34
when moist air is carried upwards and its water vapour condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except
close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
A typical tropical cyclone is characterized by the presence of rainbands, eye and
eye walls (Fig. 2.1).
Rainbands
Rainbands are bands of showers and thunderstorms that spiral cyclonically toward
the storm center. High wind gusts and heavy downpours often occur in individual
rainbands, with relatively calm weather between bands. Tornadoes often form in the
rainbands of landfalling tropical cyclones. Intense annular tropical cyclones are distinctive for their lack of rainbands; instead, they possess a thick circular area of
disturbed weather around their low pressure centre. While all surface low pressure
areas require divergence aloft to continue deepening, the divergence over tropical
Table 2.1 Details of various types of cyclones that affected the east coast of India during the
period 1891–1991
Types of
disturbances
Cyclonic
disturbances
Depressions/Deep
depressions
Cyclonic
storms
Severe cyclonic
storms
Number
1009
592
268
149
Maximum
158
131
51
38
Minimum
4
1
0
1
Yearly average
10
6
3
1.5
Percent of total
–
58
27
15
Wind speed (km/
hr)
31–>118
31–61
61–68
88–>118
Source: Shrestha (1998)
Fig. 2.1 Structure of
tropical cyclones
2 Mangroves: A Shield Against Storms and Wave Actions
when moist air is carried upwards and its water vapour condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except
close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
A typical tropical cyclone is characterized by the presence of rainbands, eye and
eye walls (Fig. 2.1).
Rainbands
Rainbands are bands of showers and thunderstorms that spiral cyclonically toward
the storm center. High wind gusts and heavy downpours often occur in individual
rainbands, with relatively calm weather between bands. Tornadoes often form in the
rainbands of landfalling tropical cyclones. Intense annular tropical cyclones are distinctive for their lack of rainbands; instead, they possess a thick circular area of
disturbed weather around their low pressure centre. While all surface low pressure
areas require divergence aloft to continue deepening, the divergence over tropical
Table 2.1 Details of various types of cyclones that affected the east coast of India during the
period 1891–1991
Types of
disturbances
Cyclonic
disturbances
Depressions/Deep
depressions
Cyclonic
storms
Severe cyclonic
storms
Number
1009
592
268
149
Maximum
158
131
51
38
Minimum
4
1
0
1
Yearly average
10
6
3
1.5
Percent of total
–
58
27
15
Wind speed (km/
hr)
31–>118
31–61
61–68
88–>118
Source: Shrestha (1998)
Fig. 2.1 Structure of
tropical cyclones
2 Mangroves: A Shield Against Storms and Wave Actions