Polar air depressions are a loosely defined class of mesoscale to subsynoptic-scale systems (a few hundred kilometres across) with a lifetime of one to two days. On satellite imagery, they appear as a cloud spiral with one or several cloud bands, as a comma cloud (see Figure 9.17 and Plate 19), or as a swirl in cumulus cloud streets. They develop mainly in the winter months, when unstable mP or mA air currents stream equatorward along the eastern side of a north-south ridge of high pressure, commonly in the rear of an occluding primary depression (Figure 9.17). They usually form within a baroclinic zone (e.g. near sea-ice margins where there are strong sea-surface temperature gradients), and their development may be stimulated by an initial upper-level disturbance.
In the northern hemisphere, the comma cloud type (which is mainly a cold core disturbance of the middle troposphere) is more common over the North Pacific, while the spiral-form polar low occurs more often in the Norwegian Sea. The latter is a low-level warm core disturbance that may have a closed cyclonic circulation up to about 800 mb or may consist simply of one or more troughs embedded in the polar airflow. A key feature is the presence of an ascending, moist southwesterly flow relative to the low centre. This organization accentuates the general instability of the cold airstream to give considerable precipitation, often as snow. Heat input to the cold air from the sea continues by night and day, so in exposed coastal districts showers may occur at any time.
In the southern hemisphere, polar low mesocyclones appear to be most frequent in the transition seasons, as these are the times of strongest meridional temperature and pressure gradients. In addition, over the Southern Ocean the patterns of occurrence and movement are more zonally distributed than in the northern hemisphere.
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