Tropical Weather Systems

DEFINITION

A hurricane is a tropical cyclone (see below) in which the maximum average wind speed near a centre or eye exceeds 74 mph or 119 Km/h. The winds rotate in a counter-clockwise spiral around a region of low pressure. In the Atlantic and Eastern Pacific such systems are they are called HURRICANES. In the Western Pacific, including the Philippines – TYPHOONS, near Australia – WILLY WILLY and in the Indian Ocean – CYCLONES. The tropical cyclones are given names for easier identification and tracking.

FORMATION AND INITIAL DEVELOPMENT STAGES OF TROPICAL CYCLONES

Tropical cyclone is the generic term used by the World Meteorological Organisation to define weather systems developing over tropical or subtropical waters in which winds exceed 34 knots or 63 km/h. Tropical cyclones also have a definite organised surface circulation.

Four conditions are necessary for the development of a tropical cyclone:

1. A warm sea temperature in excess of 26 degrees Celsius or 79 degrees Fahrenheit.
2. High relative humidity (degree to which air is saturated by vapour).
3. Atmospheric instability.
4. A location of at least 4 – 5 latitude degrees from the Equator.

PHASES

There are four phases of development which are linked to to the associated wind intensities:

* Tropical Disturbance
A weather system which gives rise to a specific area of cloudiness with embedded showers and thunderstorms.
* Tropical Depression
A tropical cyclone system with definite counter-clockwise wind circulation with maximum sustained winds of less than 38 mph (61 km/h). At this stage the system is given a number.
* Tropical Storm
A tropical cyclone system, with maximum sustained surface winds greater than 38 mph (61 km/h), but less than 74 mph (119 km/h). At this stage the system is given a name.
* Hurricane
A tropical cyclone with wind speed greater than 74 mph (119 km/h). The weather system is now better organised and the eye is well defined.

CLASSIFICATION OF HURRICANES

The Saffir Simpson Scale is used to classify Hurricanes according to intensity. In this scale, hurricanes are classified into one of five categories as shown below.

CHARACTERISTICS

Tropical cyclones are characterised by their destructive winds, storm surges and exceptional levels of rainfall which may cause flooding.

Destructive Winds

The strong winds generated by Tropical cyclones circulate clockwise in the Southern Hemisphere and counter clockwise in the Northern Hemisphere while spiraling inward and moving towards the cyclone centre. Winds speeds progressively increase towards the core. As the eye arrives, winds fall off to become almost calm but rise again just as quickly as the eye passes and are replaced by hurricane force winds from a direction nearly the reverse of those previously blowing.

Storm Surges

The rise in sea level above the normal tide. (See Table for Saffir-Simpson Scale).  As the cyclone approaches the coast, the friction of strong on-shore winds on the sea surface, plus the “suction effect” of reduced atmospheric pressure, can pile up sea water along a coastline near a cyclone’s landfall well above the predicted tide level.

Excessive Rainfall

The high humidity condenses into rain and giant cumulus clouds result in high precipitation rates when the cyclone reaches land. The rain rapidly saturates the ground and rapid run off will result in severe flooding.

LIKELY IMPACT

Physical Damage

Structures will be damaged or destroyed by wind force, storm surges, landslides and flooding.  Public utilities such as overhead power lines, water and gas distribution lines, bridges, culverts and drainage systems are also subject to severe damage.  Fallen trees, wind driven rain and flying debris can also cause considerable damage.

Crops and Food Supplies

The combination of high winds and heavy rain and flooding can ruin crops and trees. Food stocks may be lost or contaminated and it is possible that food shortages will occur.

Casualties and Public Health

There are relatively few deaths associated with the impact of high winds. However, storm surges may cause many deaths but usually few injuries among survivors. The threat to public health emerges in the aftermath of events when conditions such as water contamination or shortages, flooding and damage to sanitation facilities  may favour the spread of diseases.

Lack of Communication

Communication may be severely disrupted as telephone lines, radio and television antennas and satellite disks are blown down. Roads and railway lines may be blocked by fallen trees or debris and aircraft movements are curtailed for hours after a cyclone.