1. Air/Atmospheric Pressure -
ü The continuous physical force exerted on or against an object when something comes in contact with it, is called pressure.
ü Pressure is directly proportional to force and inversely proportional to area. And it varies inversely with volume.
ü Since air occupy space, it exert pressure on objects. The particles of air are in random motion, which also account for pressure.
ü The pressure exerted by air (that comprise of different gases) present in the atmosphere is called as atmospheric pressure.
2. Applications Of Air Pressure -
ü It creates wind. The wind is result of difference in atmospheric pressure on the earth.
ü It influences weather. Wind speed plays important role in cloud formation and patterns of monsoon.
ü It helps in predicting weather. The changes in air pressure is helpful for weather forecasting. The air pressure is measured by an instrument called barometer.
Note: When we pump air or fill air into the bicycle tube, the air molecules inside the tube collide with the walls of the tube and exerts air pressure. The air pressure exerted from inside inflates the bicycle tube. The pressure exerted by air filled in a bicycle tube keeps the tube tight and makes the bicycle tyre feel hard. If we continuously fill more and more air into the bicycle tube, the air pressure in the bicycle tube will increase too much that tube may get burst. These observations also show that air exerts pressure.
3. High Speed Winds-
ü High-speed winds are accompanied by reduced air pressure in the atmosphere.
ü Air moves from the region of high pressure to the region of low pressure.
ü The greater the difference in pressure, the faster the air moves/ higher the wind speed.
4. Expansion Of Air-
ü The term ‘expansion of air’ means the increase in the volume of air.
ü When the air is heated, its volume increases and occupies a larger space.
ü Gases or air expand more than solid and liquid on heating this is due to free arrangement of particles.
ü Air is a mixture of gases and the particles of air are always at random motion.
ü On heating, these particles of air gain kinetic energy.
ü It moves away from each other, thus occupying more space. This is due to increase in thermal forces which tends to keep particles away from each other.
ü The air becomes lighter. Due to this warm air rises up whereas comparatively cool air tends to sink towards the surface.
Note: When the air is heated, it becomes lighter and rises up in the sky. This is the reason that when we burn woods, the smoke rises along with the hot air near the place of fire. Because of this fact, air is utilized in launching hot air balloons.
5. Wind Currents-
Figure - Wind flow patterns
ü The wind is the movement of air which depends on the difference in air pressure in two regions.
ü Air moves from the region of high pressure to the region of low pressure in the atmosphere.
ü This difference in air pressure is created by uneven heating on the earth.
ü The region where the air rises, an area of low pressure is created while the region where the air sinks, an area of high pressure is created.
ü The uneven heating on the earth takes place in two situations viz-
a. Uneven heating between the equator and the poles-
ü The region of the earth which is closed to the equator of the earth gets the maximum heat from the sun. Therefore, the air in these regions gets warm and rises, creating an area of low air pressure.
ü The cooler air from the region of up to 30 degrees latitudes belt on either side of the equator moves or rushes towards the equator and replaces the warm rising air.
ü This makes the wind to blow from the North and South directions towards the equator.
ü At the poles, the air is colder than at latitudes about 60 degrees.
ü Thus, the warmer air moves upward and colder air rushes in. This makes the wind blow from the poles of the earth towards the warmer region up to about 60 degree latitudes.
b. Uneven heating of land and water-
ü During summer, the earth near the equator warms faster than the water in the oceans.
ü The air above the land gets warmer and rises up in the sky creating a low-pressure area.
ü Therefore, the winds flow from the oceans towards the land.
ü These winds carry a lot of moisture with them and bring rain.
ü It is a part of the water cycle.
ü These rain-bearing winds are called the monsoon winds.
ü During winter, the direction of the wind gets reversed.
ü The wind blows from the land to the sea due to the difference in air temperature between the land and sea. This happens so because during winter, the land cools down faster than the water in the oceans and the temperature of water in the ocean is higher than that of land.
ü The warm air over the ocean rises up creating a region of low pressure and cooler air from the land rushes towards the ocean.
ü Thus, the air flows from land to ocean carrying only a little water vapour. Hence, bring only a small amount of rain in the winter season.
Figure - Uneven heating of land and water for India
6. Thunderstorms-
ü When air moves slowly, it is called a wind breeze.
ü But when air moves violently at high speed, it is called as a storm.
ü When the pressure of air drops, it indicates the possibility of a storm.
ü The swift movement of the falling water along with the rising warm air producing sound, lightning, heavy rain and strong wind is called thunderstorm.
ü It develops in hot and humid tropical areas like India and is accompanied by heavy rains or hail. Thunderstorms are produced by the dark clouds which form at fairly low altitude in the atmosphere.
ü In the tropical area, the air gets warmed up and makes it to rise, whereas humidity provides the water vapour for the formation of cloud.
