Composition of Air
The atmosphere is a huge blanket of air that surrounds the earth. It provides us the air we breathe and protects us from the harmful effects of the sun’s rays. Atmosphere is consists of mainly nitrogen (78%), oxygen (21%), argon (0.93%), carbon dioxide (0.03%) and other gases like helium, ozone, argon and hydrogen (0.04%).
• Nitrogen is very important for plant’s survival. They cannot take nitrogen directly from the air. Bacteria that live in the soil and roots of some plants take nitrogen from the air and change its form so that plants can use it.
• Oxygen is the second most plentiful gas in the air. Humans and animals take oxygen from the air. Green plants produce oxygen during photosynthesis.
• Carbon dioxide is another important gas. Green plants use carbon dioxide to make their food and release oxygen. Humans or animals release carbon dioxide. The amount of carbon dioxide released by humans or animals seems to be equal to the amount used by the plants which make a perfect balance.
Structure of the Atmosphere
The atmosphere is divided into five layers starting from the earth’s surface. These are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.
• Troposphere: This layer is the most important layer of the atmosphere. Its average height is 13 km. The air we breathe exists here. Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
• Stratosphere: It lies above the troposphere which extends up to a height of 50 km. This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes. One important feature of stratosphere is that it contains a layer of ozone gas.
• Mesosphere: This is the third layer of the atmosphere. It lies above the stratosphere. It extends up to the height of 80 km. Meteorites burn up in this layer on entering from the space.
• Thermosphere: In thermosphere temperature rises very rapidly with increasing height. Ionosphere is a part of this layer. It extends between 80-400 km. This layer helps in radio transmission. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
• Exosphere: The upper most layer of the atmosphere is known as exosphere. This layer has very thin air. Light gases like helium and hydrogen float into the space from here.
Weather and Climate
Weather is the mix of events that happen each day in our atmosphere including temperature, rainfall and humidity. Weather is not the same everywhere. Climate refers to the average weather condition of a place for a longer period of time. The temperature is the degree of hotness and coldness of the air. The temperature of the atmosphere changes not only between day and night but also from season to season. Insolation is the incoming solar energy intercepted by the earth which influences the distribution of temperature. The amount of insolation decreases from the equator towards the poles.
The pressure exerted by the weight of air on the earth’s surface is called air pressure. The air pressure is highest at sea level and decreases with height. Horizontally the distribution of air pressure is influenced by temperature of air at a given place. Low –pressure areas where temperature is high the air gets heated and rises. It is associated with cloudy skies and wet weather. High pressure is associated with clear and sunny skies which mean areas having lower temperature, the air is cold. Heavy air sinks and creates a high pressure area. The air always moves from high pressure areas to low-pressure areas.
Wind is the movement of air from high pressure area to low pressure areas. There are three types of wind: Permanent winds; Seasonal winds; Local winds
• Permanent winds – The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
• Seasonal winds – These winds change their direction in different seasons. For example monsoons in India.
• Local winds – These winds blow only during a particular period of the day or year in a small area. For example, land and sea breeze, loo etc.
When water evaporates from land and different water bodies, it becomes water vapour. Humidity is the amount of water vapor in the air. The water vapour condenses when it raises causing formation of droplets of water. Clouds are just masses of water droplets. When these droplets of water become too heavy to float in air, then they come down as precipitation. Precipitation that comes down to the earth in liquid form is called rain. Most of the ground water comes from rainwater. There are three types of rainfall: the convectional rainfall, the orographic rainfall and the cyclonic rainfall. Rainfall is very important for the survival of plants and animals. It brings fresh water to the earth’s surface. Rainfall brings fresh water to the earth’s surface.