Dear Tom,
I often hear meteorologists talking about pressure levels like 850 millibars and 500 millibars. What are the heights of these levels?
Ross Braatz, Milwaukee, Wis.
Dear Ross,
Since the twice-daily world-wide balloon launches report temperature, humidity and wind at various pressure levels given in millibars, meteorologists have long been constructing constant millibar pressure level maps to give forecasters a three-dimensional view of the atmosphere. While the actual heights of the pressure surfaces vary considerably, there are approximate values that relate a specific constant pressure level with a height. Typically, the 850 mb level is about 5,000 feet; 700 mb, 10,000 feet; 500 mb, 18,000 feet; 300 mb, 30,000 feet; 200 mb, 40,000 feet; and 100 mb, 53,000 feet. In winter, the 850 mb temperature is used to determine whether precipitation will fall as rain or snow.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
