Tropics: Hurricanes, Cyclones, And Typhoons Essay, Research Paper
Hurricanes, cyclones, and typhoons are a major hazard in the Tropics. Every year, some 80 occurrences claim, on average, 20 000 lives and cause immense damage to property, natural vegetation, and shipping.
A hurricane is a large rotating storm around a centre of very low pressure with values of 950mb or lower. Most systems have a diameter of about 650km, less than half that of a mid-latitude depression. Wind speeds often exceed 33m per second (119km per hour) and, in the most intense hurricanes like Andrew, may reach 50m per second (1 8Okm per hour). The huge amount of heat required to create and maintain the hurricane is reflected in the height of the clouds near its centre, often up to ]2km above the Earth.
How do hurricanes form?
A number of trigger mechanisms, which rely on intensive rising convection currents, are required to transform a tropical disturbance into a more destructive hurricane. The development of a hurricane is dependent on combinations of particular.mechanisms.
§ An extensive ocean area with surface temperatures greater than 27 “C for a significant period of time.
§ Sufficient spin from the Earth’s rotation to trigger the vicious spiral in the centre of the hurricane, usually between latitudes 50 and 30cl north and south of the Equator.
§ A lack of strong horizontal air movement or wind shear, near a jet stream for example, which causes the break-up of the spiralling winds.
§ Where upper-air winds in the troposphere cause a rapid ascent of air to be sucked in at the sea surface to replace that lost above. This causes a rapid upward draught of moisture-laden air, which in turn produces huge volumes of condensation to form massive clouds.
As trade winds rush into the centre of the storm, they spiral inwards and upwards, releasing heat and moisture. Earth’s rotation twists the rising columns of air into a whirling cylinder around an eye of still, cloud-free descending air. The hurricane is fed by energy stored in water vapour brought upwards by air from evaporation at sea’-level. As this air rises, 90 per cent of the stored energy is released as heat as condensation occurs. This heat generates further uplift and instability, so long as the rising air is warmer than the surrounding air. The energy released within hurricanes is enormous.
Hurricane decay
As soon as the hurricane reaches land it loses its supply of heat and moisture and therefore its energy source. It can quickly become a mere tropical storm, especially when cold air is drawn in or when the upper atmospheric disturbance which caused the hurricane moves away.
The Saffir/Simpson Hurricane Scale
Category 1
Winds 11 8-152krn/hour (64-82 knots). Damage primarily to shrubbery, trees, poorly constructed signs, and unanchored mobile homes. No significant damage to other structures.
Storm surge 1-1.Sm above normal tide. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorages torn from moorings.
Category 2
Winds 154-176km/hour (83-95 knots). Considerable damage to shrubbery and tree foliage; some trees blown down. Extensive damage to poorly constructed signs. Major damage to exposed mobile homes. Some damage to roofing materials of buildings; some window and door damage. No major damage to buildings.
Storm surge 2-2.5m above normal tide. Coastal roads and low-lying escape routes made impassable by rising water 2-4 hours before arrival of hurricane centre. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying island areas required.
Category 3
Winds 178-209km/hour (96-113 knots). Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some window and door damage. Some structural damage to small buildings. Mobile homes destroyed.
Storm surge 2.5-3.5m above normal tide. Serious flooding at coast and many small structures near coast destroyed; large structures near coast damaged by battering waves and floating debris. Low-lying escape routes made impassable by rising water 3-5 hours before hurricane centre arrives. Flat terrain 1.Sm or less above sea-level flooded inland 13km or more. Evacuation of low-lying residences within several blocks of shoreline possibly required.
Category 4
Winds 211-25Okm/hour (114-135 knots). Shrubs and trees blown down; all signs blown down. Extensive damage to roofing materials, windows and doors. Complete failure of roofs on many smaller residences. Complete destruction of mobile homes.
Storm surge 4-5.5m above normal tide. Flat terrain 3m or less above sea-level flooded inland as far as 11 km. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes made impassable by rising waters within 3-5 hours before hurricane centre arrives. Major erosion of beaches. Massive evacuation of ail residences within 500m of shore possibly required, and of single-storey residences on low ground within 3km of shore.
Category 5
Winds greater than 25Okm/hour (135 knots), Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors with extensive shattering of glass components. Complete failure of roofs on many residences and industrial buildings. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes.
Storm surge greater than 5.Sm above normal tide. Major damage to lower floors of all structures less than 4.5m above sea-level within 500m of shore. Low-lying escape routes made impassable by rising water 3-5 hours before hurricane centre arrives. Massive evacuation of residential areas on low ground within 8-16km of shore possibly required.