Mid-Level – Geopotential Height

Maps at the mid-levels of the troposphere at 500mb are most useful for showing the geopotential height of the 500mb pressure level and the temperature advection which may be occurring in the mid-levels of the troposphere. The solid black lines representing constant geopotential height contours show how far above sea level the 500mb level is in a way which outlines the troughs and ridges in the mid-levels of the troposphere. In the above example, there is a clear 500mb trough extending from the Midwest into the Southeast United States and there is a 500mb ridge extending from the Northern Plains into the Central Plains of the United States. The axis of a 500mb trough extends down the middle of the location of the trough (in this case, the axis extends from Lake Michigan into Indiana, western Kentucky and Tennessee, and along the border of Mississippi and Alabama). This 500mb trough is synonymous with a center of low pressure and an area of lower geopotential height. Lower pressure values would imply that there is less mass within the column of air, and therefore the column of air is shorter, so this would lead to lower geopotential heights.

The flow of air in the mid-levels is an important factor in whether there is mid-level support for ascent leading to thunderstorm activity. When the flow of air in the mid-levels is cyclonic, or counter-clockwise, that will generally support rising motion and precipitation. This support for precipitation is often found along the axis of mid-level troughs or ahead of mid-level troughs where cyclonic advection (the movement of more cyclonic air into less cyclonic air) is the strongest. With this in mind, the above 500mb map shows that cyclonic advection is most likely along the east coast of the United States, so that is generally where thunderstorm activity is predicted to occur over the next several hours.

Interpreting the Geopotential Height Values

The actual values of geopotential height plotted on the above analysis are the purple numbers adjacent to each upper-air station. These numbers, typically in the 500s, represent the height above sea level of the 500mb pressure level in decameters (10s of meters). For example, a geopotential height value of 575 would translate to the 500mb level at that location being 5,750 meters above sea level. The black contours are lines of constant geopotential height. These contours are drawn for numbers which are divisible by 6 and in intervals of 6 decameters. The contours in the above example are 540, 546, 552, etc.