Atmospheric pressure

Air expands when heated and is compressed when cooled, which changes air pressure. Air moves from high to low pressure: horizontal moving air is wind. Winds spread heat and moisture around the planet, and rising moist air cools to form clouds and rain.

Atmospheric pressure is the weight of a column of air over a unit area, from sea level to the top of the atmosphere. It is measured in millibars (mb) with a mercury barometer or an aneroid barometer. Average sea-level pressure is 1,013.2 mb. Gravity makes the air densest, and pressure highest, at the surface; climbing high, the air gets thin and people feel breathless.

Vertical variation: in the lower atmosphere pressure falls fast, about 1 mb for every 10 m of height, though not always at the same rate. In the standard atmosphere: 1,013.25 mb at sea level, about 899 mb at 1 km, 540 mb at 5 km and 265 mb at 10 km. The vertical pressure difference is much stronger than the horizontal one, but it is almost exactly balanced by gravity pulling the other way.

Horizontal distribution is shown with isobars, lines joining places of equal pressure. To remove the effect of height, each station's pressure is reduced to sea level first. A low is enclosed by isobars with the lowest pressure at the centre; a high has the highest pressure at the centre.

World pressure belts:

• Equatorial low near the equator.
• Subtropical highs along about 30° N and 30° S.
• Sub-polar lows along about 60° N and 60° S.
• Polar highs near the poles.
• The belts are not fixed: they shift with the sun's apparent movement, northward in the northern summer and southward in its winter.

Example (new): a station 500 m above sea level reading 960 mb would be reduced to sea level by adding about 50 mb (1 mb for every 10 m), giving about 1,010 mb, so its pressure can be compared fairly with a coastal station's.