'Cyclogenesis' means the formation of a low, which sometimes happens rapidly, e.g. a decrease of pressure by more than 24 hPa in a day. In this case, the resulting low is called a bomb which brings destructive winds and heavy rains. Some frontal lows and east-coast lows become bombs. They appear at an early stage as a triangle of cloud on satellite images (Section 12.5), which gradually deforms into an inverted comma shape (Figure 13.5).
Factors causing rapid cyclogenesis include the following:
1 upper-level divergence due to the coincidence of a Rossby wave trough and a jet streak (Note 12.L),
2 a low-level region of steep temperature gradient, i.e. a baroclinic zone (Note 12.H),
3 a warm, moist air mass ahead of this zone, and
4 an orographic low.
Some bombs attain the intensity of a tropical cyclone.
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