MEGHAM (THE CLOUD) 4 FINAL PART Dearfriends,
I am very happy at the responses from readers on readingthe simple technical information and enjoying them and they get newinformation. Many members have written to me TO CONTINUE TO WRITE ON SUCHSIMPLE TECHNICAL ITEMS. There are a few repetitions, but I have included forsimplicity in understanding in thisposting. Sincerely, Gopala krishnan 22-2-2011. Reposted on 8-2-2021after editing and updating. R. Gopala Krishnan. 22. CLOUD FIELD A cloud field is simply a group of clouds but sometimes cloud fields cantake on certain shapes that have their own characteristics and are speciallyclassified. Stratocumulus clouds can often be found in the following forms: ACTINOFORM, which resembles a leaf or a spokewheel. CLOSED CELL, which is cloudy in the centre andclear on the edges, similar to a filled honeycomb. OPEN CELL, which resembles a honeycomb, withclouds around the edges and clear, open space in the middle. Reflectivity andcolours. 23 COLOUR OF CLOUD If we look above most of the clouds are whitish grey orvery dark grey in winter. But during sunset we see clouds like yellowish red, yellowetc. Let us see in simple language howdo it occur. The colour of a cloud,as seen from the Earth, tells much about what is going on inside the cloud. 23.1 WHITE COLOURCLOUDS Dense deep tropospheric clouds exhibit a high reflectance(70% to 95%) throughout the visible spectrum. Tiny particles of water aredensely packed and SUNLIGHT CANNOT PENETRATE FAR INTO THE CLOUD before it isreflected out, giving a cloud its characteristic white colour, especially whenviewed from the top. Cloud droplets tend to scatter light efficiently, so thatthe intensity of the solar radiation decreases with depth into the gases. As a result, the cloud base can varyfrom a very light to very dark grey depending on the cloud's thicknessand how much light is being reflected or transmitted back to the observer. Thin clouds may lookwhite or appear to have acquired the colour of their environment or background. High tropospheric andnon-tropospheric clouds appear mostly white if composed entirely of icecrystals and/or super cooled water droplets. 23.2 BLACK CLOUR CLOUDS As a tropospheric cloud matures, the dense water dropletsmay combine to produce larger droplets, which may combine to form dropletslarge enough to fall as rain. By this process of accumulation, the spacebetween droplets becomes increasingly larger, permitting light to penetratefarther into the cloud. This happens during winter rain. If the cloud issufficiently large and the droplets within are spaced far enough apart, it maybe that A PERCENTAGE OF THE LIGHT WHICH ENTERS THE CLOUD IS NOT REFLECTED BACKOUT BEFORE IT IS ABSORBED. A simple example of this is being able to seefarther in heavy rain than in heavy fog. This process of reflection/absorptionis what CAUSES THE RANGE OFCLOUD COLOUR FROM WHITE TO BLACK. 23.3 OTHER COLOURCLOUDS Other colours occur naturally in clouds. Bluish-grey is the result of LIGHTSCATTERING within the cloud. In the visible spectrum, blue and green are at the shortend of light's visible wavelengths, while red and yellow are at the long end.The short rays are more easily scattered by water droplets, and the long raysare more likely to be absorbed. The bluish colour is evidence that rain-sizeddroplets in the cloud are producing such scattering. A greenish tinge to acloud is produced whensunlight is scattered by ice. a cumulonimbus cloud emitting green is animminent sign of heavy rain, hail, strong winds and possible tornadoes. YELLOWISH CLOUDS are rare but may occur in the latespring through early fall months during forest fire season. THE YELLOW COLOUR IS DUE TO THEPRESENCE OF POLLUTANTS IN THE SMOKE. 23.4 RED, ORANGE& PINK CLOUDS We know very well thesecoloured clouds occur either during sunrise or sun set. Especially if we are going in thecar in the early morning we can see first orange coloured clouds before Sunrise. Red, orange and pinkclouds occur almost entirely at sunrise/sunset and are the result of the SCATTERING OF SUNLIGHT BY THEATMOSPHERE. THE CLOUDS DO NOT BECOME THAT COLOUR; they areREFLECTING LONG AND UN SCATTERED RAYS OF SUNLIGHT, which are predominant atthose hours. In combination with large, mature thunderheads this can produceblood-red clouds. (Mythinking was due to absorbing short and large wave length rays till Iread it from wikipedia. Now I am corrected) Clouds look darker in the near infrared because waterabsorbs solar radiation at those wavelengths. 