Not all weather occurs at the surface. The troposphere (weather "layer" of the atmosphere) and those imbedded weather systems depicted on surface weather maps as Highs and Lows may extend to heights of several kilometers above the Earth's surface. At upper levels, far above the effects of surface friction on wind flow, distinct Highs and Lows usually give way to wave-like air currents that stretch around the globe. Meteorologists have long known that the atmosphere is three- dimensional and the understanding of weather requires knowledge about the atmosphere throughout its breadth and depth.
Meteorologists routinely monitor the atmosphere using essentially horizontal upper-air charts drawn at several different levels. These charts are drawn twice daily based on data collected at 00Z and 12Z by radiosondes launched from a network of stations. Each chart is drawn at a particular pressure level. Constant pressure surfaces are used as radiosondes report the data in terms of pressure, pilots often fly aircraft using pressure altimeters, and meteorological calculations are somewhat easier when expressed in terms of pressure values. A surface of constant pressure undulates slightly in altitude from place to place primarily due to the temperatures of the underlying atmosphere.
An often-used upper-air chart is the 500-mb map. This level is about half way up in terms of atmospheric mass and occurs near an altitude of 5,500 meters (18,000 feet) in the mid-troposphere. Winds at this level correlate well with the movements of large-scale weather features that dominate the lower troposphere. Surface Lows generally travel in the same direction and at about half the speed of the 500-mb winds above them.
For a more complete look at features of various upper air charts available on the DataStreme Homepage, see the optional Thursday Supplemental Summary Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 8 Chapter Progress Response Form, under section B. Daily Summary.