Low pressure systems, like high pressure systems, are generally a reflection of systems aloft. They, too, experience shifts in location and changes in intensity and shape with height. At times, a surface system may not be evident aloft and a well developed system aloft may not reflect on a surface analysis.

The cold core low contains the coldest air at its center throughout the troposphere; that is, go-ing radially outward in any direction at any level in the troposphere, warmer air is encountered. The cold core lows (fig. 3-2-7) increase in inten-sit y with height. Relative minimums in thickness values, called cold pools, are found in such cyclones. The temperature distribution is almost symmetrical, and the axis of the low is nearly ver-tical. When they do slope vertically, they slope toward the coldest temperatures aloft. In the cold low, the lowest temperatures coincide with the lowest pressures.

The cold low has a more intense circulation aloft from 850 to 400 millibars than at the sur-face. Some cold lows show only a slight evidence in the surface pressure field that an intense cir-culation exists aloft. The cyclonic circulation aloft is usually reflected on the surface in an abnormally low daily mean temperature and in precipitation and unstable hydrometeors. 

At high latitudes the cold pools and their associated upper air lows show some tendency for location in the northern Pacific and Atlantic Oceans where, statistically, they contribute to the formation of the Aleutian and Islandic lows.

A warm core low (fig. 3-2-8) decreases in in-tensity with height and the temperature increases toward the center on a horizontal plane. The warm low is frequently stationary, such as the heat low over the southwestern United States in the summer; this is a result of strong heating in a region usually insulated from intrusions of cold air that tend to fill it or cause it to move. The warm low is also found in its moving form as a stable wave moving along a frontal surface. There is no warm low aloft in the troposphere. The tropical cyclone, however, is believed to be a warm low because its intensity diminishes with height. Because most warm lows are shallow, they have little slope. However, intense warm lows like the heat low over the southwest United States and hurricanes do slope toward warm air aloft. In general, the temperature field is quite asym-metrical around a warm core cyclone. Usually the southward moving air in the rear of the depres-sion is not as warm as that moving northward in advance of it.

A dynamic low is a combination of a warm surface low and a cold upper low or trough, or

a warm surface low in combination with a dynamic mechanism aloft for producing a cold upper low or trough. It has an axis that slopes toward the coldest tropospheric air. (See fig. 3-2-9.) In the final stage, after occlusion of the surface warm low is complete, the dynamic low becomes a cold low with the axis of the low becoming practically vertical.

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