Surface low pressure is associated with a trough on a upper level isobaric chart. I am having trouble understanding this becauseI associate surface low pressure with warm air which develops upper level high pressure? Can you help me understand this?

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Allen Gellis

MIKE MOSS SAYS:        Allen,     The relationship between upper level pressures and surface pressures can be pretty complicated in some instances, and doesn't reduce to a real simple explanation. However, in a general sense, moving low pressure centers at the surface tend to precede open-wave upper level troughs, with the location of the lowest pressure "tilting" with height toward the the location of coldest air in the lower atmosphere. The reason for this has to do with the fact that pressure decreases with height most rapidly in colder air, and less rapidly with height in warmer air. So, the area of cold air in between a surface low and a surface high to the west is often more or less beneath a trough aloft, while the warm air between a surface low and a high to the east is often located beneath a ridge aloft.

On occasion, an upper level trough will "cut off" into a closed low. This often signals a deep and mature low at the surface that has wrapped a core of cool air into its center of circulation through an occlusion process. When this happens, the lowest pressure becomes more "stacked" with height so that the upper low is in nearly the same position as the surface low, and when that happens the entire system tends to move rather slowly and to gradually weaken.

I'm not sure that brief discussion really clears things up all that well for you. You might try having a look at some of the basic meteorological principles sites on the web, where some well-designed graphics help to supplement explanations of some of these principles. Here are a couple of sites you could check out:     (especially note the "midlatitude cyclones" link along the left hand side)

and       (especially the "air pressure," "global circulations," "jet streams" and "air masses" sections)



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