CHAPTER 8 - Winds, Storms and Cyclones

AIR EXERT PRESSURE

Our earth is surrounded by a blanket of air, called the atmosphere. The atmosphere extends up to nearly 300 km above the earth's surface. This air exerts pressure on everything on the earth. This is called air pressure.

When you climb a tall mountain you will notice that your ears pop and your breathing becomes faster. As we climb a mountain, the number of air molecules decreases so air pressure decreases. This causes the ears to pop to balance the air pressure inside the ear and outside. As the number of air molecules decreases that means oxygen molecules also decrease and in each breath, we take in less oxygen, so we have to breathe more often. So mountaineers may carry an oxygen mask if they plan to climb, say the Mount Everest.

AIR EXERTED FROM TWO VALUES ALTERNATIVELY OPPOSITE TO EACH OTHER

Factors which affect atmospheric pressure are

• Altitude: Air pressure decreases with increasing altitude (height), that is, air pressure is more near the surface of the earth and decreases as we move away from the surface.

• Temperature: At a higher temperature, the air gets heated and expands reducing the air pressure while atlower temperature, the air becomes denser and air pressure increases

• Watervapour: Moist air is lighter or less dense than dry air Hence it exerts less pressure on us.

HIGH SPEED WINDS RESULT IN REDUCED AIR PRESSURE

When air moves with high speed, a low pressure of air is created. This happens due to the fact that when air moves at a high speed,the molecules also moves faster. As a result, a low pressure is created in that area.

When air moves with high speed, a low pressure of air is created. This happens due to the fact that when air moves at a high speed,the molecules also moves faster. As a result, a low pressure is created in that area.

Thus, we can say that high wind speed reduces air pressure. Look at the figure which shows wind blowing at high speed over a tin roof of a house. It will help you understand why a roof can be blown off by high-speed winds.

AIR EXPANDS ON HEATING

On heating, air expands and occupies more space. When something occupies more space, it becomes lighter. Warm air is, therefore, lighter than Cold air. That is why smoke which is warm also rises up.

As warm air rises, it leaves its place. This decreases air pressure at that place. As a result, cold air from some other place rushes in to occupy this empty space. This sets up an air current. This air current is called convection of air. Let us perform the following activity to understand this effect.

SMOKE COMING FROM CHIMNEY OF A HOUSE

SMOKE COMING FROM CHIMNEY FROM THE FACTORY

WIND GENERATION IN NATURE

We know that winds blow due to a difference in pressure between different areas. Let us try to understand how this happens in nature

Uneven heating of the earth's surface between the equator and the poles:

The regions close to the equator get maximum heat from the Sun. The air in these regions gets warm. The warm air rises, and the cooler air from the regions in the 0-30 degree. latitude belt on either side of the equator moves in. These winds blow from the north and the south towards the equator. At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at these latitudes rises up and the cold wind from the Polar Regions rushes in, to take its place. In this way, wind circulation is set up from the poles to the warmer latitudes. If the Earth stands still, these winds would blow directly from the north to the south and from the south to the north. But as the earth rotates, the winds in the northern hemisphere swerve to the right and the winds in the southern hemisphere swerve to the left.

Unequal heating of the land and water surfaces:

The regions close to the equator get maximum heat from the Sun. The air in these regions gets warm. The warm air rises, and the cooler air from the regions in the 0-30 degree. latitude belt on either side of the equator moves in. These winds blow from the north and the south towards the equator. At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at these latitudes rises up and the cold wind from the Polar Regions rushes in, to take its place. In this way, wind circulation is set up from the poles to the warmer latitudes. If the Earth stands still, these winds would blow directly from the north to the south and from the south to the north. But as the earth rotates, the winds in the northern hemisphere swerve to the right and the winds in the southern hemisphere swerve to the left. The land gets heated faster during the day but also loses its heat fast after sunset, whereas water takes a much longer time to get warm but also a long time to cool. Land and sea breezes influence only a narrow strip of the coast. During hot summer days, the land gets heated up quickly compared to the water in the sea. The air above the land gets heated and rises up and is replaced by the cool air from the sea giving rise to the sea breeze. At night, the land cools faster than the sea. The air above the sea, being warmer, rises up and is replaced by the cool air from the land giving rise to the land breeze.

