The cold low (or cold pool) is usually most evident in the circulation and temperature fields of the middle troposphere. Characteristically, it displays symmetrical isotherms about the low centre. Surface charts may show little or no sign of these persistent systems, which are frequent over northeastern North America and northeastern Siberia. They probably form as the result of strong vertical motion and adiabatic cooling in occluding baroclinic lows along the Arctic coastal margins. Such lows are especially important during the Arctic winter in that they bring large amounts of medium and high cloud, which offsets radiational cooling of the surface. Otherwise, they usually cause no 'weather' in the Arctic during this season. It is important to emphasize that tropospheric cold lows may be linked with either low- or high-pressure cells at the surface.
In middle latitudes, cold lows may form during periods of low-index circulation pattern (see Figure 7.23) by the cutting off of polar air from the main body of cold air to the north (these are sometimes referred to as cut-off lows). This gives rise to weather of polar airmass type, although rather weak fronts may also be present. Such lows are commonly slow moving and give persistent unsettled weather with thunder in summer. Heavy precipitation over Colorado in spring and autumn is often associated with cold lows.
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