ATMOSPHERE

NATURE OF THE ATMOSPHERE :- The atmosphere is as much a part of the earth as land or water, although we may not feel, it, except when it moves as wind, it is not as dense as either land or water, but it has weight and exerts pressure. The atmosphere is mobile, elastic and both compressible and expansible. Held to the earth by gravitational attraction, this envelope is densest at sea level and thins rapidly upward. Composition of Atmosphere: The atmosphere constitutes a mixture of gases, the (composition and ratio of which vary somewhat with height. About 21 percent of it consists of oxygen which helps burning and breathing and without which we cannot live. The bulk of the atmosphere is made up of an inert gas, nitrogen, which dilutes the oxygen and slows down the process of oxidation. It is also necessary for the growth of plant life. There is a small amount of carbon dioxide which the plants utilise during the process of photosynthesis. This gas absorbs heat and thus allows the lower atmosphere to be warmed .by heat radiation coming from the sun and from the earth's surface. It is the heaviest of the gases of the air and, therefore, the lower layers of the troposphere contain much more CO,2 than the upper layers. There are also traces of argon, ammonia and water vapor. The atmosphere protects us from the millions of meteors and meteoric particles which fall toward the earth every day. It filters a great deal of the ultra-violet light coming from the Sun. It acts as a huge air-conditioner moderating the extremes of heat and cold. It is because of the atmosphere that we have winds and rain and other phenomena on which all plant and animal life depends. The main reason why the moon is uninhabitable is its lack of suitable atmosphere. Layers of Atmosphere: The atmosphere that surrounds the earth is not of the same thickness at all levels. It is built more like a four layer cake each layer having its own characteristics. The Troposphere is the lower layer of the atmosphere 'and extends upto 8 km at the equator. About 90 % of the atmosphere's total mass is contained within this layer. The troposphere acts as a warm blanket to moderate the extremes of outer space. On the average temperature decreases everywhere with the height at the rate of 6°C per kilometer. The Stratosphere lies above the troposphere. Within it the temperature does not decrease with altitude as it does in the troposphere nor is there much vertical movement of the air. The stratosphere expends upto 80 km above the surface of the earth. The Ionosphere forms the next layer of the atmosphere from 80 km to 480 km. Radio waves used in long-distance radio communications are reflected back to earth by the ionosphere. In this way, radio messages can be transmitted round the curve of the Earth. It also benefits man by absorbing the sun's deadly x-rays. The northern lights or aurora borealis are in this zone. The outermost layer of atmosphere is known as the Exosphere. It lies somewhere between 480 and 960 km above the Earth. WEATHER AND CLIMATE :- Climate is one of the basic elements in the natural environment. It affects land forms, soil types and vegetation. Its influence on man is very great. The kind of clothes that he wears, the food he eats and the house in which he lives and works are intimately related to climate. The term "climate" should not be confused with "weather". Weather is the day to day condition of the atmosphere at any place as regards temperature, rainfall, winds, humidity, sunshine and cloudiness and such other elements. Climate is generally defined as the average state of weather. The elements to be considered are the same while studying the climate or the weather conditions of a place, but, weather refers mainly to short periods like a day, a week, a month or a little longer while climate is concerned with average conditions determined by observations over long periods. Both weather and climate are affected by such things as directions of the sun's rays and the length of day, altitude, distribution of land and water bodies, direction of mountain ranges, air pressure, winds and ocean currents. FACTORS AFFECTING AIR TEMPERATURE OVER THE EARTH'S SURFACE :- 1. Latitude: This midday sun is almost overhead within the tropics, while outside the tropics the sun's rays fall obliquely. ,Within the tropics the rays are concentrated on a smaller area and pass through less atmosphere. On the other hand, outside the tropics, the rays are spread over a larger area and pass through a longer distance and much of the heat is absorbed by clouds, water vapor and dust particles. 2. Altitude: The atmosphere is mainly heated by conduction, namely, by the land or water with which it is in contact. The effects of the heat are felt more in the layers of the atmosphere near the surface of the earth. Hence, places nearer to the earth's surface are warmer than those higher up. 3. Distance from the S'ea: Due to the phenomena of land and sea breezes the temperature of the coastal margins is comparatively cooler than that of a place situated far away from the sea. 4. Ocean Currents and Winds: Warm and cool ocean currents raise or lower the temperatures, of land surfaces along the coastal margins. This effect is felt all the more if the winds are on-shore i.e. blowing from the sea towards land. 5. The Slope of the land: Slopes facing south in the Northern Hemisphere are warmer than those facing north. This is because the rays of the sun strike the south-facing slopes at a steeper angle than they do the northern slopes. The severe cold of Siberia is largely due to the fact that the country slopes towards the Arctic Circle. North of the Equator, the northern slopes of the east-west valleys have a less rigorous climate than the southern, because they get the full benefit of the sun. 6. Nature of soil: Alluvial oil can retain water; on rocky soil the rainfall runs quickly off the surface and is lost. The more water a soil can retain, the less rapidly it heats or cools. Rocky and sandy soil is heated more rapidly and also cooled more rapidly. This increases the day temperatures and decreases the night temperatures. The great diurnal range of temperature in the Deserts is partly due to the nature of soil. Dark colored soils and surfaces absorb more of the sun's heat than light colored ones. 7. Vegetation: Forests are like sponges which retain moisture in the soil. They also prevent the air from being heated rapidly during the day and from being cooled quickly during the night. The dense vegetation of the Equatorial forests cuts off much of the incoming isolation and in many places sunlight never reaches the ground. For this reason, it is cool in the jungle and its shade temperature is a few degrees lower than that of open spaces in similar latitudes elsewhere. 8. Clouds and Rainfall: Clouds check solar radiation by day and ground radiation by night. The intense heat in the Savanna regions is partly due to the lack of clouds. There are few clouds in the tropical deserts so that sunshine is abundant. This makes these regions much hotter and drier than the equatorial regions. The nature of clouds, whether cirrus, cumulus or stratus and its position in the sky also affect the sunshine hours.