Geopotential height approximates the actual height of a pressure surface above mean sea-level. Therefore, a geopotential height observation represents the height of the pressure surface on which the observation was taken.
Since cold air is more dense than warm air, it causes pressure surfaces to be lower in colder air masses, while less dense, warmer air allows the pressure surfaces to be higher. Thus, heights are lower in cold air masses, and higher in warm air masses.
A line drawn on a weather map connecting points of equal height (in meters) is called a height contour. That means, at every point along a given contour, the values of geopotential height are the same. An image depicting the geopotential height field is given below.
Geopotential Height Anomalies
Geopotential height anomalies consist of deviations in the geopotential height field from average values. In Figure 2 below, (which displays the geopotential height anomalies from Figure 1), it is evident that areas with lower geopotential heights correlate with negative geopotential height anomalies. The height anomalies map below indicates that the geopotential heights are much below average for this time of year over the eastern United States, and implies colder than average temperatures across this region.