Such disturbances fall into the following types which have been briefly described below: tropical depressions, thermal lows, lee depressions, and polar air depressions.
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Tropical depressions:
On occasions tropical disturbances in the last stage of their movement enter into the west wind belt. As they move onward they get involved into the extra tropical cyclones (or wave cyclones) found in their vicinity and acquire fronts and contrasting air masses.
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However, some of the tropical depressions soon after entering into the west wind belt disintegrate. On the contrary, a few of them may also redevelop into strong extra tropical storms.
Thermal lows:
These depressions originate exclusively in summer and are the result of intense daytime heating of the continental areas. These weather disturbances are also called heat lows.
In summer, the intense heating of continents, particularly the desert areas, produces shallow depressions which hardly move out of their source regions. The weather changes produced by these storms are only short-lived.
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Strong convective activity sometimes produces cumulonimbus clouds which give thundershowers. Areas, where there is no polar front in summer in the temperate zone, may experience thermal lows.
The summertime thermal lows in the vicinity of Persian Gulf are typical examples of this type of weather disturbance. The weather produced by these storms does not exhibit the characteristics of a true extratropical frontal cyclone.
Lee depressions:
Such weather disturbances are produced on the lee side of the mountain barriers. When the westerlies are blocked by the north-south mountain barrier, the air flow undergoes contraction over the ridge and expansion on the lee side.
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Thus, there is a convergence and cyclonic curvature in the lee of the barrier. Such depressions are common in winter to the south of the Alps and the Atlas mountains. Other regions where such depressions originate in winter are Colorado and Texas situated to the east of the Rockies.
When these depressions move out, they attract polar air masses and acquire fronts. Thereafter they develop all the characteristics of an extra tropical depression. Dark nimbus clouds, showery rainfall or snowfall, and high velocity winds are some of the characteristic features of the weather associated with them.
Polar-air depression:
These storms are wintertime phenomena produced by unstable maritime polar air currents moving southward on the eastern side of a ridge of high pressure generally in the rear of an occluding depression. Polar lows originating over the North Atlantic affect the weather of northwestern Europe in winter. Sometimes they also acquire fronts.
From the foregoing discussion it is clear that not all depressions of the temperate zone originate on the polar fronts, nor do they have all the characteristics of a typical wave cyclone. However, it is true that all the weather disturbances described above have low pressure at their centers, and all produce unsettled weather.