Air Pressure
Air has weight and exerts pressure in all directions. Atmospheric pressure is the pressure exerted by the weight of the column of air above a surface. At sea level, atmospheric pressure is approximately 101,325 Pa (about 1 atm).
Key fact: Air pressure decreases with altitude (higher up = less air above = less pressure). This is why ears pop in aeroplanes and mountain climbers use oxygen cylinders.
A barometer measures atmospheric pressure. Mercury barometers show ~760 mm of mercury at sea level.
Bernoulli's Principle: Where the speed of a fluid (liquid or gas) is higher, the pressure is lower. Applications: aircraft wings (aerofoil shape makes air move faster over the top surface, creating lower pressure above the wing, generating lift), perfume atomisers, Venturi meters.
Wind
Wind is the horizontal movement of air from a region of high pressure to a region of low pressure. The greater the pressure difference, the stronger the wind.
- Why does air heat unevenly?
- Land heats and cools faster than water.
- Heated air expands, becomes less dense, and rises → creates a low-pressure area.
- Cooler, denser air sinks → creates a high-pressure area.
- Air moves from high pressure to low pressure — this movement is wind.
Sea breeze (daytime): During the day, land heats up faster than the sea. Air over land rises (low pressure). Cool air from the sea moves toward land. Wind blows from sea to land.
Land breeze (nighttime): At night, land cools faster than sea. Air over sea is warmer, rises. Cool air from land moves toward sea. Wind blows from land to sea.
Monsoon Winds
In summer, the Indian subcontinent heats up strongly, creating a vast low-pressure area. Moist air from the Indian Ocean (high pressure) flows toward India, bringing the South-West Monsoon — India's main source of rainfall (June–September).
Thunderstorms
Thunderstorms develop when warm, moist air rises rapidly to great heights. Water vapour condenses, forming cumulonimbus (thunderhead) clouds. Ice crystals in clouds collide, generating static electricity. When charge builds up sufficiently, a lightning discharge occurs. The rapid heating of air by lightning causes it to expand explosively, producing the boom of thunder (sound travels slower than light, so thunder is heard after lightning is seen).
Safety during thunderstorm: Stay indoors; avoid tall trees (lightning conductors), open fields, and bodies of water.
Cyclones
- A cyclone is a large-scale atmospheric weather system characterised by:
- A low-pressure centre (eye of the cyclone) with rising warm air.
- Very strong winds spiralling inward (anticlockwise in Northern Hemisphere; clockwise in Southern Hemisphere).
- Heavy rainfall.
Formation: Warm ocean water (above 27 °C) evaporates rapidly → warm, moist air rises → low pressure forms → surrounding air rushes in → Earth's rotation (Coriolis effect) causes the air to spiral → cyclone intensifies.
Eye of the cyclone: The calm, clear centre with the lowest pressure. Winds are light inside the eye, but very strong in the eye wall surrounding it.
- Cyclone names in different regions:
- Hurricane (Atlantic, Eastern Pacific)
- Typhoon (Western Pacific)
- Cyclone (Indian Ocean, South Pacific)
---
Barometer reading — A student climbs a mountain. At the base, the mercury barometer reads 760 mm Hg. At 1000 m, it reads about 674 mm Hg. At greater heights, there is less air above, so atmospheric pressure is lower.
Sea breeze — On a hot summer afternoon at a beach, the sand feels hot but the sea is cool. Air above the hot sand rises, and a refreshing breeze blows in from the sea. This is the sea breeze driven by the pressure difference between land (low) and sea (high) during the day.
Bernoulli and aircraft wings — An aerofoil (wing cross-section) is designed so that the top surface is more curved. Air travels farther (and therefore faster) over the top than the bottom. By Bernoulli's principle, higher speed → lower pressure on top. The higher pressure below pushes the wing upward — generating lift.
Thunderstorm safety — During a severe thunderstorm, Raj hears the thunder 6 seconds after seeing the lightning. Speed of sound ≈ 340 m/s. Distance = 340 × 6 = 2040 m ≈ 2 km away. He knows the storm is 2 km from him.
Cyclone Amphan (2020) — A powerful cyclone formed over the Bay of Bengal with wind speeds over 180 km/h. Early warning by the IMD (India Meteorological Department) allowed evacuation of millions of people from coastal areas, saving many lives. This shows the importance of meteorological monitoring.
Monsoon winds — In June, the Indian landmass heats intensely, creating a low-pressure zone. Moist air from the Arabian Sea and Bay of Bengal is drawn northward. When this moisture-laden air rises over the Western Ghats, it cools, condenses, and brings heavy rainfall to the west coast of India.
Tornado vs cyclone — A tornado is a narrow, violently rotating column of air (funnel-shaped) extending from a thunderstorm to the ground, typically 100–500 m wide. A cyclone is thousands of kilometres wide and forms over warm oceans. Both have low pressure at their core and destructive winds.
Key Formulas
- Pressure: P = F/A
- Wind direction: High pressure → Low pressure
- Distance to lightning: d = speed of sound × time delay (340 m/s × seconds)
Common mistakes
Students confuse the direction of sea breeze and land breeze. Remember: during the day (sea breeze) wind blows from sea to land; at night (land breeze) wind blows from land to sea. Also, the eye of a cyclone is calm, not the most dangerous part — the eye wall is most violent.
Summary
Atmospheric pressure decreases with altitude. Winds blow from high to low pressure. Unequal heating of land and water causes sea and land breezes and monsoons. Thunderstorms involve rapidly rising warm air and lightning. Cyclones are intense low-pressure weather systems with spiralling winds; they form over warm tropical oceans.