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Each 10mb in height represents about 310 feet of altitude. 1000 mb pressure is usually between 0 and 350ft. As overall pressure changes, these constant pressure heights will move up and down in altitude above sea level.

When the surface pressure is less than 1000mb, it is a fairly deep low pressure. Standard sea level pressure is 1013mb. For a full explanation of pressure levels see Haby Hints.

The actual altitude for a given level can be estimated by where the level hits the known mountains (the color is the same as the border of the map). Blanchard is 2200' Stewart is 3000', Black is 4800. The contour maps used by the forecasting model are smoothed and often poor representations of important local features that make a big difference in our ability to fly a given site on a given day.