Any non-meteorologist who has stumbled across numerical weather models online may have been puzzled by the fact that upper-level maps of the atmosphere are not maps of specific heights in the air, but of specific pressures. We do not look at a map of the weather conditions at 18,000 feet. Instead, we look at a map of 500 millibars. This is done because the computations of forecasts of those layers are much simpler if the result is a map of a pressure surface rather than a height.

In fact, the meteorological world uses a coordinate system with linear distances in the X and Y planes (north/south and east/west) but the vertical measurement is one of air pressure. This works because the air pressure always lower, higher up.

One of the most fundamental principles of numerical weather forecasting is that the distance between two pressure surfaces (thickness) is proportional to the average temperature between those surfaces. This is a small part of how the weather models work.

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