Class 7 Science
Air: Air is a mixture of gases.
Wind: Moving air is called wind.
Air exerts pressure: The fact that Air exerts pressure can be understood by many examples.
- Tube of bicycle gets inflated when air is filled in it. This happens because air exerts pressure.
- It is difficult to move the bicycle against the direction of wind. Similarly, a sailor feels it difficult to sail his boat against the direction of wind. This happens because air exerts pressure.
- When boiled water is filled in a tin can and its mouth is closed with the lid; followed by cold water being poured over it, the can gets distorted. This happens because air inside the can gets condensed and creates low pressure. The higher pressure from outside distorts the shape of the can.
- You can fly a kite because air exerts pressure from the rear portion of the kite.
- Aeroplane can fly because air exerts pressure.
- Birds can fly because air exerts pressure.
- High speed wind creates area of low air pressure:
- Thatched roof is blown off because of high speed wind. This happens because high speed wind creates an area of low pressure. Due to this, the thatched roof is sucked up and finally gets blown away. Thus, increase in wind speed is associated by decrease in air pressure.
Movement of air:
Air moves from high pressure zone to low pressure zone of air. Air; from high pressure zone; rushes towards the low pressure zone; to fill the gap. Thus, greater is the pressure difference greater will be the speed of wind and vice-versa.
Cause of Wind Movement:
Pressure difference in the air is cause by differential heating. Warm air rises up because it becomes lighter. This creates an area of low pressure below the rising air. The area of low pressure is quickly filled in by cooler air from around. That is how the wind movement is created on the earth. Thus differential heating of air creates pressure difference; which makes the wind movement.
Movement of Air Because of Uneven Heating on Earth:
Movement of Air in Equatorial Region: The Equatorial region gets the highest amount of sunshine throughout the year. Due to this, the air near the equatorial region gets heated. The warm air rises up and creates an area of low pressure. The cooler air; from the higher latitudes; rushes towards the equator to fill the gap. This results in the wind movement from tropical zones towards the equator. Similarly, wind movement is caused in other parts of earth.
Movement of Air Because of Uneven Heating of Land and Water:
Sea Breeze: In coastal areas, the land warms up faster than the ocean surface. Warm air from the land rises up and creates an area of low pressure near the ground. The low pressure zone is filled in by cooler air from the ocean surface. This gives rise to the sea breeze.
Land Breeze: During night, the land cools down faster than the ocean surface. The warmer air from the ocean surface rises up and creates an area of low pressure near the surface. This is filled in by cooler air from the land. This gives rise to the land breeze.
Monsoon: The flow of moist air from ocean towards land in summer is called Monsoon or Monsoon wind. The word Monsoon came from Arabic word ‘mausim’, which means weather. The term ‘Monsoon’ had traditionally been used to describe the typical climate of the Indian Peninsula during the rainy season. Now-a-days, the term ‘Monsoon’ is being also used to describe the similar climate in other parts of the world.
Monsoon winds carry water vapour which falls over the land in the form of rain after cooling down. In India, the harvest depends on the monsoon wind because it is the major factor towards bringing rainfall in India.
Thunderstorm: Many a times, rain is associated with lightning. Rain with lightning is called thunderstorm. This happens mostly in the summer. Thunderstorm occurs frequently in hot and humid tropical areas, such as India.
In summer, the air rises up along with lot of moisture. After rising up to a certain height water vapour cools down and starts coming down in the form of water droplets. The swift drifting of water droplets creates electricity among clouds. This manifests in the form of lighting and sound. This is known as thunderstorm.