ANSWER:
Most people are familiar with the midlatitude surface low-pressure systems that march across the country over several days, usually from west to east. By definition, a low occurs at any point on a horizontal surface where the pressure is less than at any surrounding point.
In a well-developed surface low, the wind tends to spiral in toward the center--counterclockwise in the Northern Hemisphere--propelled by the pressure gradient, i.e., the difference in pressure between the center of the low and a point well removed from the center. The stronger the pressure gradient, the stronger the winds that spiral toward the center.
An upper-level low is merely a low well above the surface, say, at an altitude of 10,000 or 20,000 feet. Accompanied by a closed wind circulation that is sometimes nearly circular, these lows tend to move more slowly than wavelike disturbances in the westerlies, and they usually have a surface low underneath them. Weather around upper-level lows is usually inclement. In summer, upper-level lows help spawn thunderstorm development, primarily because they are accompanied by cold air aloft, which contributes to atmospheric instability.
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