Geopotential height
Air pressure Height field Geopotential altitude
Geopotential height describes height while accounting for gravity. On weather charts it usually shows how high a chosen pressure surface lies.
Height with gravity taken into account
Gravity is not exactly the same everywhere on Earth. Geopotential height allows for this: it is the energy needed to lift one kilogram, divided by a standard gravity value. In the lower atmosphere, it is close to ordinary height above sea level.
Reading an upper-air chart
On a 500 hPa chart, pressure is 500 hPa everywhere. The contour lines show the height at which that pressure is reached. Some charts use metres, others decametres: 560 dam equals 5,600 geopotential metres.
Why warm air lifts the contours
A warm layer of air is more expanded than a cold one, so the distance between two pressure surfaces is greater. A bulge of high values is called a ridge; an elongated area of lower values is a trough.
Closely spaced height contours often indicate strong upper winds away from the equatorial region. A trough may favour unsettled weather and a ridge more settled conditions. A single height value, however, cannot tell you whether rain will reach the ground.