ü Therefore, it can be said that to occur a thunderstorm, it requires moisture, rapidly rising warm air and sea breeze or mountains.
7. Cyclone-
ü The centre of a cyclone is a calm area and is called the eye of the storm.
ü The diameter of the eye varies from 10 to 30 km.
ü It is a region free of clouds and has light winds.
ü Around this calm and clear eye, there is a cloud region of about 150 km in size.
ü In this region, there are high speed winds (150-250 km/h) and thick clouds with heavy rains.
ü Away from this region, the winds speed gradually decreases.
ü Formation of Cyclones -
n Factors like temperature, humidity, wind speed, wind direction and rotation of the earth, contribute to the development of a cyclone.
n The energy required to form and sustain a cyclone comes from the heat of condensation of water vapour present in moist air rising from the surface of hot seawater that condenses at altitudes to form a cloud.
n When the air above the seawater is heated, a region of low pressure is created because warm and moist air rises up. The cool air rushes in forcing up more hot air.
n The process of moving warm air up and its replacement by cool air is repeated again and again making or setting up a cycle or air current.
n Once the cyclone is formed, it begins to move over the surface of the sea. The strongest wind and the heaviest rain occur in the towering thunder clouds about 20-30 kilometres from the centre of the cyclone.
n Cyclone comes to an end quickly if a cyclone moves over land because it no longer receives heat energy and moisture from warm seawater.
Note: A cyclone is known by different names in different parts of the world. It is called a hurricane in the American continent. In the Philippines and Japan, it is called a typhoon.
Figure - Formation of cyclone
8. Tornadoes-
ü A tornado is a violent spinning storm in the shape of a dark funnel with a narrow end on the land.
ü These are formed over the sea and are called water spouts.
ü Tornado develops from the thunderstorm and is formed mostly on the land.
ü These are violent and can reach the speeds of more than 500 km/h destroying everything in their path.
NCERT EXERCISE QUESTIONS
1. Fill the missing word in the blank spaces in the following statements:
(a) Wind is——————air.
(b) Winds are generated due to——————heating on the earth.
(c) Near the earth’s surface __________air rises up whereas ___________ air comes down.
(d) Air moves from a region of ——— pressure to a region of——— pressure.
Ans.- (a) moving (b) uneven (c) warm; cold (d) high; low
2. Suggest two methods to find out wind direction at a given place.
Ans.- (a) By noticing the direction of dry leaves after they are released in the air. (b) By noticing the direction in which it is difficult to ride the bicycle.
3. State two experiences that made you think that air exerts pressure (other than those given in the text).
Ans.- (a) When we fill the air in the tyre, it becomes tight and if we try to fill more air it will get burst which proves air exerts pressure. (b) Blowing wind causes curtains, doors and windows to move with the direction of the wind. This demonstrates air exerts pressure.
4. You want to buy a house. Would you like to buy a house having windows but no ventilators? Explain your answer.
Ans.- It is not at all a good idea to buy a house without ventilators because warm air is lighter than cold air hence warm air passes out of houses through ventilators. Hence, without ventilators, warm air will not pass out, and the house will remain warm.
5. Explain why holes are made in hanging banners and hoardings.
Ans.- Holes in the banners and hoardings helps air to pass through it, thereby it allows the banner to withstand the pressure of the air and remains intact.
6. How will you help your neighbors in case cyclone approaches your village/town?
Ans.- (a) By calling emergency services such as fire station, police and medical centres. (b) By helping them to shift their animals, necessary households and vehicles to a safer place. (c) By helping them shift to a safer place.
7. What planning is required in advance to deal with the situation created by a cyclone?
Ans.- (a) We should not ignore the warnings issued by the meteorological department through TV, radio, or newspapers. (b) We should make necessary arrangements to shift the essential household goods, domestic animals and vehicles, etc. to safer places. (c) Avoid driving on roads through standing water, as floods may have damaged the roads. (d) Phone numbers of all emergency services like police, fire brigade, and medical centers should be handy. (e) Always store drinking water for emergencies. (f) Do not touch wet switches and fallen power lines. (g) Do not pressurize the rescue force by making undue demands. (h) Cooperate and help your neighbor and friends.
8. Which one of the following places is unlikely to be affected by a cyclone.
(i) Chennai (ii) Mangaluru (Mangalore) (iii) Amritsar (iv) Puri
Ans.- (iii) Amritsar
9. Which of the statements given below is correct?
(i) In winter the winds flow from the land to the ocean.
(ii) In summer the winds flow from the land towards the ocean.
(iii) A cyclone is formed by a very high-pressure system with very high-speed winds revolving around it.
(iv) The coastline of India is not vulnerable to cyclones.
Ans.- (i) In winter, the winds flow from the land to the ocean.
Junior Science Topic - Winds, Storms And Cyclone. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 08)
By - EduPorm India - The Educational Platform
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