Constant pressure maps (upper air maps)

• Constant pressure map (upper air map) - map of the height above sea level at which the pressure is a specific values.
• For example, the 700 mb map has values of the height above sea level at which the pressure is 700 mb.  So, everywhere on the map the pressure is 700 mb, but the height above sea level is generally different from place to place.
• The data for this map comes from balloons that are released twice a day at stations all over the globe.  There are much fewer of these stations than the surface reporting stations.
•  More about constant pressure surface
•  500 mb map for Sept. 13, 1999 @ 5 A.M.  with contours of the height above sea level of the 500 mb pressure every 60 meters.
• Although this is a map of the altitude of a certain pressure level (the pressure everwhere on this map is the same; it is just the altitude that is different), regions of lower pressure correspond to regions of lower height values. Likewise, regions of higher height values correspond to regions of higher pressure.