time the tropopause is reached. The subtropical highsare good examples of this type of high. Therefore,anticyclones found in tropical air are always warm core.Examples of warm core highs are the Azores orBermuda High and the Pacific High.VERTICAL STRUCTURE OF LOW-PRESSURESYSTEMSLow-pressure systems, like high-pressure systems,are generally a reflection of systems aloft. They, too,experience shifts in location and changes in intensityand shape with height. At times, a surface system maynot be evident aloft and a well-developed system aloftmay not reflect on a surface analysis.Cold Core LowsThe cold core low contains the coldest air at itscenter throughout the troposphere; that is, goingoutward in any direction at any level in the troposphere,warmer air is encountered. The cold core low (figure3-20)increasesintensitywithheight.Relativeminimums in thickness values, called cold pools, arefound in such cyclones. The temperature distribution isalmost symmetrical, and the axis of the low is nearlyvertical. When they do slope vertically, they slopetoward the coldest temperatures aloft. In the cold low,the lowest temperatures coincide with the lowestpressures.The cold low has a more intense circulation aloftfrom 850 to 400 millibars than at the surface. Somecold lows show only slight evidence in the surfacepressure field that an intense circulation exists aloft.The cyclonic circulation aloft is usually reflected on thesurface in an abnormally low daily mean temperatureoftenaccompaniedbyinstabilityandshoweryprecipitation. A cold core low is accompanied by a lowwarm tropopause. Since the pressure surfaces are closetogether, the tropopause is reached at low altitudeswhere the temperature is relatively warm. Goodexamples of cold core lows are the Aleutian andIcelandic lows. Occluded cyclones are generally coldcore in their later stages, because polar or arctic air hasclosed in on them.At high latitudes the cold pools and their associatedupper air lows show some tendency for location in thenorthernPacificandAtlanticOceanswhere,statistically, they contribute to the formation of theAleutian and Icelandic lows.Warm Core LowsA warm core low (figure 3-21) decreases intensitywith height and the temperature increases toward thecenter on a horizontal plane. The warm low isfrequently stationary, such as the heat low over thesouthwestern United States in the summer; this is aresult of strong heating in a region usually insulatedfrom intrusions of cold air that tend to fill it or cause itto move. The warm low is also found in its moving formas a stable wave moving along a frontal surface. Thereis no warm low aloft in the troposphere. The tropicalcyclone, however, is believed to be a warm low becauseits intensity diminishes with height. Because mostwarm lows are shallow, they have little slope. However,intense warm lows like the heat low over the southwestUnited States and hurricanes do slope toward warm airaloft.Ingeneral,thetemperaturefieldisquiteasymmetrical around a warm core cyclone. Usually thesouthward moving air in the rear of the depression is3-19HH800MB800MB600MB 700MB900MB 1000MB1000MB 900MB700MB 600MBAG5f0319Figure 3-19.—Warm core high.LL600MB600MB700MB700MB800MB800MB900MB900MB1000MB1000MBAG5f0320Figure 3-20.—Cold core low.HL600MB600MB700MB700MB800MB800MB900MB900MB1000MB1000MBAG5f0321Figure 3-21.—Warm core low.
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