Published: 2007-08-29 12:32:44
Updated: 2007-08-29 12:32:44
Posted August 29, 2007
MIKE MOSS SAYS: Michael, I don't believe it has been asked, and it is a very good question, one that doesn't actually have as cut and dried, well-defined an answer as you might expect. There aren't sharply defined and agreed upon definitions for "surface," "mid-level," and "upper-level" features and disturbances, but in a good general sense you can figure that a meteorologist talking or writing about about surface features is usually referring to those that are evident on either a map of sea-level pressure contours or a map of 1000 millibar pressure surface height contours (think of this as an imaginary "sheet" at which the pressure everywhere is 1000 millibars - near a low pressure center, this sheet will have a lower altitude than surrounding areas, and vice versa for high pressure centers). Effectively, both of those maps show pressure patterns near the earth's surface and give decent representations of "surface" high and low pressure centers, troughs, frontal boundaries and so on.
You will also see references to mid-level and upper-lebvel disturbances, which typically are describing either short wavelength low pressure troughs, jet streaks (concentrated areas of higher-than-surrounding winds aloft) and vorticity centers (areas of enhanced cyclonic rotation aloft), the three of which often exist together in combinations of two or more. The altitude definitions, as I mentioned, are a little fuzzy and may overlap a bit, but typically a "mid-level" system exists between about 8,000 and 20,000 feet above the ground, while an "upper-level" disturbance would be somewhere in the range of about 18,000 to 35,000 feet up. Mid-level disturbances would usually be analyzed on maps of 700 millibar pressure surface height (around 10,000 feet up) and sometimes on 500 millibar maps (about 18,000 feet), while upper-level disturbances might also refer to the 500 millibar surface or to features evident on 300 millibar (roughly 30,000 feet) or 250 millibar (roughly 35,000 feet) analyses and forecasts. Since these systems can have substantial vertical depth to them at times, the term upper-level disturbance will sometimes be used as a catch-all for something that could just as well be referred to as a mid to upper level disturbance, and vice versa.