The Wind

Wind High Speed Winds are accompanied by Reduced Air Pressure Changes in air pressure bring changes in the weather and make winds blow. Air usually moves from areas of high pressure to areas of low pressure and this produces winds. High air speed causes low pressure and low air speed causes high pressure. Activity Take two balloons and fill a part with water and rest with air. Place the balloons in front of the electric fan with some distance between them. Switch on the fan at lower speed. You will notice that the distance between balloon decreases slightly. Switch off the fan you will see that the balloons come back to their original place. Why did that happen? When air blew between the balloons the pressure between the balloons reduces and pressure outside the balloons is higher and it pushes them closer. Can you think what will happen if, speed of the fan is further increased? The balloons will come even closer to each other. We conclude that high air speed causes low pressure and low air speed causes high pressure. Air expands on Heating Air condenses as it is cooled and hence requires less space. That means air contracts on cooling. Warm air therefore takes up more space than the same amount of cold air; it also weighs less than cold air occupying the same space. On heating, the molecules begin to move faster and move apart from each other leading to expansion. Activity Air expands on heating and contracts on cooling. Take a glass bottle and a balloon. Now fill half the bottle with water and the balloon has to be fixed over the mount of the water bottle, with the help of rubber band. Now place a flame under the bottle for heating the water. What do you see? The balloon expands. This is because when you heat the air inside the bottle, it expands. When it expands it needs more space to fill out. This expanding hot air finds empty space inside the balloon, so the air moves inside the balloon and hence the balloon expands. Let us put this bottle into a jar having ice cubes. What will you see? The air inside the balloon moves toward the bottle and the balloon contracts. Air condenses as it is cooled and hence requires less space. That means air contracts on cooling. The continuous contraction of air inside the balloon will result in a completely deflated balloon. Some applications ¾ Smoke from a fire moves upward. This is because fire heats the air above it, and this hot air moves up. The air carries the smoke along with it, thereby making it rise up. ¾ In a hot air balloon, there is a balloon fixed over a basket and a flame just below the balloon. As the air inside the balloon gets heated up, it becomes lighter and rises up. Wind Currents Are Generated Due To Uneven Heating on the Earth The winds blow from a region of low temperature to a region of high temperature because of a difference in the atmospheric pressures. A region with a high temperature has a low-pressure condition because of low air density. That means when a place is hot, air above the place will also get heated up and hence will rise up thus decreasing the air density at that place whereas a region with a low temperature has a high-pressure condition because of high air density. The pressure differences are caused due to following reasons: (a) Uneven heating between the equator and the poles land and water. Because of the tilt of the Earth, both the poles receive least amount of sunlight as compared to equator region and hence are very cold. Due to this air in these regions also remains cool forming a High-Pressure region. The air between the 30° to 60° latitude belts is warmer in comparison to the air near the poles. Hence, this warm air rises up and cold air from the Polar Regions rush in to fill the empty space. (b) Uneven heating of land and water. Sea breeze In day time, the surface of the land is warmer than the surface of sea. The warm air over the land moves upward. Once it does that. There is decrease in air pressure here. So, there is vacant space. The air from the sea blows toward the land to occupy the space. This blowing air is called a sea breeze. Now this sea breeze brings cold air from the sea. So, it reduces the temperature of the surrounding area. Land breeze In night time, exactly the reverse happens. The land absorbs the heat in the day and releases it very quickly in the night as compared to water which absorbs the heat late and releases it late! The air over the land is cooler and denser than the air over the sea. So now the reverse is happening, the warm air over the sea moves upward. So there is a low pressure area here and it is waiting for the air from the land to rush in and take its space. This is known as the land breeze. That is the wind blowing from the land toward the sea. This is the reason that the temperature of the beach is neither too hot and nor too cold. During the summer months, the land at equator absorbs heat and the temperature of the land becomes higher than that of the oceans. This makes the hot air from the land to rise up and the cold air from the oceans blow towards the land. This results in the formation of the monsoon winds. During winter months, the temperature of the land becomes less than that of the oceans. As a result, winds start blowing from land to the oceans. Monsoon rain Monsoons are seasonal winds that change their direction with changing seasons. They blow from the oceans towards land in summer and from the land to oceans in winter. When blowing from oceans to land, these winds carry water droplets with them and are responsible for the rains in India.