Competitive Exams: Atmospheric Science Atmosphere
The thin gaseous envelope surrounding the earth is called atmosphere and it acts like an insulating blanket protecting the earth. It softens the intense light and heat of the sun. Its ozonic (03) layer absorbs most of tile very deleterious ultraviolet-rays from the sun and thus protects living organism from extinction.
Atmosphere contains about 5.0 × 1015 tonnes of gases, a small amount of water vapour and some dust particles. A column of air weighing about one tonne is pressing downwards on our shoulders but we do not feel this pressure as it is counter balanced by the saline pressure from within our bodies. The atmosphere is bound to the Earth by gravity. Satellites which have very low gravitiational power, cannot and do not hold an atmosphere. Air has very little weight.
A litre of air weights around 1.3 × 103 mg. At the sea level the air pressure is 1033.6 gram per square em and this pressure usually termed as one atmosphere. The dry air of the atmosphere is composed of various gases and water vapour from. The Earth surface and upto about 50 km the atmosphere comprises of about 78 percent nitrogen~ 21 percent oxygen and 0.93 percent of argon, carbon dioxide, neon, helium and methane in the above said order. The amount of carbon dioxide varies from place to place being greatest around the cities and smallest in the village area. Atmosphere also contains tiny particles of dust and some other substance. Water vapour also varies from place-to-place. Water vapour is present in the lower atmosphere, its amount ranging from 0.01 percent to 1 percent. Though the amount of water vapour in the atmosphere is very small but its importance is very great. If there is no water in the atmosphere, there would be no weather on Earth. Clouds. Are made of water vapour that has evaporated from the Earth. They are tiny droplets of microscopic size and are light to fall down as rain. So they ride on the air waves until they condense and then fall down as rain. Clouds are distinguished according to their shape. These are the following: 1. Cirrus clouds-It is shaped like ringlets and goes upto 12 km in height. 2. Cumulus clouds-It is risen in heaps. 3. Stratus clouds-It is scattered. 4. Nimbus clouds-It is the menacing rain storm clouds. And there are various kinds of clouds which is often found mixed together like the cirro-cumulus, cirrostratus, cumulo-nimbus … Etc. Clouds are the surge of electricity from the earth that makes lightning. The lead, however is taken by the clouds which send down a rather weak stroke called the leader stroke. Dry air is highly resistant to electricity. When the air is loaded with water vapour it becomes an easier conductor. Enough power is required far the stroke to rip through the air. This excessive discharge of electricity heats up the air around the passage of the stroke to incandescent temperatures upto 1.0 × 104 C. It is this growing air that we see as lightning flash. The heat also causes a sudden expansion of air which as the heat disappears, contracts quickly again. This sudden expansion and contraction produce the familiar thunder clap. Though both occur at the same time, we! lee the light first because light travels faster than that of sound.
Atmospheric Layers: The character and composition of the atmosphere changes as we go higher and higher and it comes in layer which is called atmospheric layers and are layers of air into sub-sphere with 3 pauses. They are
- Troposphere: It is the nearest to the earth's surface and extends to a distance of about 12 km. The temperature generally decreases as height increases in troposphere. It is the densest of all layers and contains water vapour, moisture and dust. It also profoundly influences earth's climate since 80 percent of the mass of air comprising the entire atmosphere is concentrated in this zone.
- Tropopause: It is the layer that joins troposphere, the lowest layer with the upper layer stratosphere. The height of tropopause varies with latitude.
- Stratosphere: It is the region above tropopause. It is about 30 km thick. It is free from the violent weather changes which occurs below. So, it is preferred by our jet liners. Jet liners however face another means in stratosphere, namely Jet streams. Jet streams are high velocity air currents.
- Mesosphere: Its a very cold region above the ozone rich layer of stratosphere.
- Ionosphere: It comes just above mesosphere, extends from about 60 km to 500 km above the earth. It includes the thermosphere and exosphere. The region contains an electrically charged air and reflects radio waves facilitating wireless communication between distant places. The ionised air also protects those on earth from the falling meteorites most of which are made to bum out at this region.
- Thermosphere: It constitutes the middle layer of ionosphere and has a temperature of 212° F or 100° C.
- Exosphere: It is the uppermost region of the atmosphere where the air-density is so low that an air molecule moving rapidlly straight upward is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules.