Class 8 Science · Chapter 6 NotesPressure, Winds, Storms, and Cyclones

Pressure is defined as the force acting perpendicular to a surface per unit area. The formula is pressure = force / area. The SI unit of pressure is newton per square metre (N/m²), also called pascal (Pa). When the same force acts over a smaller area, the pressure is greater. This is why a sharp knife cuts better than a blunt one, and why a nail is easier to drive using its pointed end. Broad straps of a bag reduce pressure on the shoulders, making it more comfortable to carry. Similarly, a broad handle on a bucket reduces pressure on the hand. People carrying loads on their heads often place a cloth pad to increase the area and reduce pressure.

• Pressure = force / area
• SI unitN/m² = pascal (Pa)
• Smaller area → greater pressure

Liquids exert pressure on the bottom and walls of their container. The pressure depends on the height of the liquid column, not on the shape or width of the container. This is shown by an activity where two pipes of different diameters are filled with water to the same height; the balloons attached at the bottom bulge equally. If the water level is increased, the bulge increases, showing that pressure increases with depth. This is why overhead water tanks are placed at a height to provide greater pressure at taps. Liquids also exert pressure sideways, as seen when water flows out of holes in a bottle. Dams have broader bases to withstand the greater pressure near the bottom.

• Liquid pressure depends on height of column
• Liquids exert pressure in all directions
• Overhead tanks are placed high for good water flow

The envelope of air around the Earth is called the atmosphere. Air exerts pressure on all objects; this is called atmospheric pressure. A rubber sucker sticks to a smooth surface because the air pressure outside is greater than the reduced pressure inside. The atmospheric pressure is very large: the force on an area of 15 cm × 15 cm is about 2250 N, roughly the weight of a 225 kg object. We are not crushed because the pressure inside our bodies balances the external pressure. The practical unit of air pressure is millibar (mb), where 1 mb = 100 Pa. Another unit is hectopascal (hPa), also equal to 100 Pa.

• Atmospheric pressure is the pressure exerted by air around us.
• 1 millibar (mb) = 100 Pa
• 1 hectopascal (hPa) = 100 Pa

Air moves from a region of high pressure to a region of low pressure. This movement of air is called wind. When air is heated, it expands, becomes lighter, and rises, creating a low-pressure area. Cooler air from surrounding high-pressure areas rushes in to take its place. This is how winds form. During the day, land heats up faster than water, so air above land rises, creating low pressure; cooler air from the sea moves in, causing a sea breeze. At night, the sea is warmer than the land, so air above the sea rises, and wind blows from land to sea, causing a land breeze.

• Wind blows from high pressure to low pressure.
• Sea breezeday, from sea to land.
• Land breezenight, from land to sea.

High-speed winds are accompanied by reduced air pressure. When you blow between two hanging balloons, they move towards each other because the fast-moving air creates a low-pressure region between them. Similarly, when strong winds blow over a house, the pressure above the roof becomes lower than the pressure inside the house. This pressure difference can lift the roof off. To prevent this, it is safer to keep doors and windows open during storms so that the pressure difference is reduced. This principle explains why roofs are sometimes blown away in strong storms.

• Fast-moving air → lower pressure.
• Pressure difference can blow off roofs.
• Keep doors and windows open during storms.

Storms are strong winds accompanied by rain. In hot, humid regions, warm moist air rises, cools, and water vapour condenses to form clouds. When water droplets become heavy, they fall as rain, hail, or snow. In thunderstorms, strong up-and-down winds cause ice particles and water droplets to rub against each other, generating static electric charges. Positive charges collect at the top of the cloud and negative charges at the bottom. When the charge build-up is large, air's insulation breaks down and a sudden flow of charges occurs, producing lightning. Lightning heats the air, causing it to expand and produce thunder. Lightning can be dangerous, so we must take precautions such as staying away from tall objects and crouching low.

• Thunderstormstorm with lightning and thunder.
• Lightning conductor protects buildings.

Cyclones are large storms that form over warm ocean waters. Warm, moist air rises, and water vapour condenses to form raindrops, releasing heat. This heat makes the air rise further, creating a very low-pressure area. Air from surrounding regions rushes in and also rises. The Earth's rotation causes the moving air to spin, forming a cyclone. The centre of a cyclone is called the eye, where winds are calm. Around the eye, there are strong winds and heavy rain. Cyclones can cause great destruction through strong winds, storm surges (walls of water 3–12 m high), heavy rain, and flooding. The India Meteorological Department (IMD) monitors and issues warnings. Safety measures include staying updated on weather reports and moving to cyclone shelters.

• Cyclonespinning storm over warm ocean.
• Eyecalm centre.
• Safetyfollow IMD warnings, go to shelters.