Pressure, Winds, Storms, and Cyclones

Class 08 Science

When we carry a bag, we feel its weight because of the force of gravity acting on our shoulders. The weight of the bag with narrow straps acts on a smaller area of our shoulders, whereas the weight of the bag with broad straps is spread out over a larger area of our shoulders.

It is due to this reason that we feel more comfortable carrying a bag with broader straps than one with narrow straps, although both bags have the same weight. Since the area over which the force acts is involved, we define a quantity called pressure.

Pressure is the force per unit area.

$$ \text{Pressure} = \frac{\text{Force}}{\text{Area}} $$

Pressure is defined as the force per unit area. The SI unit of force is newton and that of area is metre². Therefore, the SI unit of pressure is newton/metre² (N/m²). This unit is also called a pascal, denoted by Pa.

Example: If a force of 100 N is applied on a cardboard of area 2 m², then the pressure applied on the cardboard will be

$$ \text{Pressure} = \frac{\text{Force}}{\text{Area}} = \frac{100\, \text{N}}{2\, \text{m}^2} = 50\, \text{Pa} $$

When the area over which a force applied is smaller, the resulting pressure is higher. This is why it is easier to drive a nail using its pointed end, and it is easier to cut an apple with the sharp edge of a knife.

The pressure exerted by a liquid in a vessel depends on the height of its column. This is the reason why overhead tanks are placed at a height so that the pressure in the taps is increased, resulting in a good stream of water from the taps.

Water also exerts pressure on the sides of a container. Liquids exert pressure not only at the bottom of the container, but also on its sides. In fact liquids exert pressure in all directions.

Pressure Exerted by Air

The envelope of air surrounding the Earth is called atmosphere. The atmospheric air contains nitrogen, oxygen, argon, carbon dioxide, and other gases in small quantities. The atmosphere extends up to many kilometres above the surface of the Earth.

The pressure exerted by air around us is known as the atmospheric pressure.

The force exerted by the atmospheric air column over an area 15 cm × 15 cm is nearly equal to the force of gravity on an object of mass 225 kg (2250 N).

Formation of Wind

Air moves from a region of high air pressure to a region of low air pressure.

As land gets heated faster than water during the day, the air above the land becomes warmer and lighter. Hence, it rises, creating an area of low pressure. The air from the high pressure region of the sea blows to the low pressure region which develops on the land, resulting in a sea breeze. At night, the water is warmer than the land. Therefore, a low pressure area develops above the sea. As a result, wind blows from the land to the sea, giving rise to land breeze. Thus, the phenomenon of land breeze and sea breeze is mainly due to the pressure differences over the land and the sea.

Storms, Thunderstorms, and Lightning

When land gets heated, the warm and moist air, being lighter, rises, thereby creating a low pressure area. Cooler air from the surrounding high-pressure areas flows to take its place. This air, in turn, gets heated and rises. This results in a continuous process of wind circulation.

As the rising air expands, it cools and moisture in it condenses to form water droplets, creating clouds. The water droplets merge to form heavier drops, which come down as rain, hail, or snow. The strong winds accompanied by rain is called a storm. In hot, humid, and tropical regions like India, storms are more frequent. Under certain conditions, warm air rises to great heights that the low temperature there converts water droplets into ice particles.

Strong winds blowing upwards and downwards facilitate rubbing between water droplets and ice particles. Strong winds blowing upwards and downwards and rubbing against each other cause static electric charges to develop within the clouds.

The positively charged lighter ice particles move upwards and occupy the upper part of the clouds. The negatively charged heavier water droplets occupy the lower part of the clouds. Thus, a charge separation within the cloud takes place. Also, when the negatively charged lower part of the cloud moves closer to the ground, it causes the ground and nearby objects, such as trees or buildings, to become positively charged.

Normally, air acts as an electrical insulator and does not let opposite charges meet. But when the build up of charges becomes very large, the insulating property of air breaks down. A sudden flow of charges takes place, producing a bright flash of light called lightning.

Lightning can occur as opposite charges collide within a cloud, between clouds, or between clouds and the ground. Lightning rapidly heats up the air around it, causing the air to expand and produce a loud sound known as thunder. A storm accompanied by lightning and thunder is called a thunderstorm.

Cyclone

Cyclones are large storms that form over warm ocean waters. As the ocean water gets heated, the warm and moist air above it rises. As the moist air rises, the water vapour condenses to form raindrops. During evaporation, water takes up heat to change into vapour. When this water vapour condenses into raindrops, heat is released back into the atmosphere. This causes further warming of the ascending air leading it to rise even further, creating an even lower pressure. Air from the surrounding regions rushes in and it also starts rising. Earth’s rotation causes the moving air to spin. This cycle is repeated, resulting in the creation of a very low-pressure area with high-speed winds revolving around it. This spinning system of clouds, winds, and rain is called a cyclone.

In a cyclone, the region of lowest pressure is at the centre, known as the eye of the cyclone. At the eye of the cyclone, the wind is calm, but the surrounding region experiences strong winds and heavy rainfall. As a cyclone moves from the ocean towards the land, it generates higher wind speeds compared to the wind speeds produced by regular thunderstorms. Once the cyclone reaches land, the source of moist air is cut off and it gradually loses its strength.