A tropical disturbance is any persistent area of
thunderstorms that forecasters can track for several days in
satellite pictures. When such a system develops a closed
cyclonic circulation (south wind on the east side, east wind on
the north, north wind on the west and, hardest to find, west
winds south of the center) it becomes a depression and is
numbered. When the maximum winds reach 35 kt, often with
pressures still above 1000 mb, it becomes a tropical storm and
receives a name. When it reaches 65 kt (74 mph) it is a
hurricane. Hurricanes are classified by intensity into five
categories:
Saffir-Simpson Scale
Category | Pressure (hPa) |
Wind (mph) | Damage |
ONE | above 980 |
74 to 85 | Minimal |
TWO | 979 to 965 | 86 to 110 |
Moderate |
THREE | 964 to 945 | 111 to 130 |
Extensive |
FOUR | 944 to 920 | 131 to 155 |
Extreme |
FIVE | below 920 | more than 155 |
Catastrophic |
Source:Simpson, R. H., 1974: The hurricane disaster--potential scale.Weatherwise, 27, 169. |