Points to Remember : NCERT class VII : Our Environment – Air – Chapter 4
Air we take in while breathing is actually a mixture of many gases.
Plants need nitrogen for their survival.
Humans and animals take oxygen from the air as they breathe.
Green plants use carbon dioxide to make their food and release oxygen.
When air is heated, it expands, becomes lighter and goes up. Cold air is denser and heavy.
Our atmosphere is divided into five layers starting from the earth’s surface-
- Troposphere: most important layer of the atmosphere, 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: extends up to a height of 50 km. This layer is almost free from clouds and associated weather phenomenon, making conditions .it contains a layer of ozone gas which protects us from the harmful effect of the sun rays.
- Mesosphere: extends up to the height of 80 km. Meteorites burn up in this layer on entering from the space.
- Thermosphere: Ionosphere is a part of this layer. It extends between 80-400 km, helps in transmission of radio waves.
- 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 is a day to day phenomenon. However, the average weather condition of a place for a longer period of time represents the climate of a place.
The degree of hotness and coldness of the air is known as temperature.
An important factor that influences the distribution of temperature is insolation. Insolation is the incoming solar energy intercepted by the earth. The amount of insolation decreases from the equator towards the poles.Therefore, temperature decreases in the same manner.
Temperature is measured in degree Celsius. It was invented by Anders Celsius. On the Celsius scale the water freezes at 0°C and boils at 100°C.
Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface. As we go up the layers of atmosphere, the pressure falls rapidly. 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. In areas where temperature is high the air gets heated and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.
The air always moves from high pressure areas to low pressure areas.
The movement of air from high pressure area to low pressure areas is called wind.
Winds can be broadly divided into three types.-
- 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 blow only during a particular period of the day or year in a small area. For example, land and sea breeze.
Moisture in the air at any time, is known as humidity.
Precipitation that comes down to the earth in liquid form is called rain.
