Tropical Revolving Storms
...slow moving systems of extremem low pressure - usually 200-700km in diameter, characterised by relatively uniform temeratures, pressures and humidity
In the northern hemisphere, by late summer, the ITCZ has moved north, heating wide expanses of ocean to great depth - leading to the warming of air above. The convergence of air at low levels and uplift creates very low pressure and strong winds - eventually leading to a storm.
- Once generated, they tend to move west adn are most destructive in: The caribbean sea/ Gulf of Mexico, East Pacific, The Arabian Sea, Bay of Bengal, Southeast Asia, Madagascar, Northwest Australia and South Western Pacific.
- Measured by the Saffir-Simpson Scale - 5 levels based on central pressure, wind speed, storm surge and damage potential
- Every year 70-75 tropical storms develop around the world and around 50 intensify to become tropical revolving storms
Characteristics of Formations
- Oceans heated to 27dc at a depth of 70m to ensure sustained heating over a wide area - heat source to create a large mass of warm, unstable air.
- Autumn - sea temperatures are highest having built up over summer
- 5-20dN/S of the Equator
- Travel unpredictably to the west
- On land they travel polewards - transferring surplus energy away from the tropics
- Away from the ocean heat source, they lose power and downgrade to storms and then depressions -> The storm exists while there is a supply of latent heat and moisture to provide energy adn low frictional drag on the ocean surface
1) Rising air currents must be sustained bya constant supply of heat and moisture
2) As winds sweep over the ocean, they increase evaporation rates and the latent heat needed to transform liquid to vapour is transferred to the rising air - the moist air will condense to form clouds and heavy rainfall, releasing latent heat, driving the storm
3) Once the system reaches maturity, a central eye develops - A 10-50km area in diameter with calm conditions, clear skies, high temperatures and descending air which increases instability by warming. This intensifies the storm - winds of 300km/h have been recorded by the eye.
4) Winds cause storm surges - broad waves of water pushed ahead of the storm, exacerabted by the rise in sea level caused by the intense low pressure of the storm.
5) Intense rain leads to run-off on land to swollen rivers which often have their outlet impeded by the inundation of seawater by storm surges into eustaries and low-lying land.
6) On land, they rapidly decline in terms of energy as the storm loses its source of heat and moisture over land - increased friction also slows the storm
7) If it moves away from the tropics ocer the sea, the increasingly cooler water restricts the amount of energy avaliable, ultimately reducing the pressure differences