainfall is the major source of fresh water for waterbodies on the surface of the Earth. It is therefore essential for the survival of plants and animals. However, if rainfall is too heavy, inundation and flooding take place and if it fails, then droughts occur. > Most groundwater resources come from rain as rainwater percolates intothe ground. Soit is important Snowfal, hailstones and sleet. to dig percolation pits to collect and conserve rain Sleet can form a water. This is known as rainwater harvesting. surface over roadsslippery and can > Rainfallis measured with the help of a rain gauge and therefore be dangerous for a disdrometer. people who are driving, Types of rainfall There are three main types of rainfall-convectional rainfall, orographic rainfall and cyclonic rainfal. Convectionalrainfall " Convectional rainfall s common in places with equatorial and tropical climate that experience intense heating of the Earth's surface takes place during the day. During the course of the day, the air above the land becomes heated.This makes the air less dense causing it to rise. 60  rises,it cools. Since cool air cannot hold air as muchwater Vapour as warm air, condensation the Cumulonimbuclouds Water droplets As OCcurs. are formed. Toweringchunderstorm clouds (cumulonimbus smulate and fa as rain acOumu due to gaity Houds)areformed Whentche weight ofthe water in these clouds becomes Air cools andcpndenses glouds. with height, fotrning thanthe cloud can hold, the water comes Dew point more down huge downpour. 11Warmer ait sises orographic asa rainfall The sun heats up the ground Orographicrainfallorrelieffrainfall is caused by the relief land. ororographyoffthe or moisture-laden winds encounter When mountainsor hills, they are forced to rise Warm, moist Air cools, the aír is forced vapour in it to rise over alongthe slope. When air rises and condenses and high areas forms couds. up cools,the water vapour in it condenses andformsclouds, oCcurs when air reaches the Heavyrain. point and the clouds are no longer dew SRain holdthe water. shadow ableto the India, one of best examples of. In. orographicrain is the Monsoon rains Sea Air descends, that occur along the Western Ghats warms and andthe Himalayanfoothills. When the It rains becomes drier moisture-laden Monsoon winds hit the Western Ghats and the Himalayas, the air mass is forced to riseaand reach its dew point. The Monsoon winds bring copious rainfallto the windwardIside of these mountain ranges. The areas which lie on the leeward side of the Western Ghats, like Coimbatore, get less rain. Theyare called the rain shadow regions. Cyclonic or frontal rainfall This type of rainfall Is more common in the tropical and Huge cumulonimbus temperate zones, where cold air mixes with warm ànd clouds are formed moist air..ltis associated with cyclones in the tropical regions and depressions in the temperate zones. When an area of low pressure (or a depression) is Cold air is sucked created, winds rush into the pocket of low pressure from Descending cold in to replace the rising air n tac allsides. At times, there is a mixing of coldair masses alr forms Witn his air too becomes warm and the with warm air masses within this low pressure system. "eye starts rising. "The point where the warm mass of air meets the cold air mass is called a front.When they meet, warm air, Water over 26.5°C warms the air above It, causing it to rise rapldly which is lighter, is forced to rise above the denser, heavier cold air. " As the warm air is forced upwards,it cools. When the temperature drops to below the dew point, the water vapour in the air starts to condense and form clouds that cause rain. Such rain is called frontal or cyclonic rainfall. 61