LAND BREEZE

SEA BREEZE

Monsoon wind

The winds blowing from the sea towards the land are laden with moisture and bring rain. These winds come from the southwest direction and bring moisture with them from the Indian Ocean. They bring rain to almost all parts of India. In the winter months, the reverse happens and dry cold winds blow from the land to the sea causing rain in coastal Tamil Nadu whereas the rest of the country receives less rainfall. These winds are called the winter monsoon winds. Only a part of these winter winds which flow from eastern India over the Bay of Bengal is able to absorb moisture from the bay and bring rain, to the western region of southern India. These winds are known as the Monsoon winds.

CHARACTERISTICS OF WIND

The wind has two characteristics-directions and speed-which can be determined with the help of suitable instruments.

WIND DIRECTION

Wind direction: The direction of wind can be observed with the help of a 'wind vane' commonly called 'the weather cock' because it has a cock sitting on top. This arrow-shaped vane is mounted at its centre to move freely about its vertical axis. The arrow aligns itself and its tip points towards the direction of the wind. There are four directions- north, south, east and west. They are marked on the wind vane.

For obtaining accurate direction, the wind vane is mounted high above the ground so that it is free from obstacles such as tall trees and high buildings.

SPEED OF THE WIND

The speed of the wind depends upon the air pressure difference between two areas. When the difference is high, the wind rushes from high pressure to low pressure area. The speed of the wind is usually measured with an instrument called an anemometer. Cup anemometers are mainly used by meteorological stations. It consists of 3-4 metal or plastic cups on arms that spin around a vertical pole. A dial at the base records the number of revolutions made by the cups in a given period of time, which indicates the speed of the wind.

STORMS

You must have observed wind blowing with great speed creates disturbances in the atmosphere. They can raise a lot of dust, uproot huge trees, blow away the roof of houses and cause large scale destruction to life and property. Such strong winds are called storms. They are caused when warm and light air rises quickly into the higher altitude. They are called by different names as cyclones, hurricanes and typhoon according to the geographical zone in which they form.

THUNDERSTORM

The thunderstorm is caused by violet air current inside the cumulus (thunder) clouds. Inside the clouds, warm and moist air rapidly rises up. In doing so, the water vapour in it rapidly condenses to form tiny droplets of water, which freeze to form small particles of ice. During this condensation and freezing, a large amount of heat energy is released. The water and ice particles rub against each other in rapidly rising air. This builds up a negative electric charge in the cloud by the stroke of lightning. This also produces a crash or thunder similar to a loud clap sound. The thunderstorms are accompanied by heavy rains and high-speed winds which can damage weak buildings, uproot trees, submerge crops and can cause flash floods

CYCLONE

Cyclonic winds are irregular winds of short duration. The term cyclone is associated with circulatory winds in the atmosphere. A cyclone is created, when a low-pressure area is developed with higher pressure all around. The wind tends to blow towards the low-pressure centre and circulates around the centre. Tropical cyclones are of a seasonal nature and generally occur in the later part of the summer season. Cyclones:

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TROPICAL RAINFORESTS

The tropical rainforest is a forest of tall trees, in a region of year-round warmth, the temperature ranging from 15°C to 40°C. Because of continuous warmth and rain, this region supports a wide variety of plants and animals. Climate condition of the rainforest supports a variety of flora and fauna all adapted to its humid climate. To avoid competition for food, the animals confine themselves to different layers of the forest.

The different layers of the forest are discussed below:

• Emergent is the topmost layer of the forest. It consists of various tall trees and animals such as eagles, monkeys, bats and some insects.

• Below the emergent is a dense layer of tree crowns growing in a close space. This layer is called the canopy. Animals such as sloths, snakes, lizards, insects and birds like toucan are found in this layer.

• Below the canopy is the understory. Here, very little sunshine reaches and the plant leaves are broad. Animals such as jaguars, red-eyed tree frogs and leopards are found in this layer.

• Below the understory is the forest floor. In this layer, there is no vegetation. Animals such as anteaters, snakes, mice and insects are found in this layer.

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