Atmospheric pressure

Before a thunderstorm the air sometimes feels heavy. That is to do with atmospheric pressure: the push from the weight of all the air above and around us.

Think of a tall bottle of water. The water at the bottom carries the weight of all the water above it; the water at the top carries almost none. Air is the same. Pressure is higher at the sea coast and lower high up in the mountains, where there is less air above you.

Up a mountain the air is thinner, so there is less oxygen with each breath. Your body has to work harder, and people may feel breathless, dizzy or tired. Travellers are told to stop on the way up to acclimatise (let the body adjust). Our soldiers serve at places like Khardung La in Ladakh, over 5600 m high, where the pressure is only about 650 millibars.

Pressure can also drop for other reasons. A big drop makes a depression (a low-pressure system), which can grow into a storm or even a cyclone. High pressure usually means clear, calm weather.

Pressure is measured with a barometer, in millibars (mb). Normal pressure at the coast is about 1013 mb; below 1000 mb means a depression. A falling barometer is often the first warning of a storm.