A general term for a cyclone that originates over a tropical ocean.
These cyclones are classified as follows:
Tropical depression–winds from 35 to 34 knots;
Tropical storm–winds from 35 to 64 knots,;
Hurricane (Atlantic Ocean) or Typhoon (Pacific Ocean)–winds 65 knots or greater.
Structure of Tropical cyclones
- Warm core vortex of tropical origin with small diameter,
- Circular shape, minimum surface pressure accompanied by high velocity winds spiralling inwards from all directions.
- The eye of the cyclone is circular, 5-50 km in diameter, has low pressure and clear skies (subsidence of air the by troposphere).
- Around the eye is eye wall, where most intense winds are found.
- The wind bands rotate with storm in the form of spiral galaxy producing very heavy rainfall.
- The air which rose due to low pressure, diverges at upper troposphere, subsides around cyclone in annular zone.
- Outside cyclone also a convective cloud is formed.
Tropical cyclones are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
Tropical cyclone is the result of :
- Some trigger in presence of suitable deep warm waters (source of sensible and latent heat)
- Presence of pre-existing low-level disturbances (low pressure)
- Upper atmospheric disturbances (dying remains of westerlies)
- Presence of coriolis force
- Presence of minimal vertical wind shear, deepening trough of upper westerlies and divergence of rising air by it, and high humidity level.
Tropical cyclones are very disastrous and cause damage to buildings and mankind.