24. WHETHER OTHER PLANETS TOO HAVE CLOUDS? Within our Solar System,any planet or moon with an atmosphere also has clouds. Venus's clouds are composed ofsulphuric acid droplets. Mars has high, thin clouds of water ice. Both Jupiterand Saturn have an outer cloud deck composed of ammonia clouds, an intermediatedeck of ammonium hydro sulphide clouds and an inner deck of water clouds.Uranus and Neptune have cloudy atmospheres dominated by methane gas. Saturn's moon Titan hasclouds believed to be composed largely of droplets of liquid methane. The Cassini–Huygens Saturn missionuncovered evidence of a fluid cycle on Titan, including lakes near the polesand fluvial channels on the surface of the moon. 24 HOW RAIN COMEFROM CLOUDS To get rain, the water condensing in the clouds has tobecome heavy enough to fall to Earth. The tiny droplets just aren't heavyenough to fall. Just like fog or in the shower, they go whichever way the windand eddy currents blow them, or they just hang there, suspended in the air. This explains why rainis not there, when we see cloud all the time. To become heavier, the droplets need to grow into drops. Todo this they have to acquire more water and become larger. Some will collide with otherdroplets and become larger, and others will grow as water condenses out the airdirectly into the droplet. Others will grow by both methods. It's abit like watching drops of rain water on a window - small drops fall, they joinwith other small drops, become larger drops, and so on. I had referred surface tension, this processof formation and falling when surface tension becomes less than the weight ofthe droplet. In the right clouds, this process will be happening tomillions of tiny droplets, all growing at the same time, but at differentspeeds. Eventually, if the droplets keep growing, they will reach amass where they can't stay floating in the cloud because they are too heavy -and will start to fall. Some may get caught in upward blowing winds and get blownback into the clouds for a while, but once they are heavy enough to overcomethe force of the wind, they will fall to earth - as rain! 25. HOW LONG IT WILLRAIN It will keep rainingas long as the conditions are right to make the clouds and let the waterdroplets grow heavy enough to fall. 26. WHY CLOUD HAS TOBE COOLED FOR RAINING The cooling the air reduces its ability to hold watervapour, and triggers the formation of water droplets. To create clouds,and to have rain, the air has to be cooled. There are three main ways in whichthis happens inthe atmosphere, and every geography student needs to know, and understand, allthree of them. 26a ConvectionalRain 1) On a warm day thesunshine heats up the ground. Air above the warmed ground also becomes warmer,and it begins to rise (because warm air is less dense than the air around it). As it warms up itabsorbs more water because, as you should remember, warm air can hold morewater than cool air. 2) The atmosphere gets cooler as you go higher - by roughlyone degree centigrade cooler for every 100m of altitude in dry air - so as thewarm air rises it becomes cooled again by the colder air around it. 3) EVENTUALLY THEAIR REACHES A HEIGHT WHERE THE TEMPERATURE FORCES THE WATER VAPOUR IN THE WARMAIR TO START CONDENSING. This is called the condensation point, and is where theclouds begin to form. The typical cloud formed this way is called a cumuluscloud, or a cumulonimbus cloud when it has a grey bottom and 'looks like rain'. 4) The rain formingprocess starts, and usually leads to very heavy rain, perhaps with thunder and lightening. Convectional rain isoften experienced at the end of a hot summer day and associated with torrentialdownpours with large drops and towering dark grey clouds. In summer months,look out for clouds forming over fields of corn, large car parks andconcentrations ofbuildings. All these features heat up quickly and cause warmed air to rise. 26b.Frontal Rain 1) Two bodies of air are involved - an area of relativelywarm air and an area of relatively cool air. 2) The warmer air is less dense, so when it meets thecooler air it rises up over the cooler air mass. The cooler air stays where itis, and lies underneath the warmer air. 3) As the warmer air rises over the cold air it starts tocool. 4) As the air-cools, water vapour is precipitated and thecloud forming process begins, leading to rain. Sky's are typically grey andcloud covers almost all the sky. 26c Relief Rain 1) A physicalobstruction of some kind is needed, so that warm moist air is forced to rise upover it. Mountainranges, big hills and even cliffs along the coast are large enough to force theair to rise. 2) As the warm airrises over the obstruction it cools and clouds form. Rain falls from theclouds, or if thedroplets don't grow large enough, fog may form over the hilltops. FOG IS BASICALLYJUST CLOUDS AT GROUND LEVEL. 3) The air passes over the obstruction and can sink again,gaining warmth as it does so. This air is drier than it was before it lostwater as rain, so any clouds left will evaporate again, leaving clear skies.This area has only a little rain because the cloud making process isn'tworking. Because there is so little rain, the area is said to be in a 'rainshadow'. Relief rain is common in uplandand mountainous areas where is can lead to extraordinary local rainfall patterns. It'scommon for one side of a mountain to be in warm sunshine, yet only a fewhundred meters away it's raining on the other side of the mountain ridge. Thiscan be important to the development of settlements, agriculture and tourism. Forexample, in summer most people want to be on the sunny side of a mountain withlittle precipitation, but in winter, when rain is replaced by snow, they wantto be on the side where the precipitation does fall. (CONCLUDED) -- You received this message because you are subscribed to the Google Groups "Thatha_Patty" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/thatha_patty/1310664562.956294.1612794540613%40mail.yahoo.com.
