Tropical Cyclones

Tropical cyclones are violent whirling storms, several hundred of kilometers in diameter, that develop over tropical waters. Viewed from satellites high above the earth, a tropical cyclone appears as a powerful, tightly coiled system with spiral cloud bands emanating outwards. In a mature tropical cyclone, there exists a central region of relatively calm air and little cloudiness known as the eye.

Tropical cyclones are classified as follows:

  • Tropical Depression – An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 63 kmph or less.
  • Tropical Storm – An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 64 – 117 kmph.
  • Typhoon – An intense tropical weather system with a well defined circulation and maximum sustained winds of 118 kmph or higher.


How are tropical cyclones formed?

Tropical cyclones are formed over oceanic regions where the sea surface temperature is 26o C or higher. In the Asian region, tropical cyclones, usually referred to as typhoons, are common during the Northern Hemisphere summer when temperatures over the Pacific Ocean are warm and favourable for convective activity. In the warm tropical ocean, low pressure systems are formed, move westwards and develop from tropical depressions into storms and finally evolve into mature typhoons.

The development from tropical depressions to typhoons depends on three conditions:

  • Warm waters
  • Moisture
  • Wind pattern near the ocean surface that spirals air inwards (cyclonic flows).

Developing cyclones gather heat and energy through contact with warm ocean waters. Heat and moisture by evaporation from the sea surface powers them like giant heat engines. Bands of thunderstorms form allowing the air to warm further and rise higher into the atmosphere.

The eye, whose diameter ranges from 30 to 100 kilometres, coincides with the low pressure centre of the storm. The most violent activity takes place in the area immediately around the eye, called the eye wall. Here, air spirals upwards and outwards at increasing speeds. Some of the air moves inwards and sinks into the eye, creating a cloud-free area.


When typhoons are most frequent?

Typhoons begin to appear in the Western North Pacific from early May, and in increasing numbers until attaining a peak frequency in September. The majority of typhoons develop in the area bounded by latitudes 5oN and 20oN and longitude 130oE and 170oE. The normal passage of typhoons is westwards across the Philippines, recurring northeastwards as they approach the Asiatic land mass.


Typhoon Related Calamities

Typhoons are undoubtedly one of the mightiest and most devastating forces of nature. They travel great distances and last long enough in the atmosphere to wreck a path of fear and destruction in their wake.

Some of the dangers associated with typhoons are:

  • Storm surge – Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a typhoon makes landfall. The stronger the typhoon and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property.
  • Heavy rain – Widespread torrential rains can produce deadly and destructive floods.
  • High winds – Typhoon force winds can destroy poorly constructed buildings and other structures. Debris becomes flying missiles in typhoons. Strong gusts can down trees and power lines causing massive disruption.


Weather Advisories and Warnings

The Malaysia Meteorological Department maintains a constant vigilance on the weather throughout the year. During the “typhoon season”, monitoring efforts are intensified as the position and behaviour of all typhoons in the western North Pacific region are meticulously tracked. This information forms an integral part of the flight weather information provided to the aviation and shipping sectors.

The different categories of weather statements, outlooks, advisories and warnings issued to the mass media are:

  • Heavy Rainfall Advisories – issued when there is a possibility of heavy rainfall occurrence within 24 to 48 hours.
  • Heavy Rainfall Warning – issued when latest information received indicate that heavy rainfall is expected.
  • Strong Wind and Rough Sea Advisories – issued when there is a possibility of strong wind and rough seas within Malaysian waters within 24 to 48 hours.
  • Strong Wind and Rough Seas Warnings – issued when latest information received indicate that strong wind and rough seas are expected


Important Points to Note

  • Listen to the radio and television for regular broadcasts of weather bulletins and other announcements.
  • Take appropriate measures to avoid possible damage or loss of life due to flooding if a tropical storm develops or moves into your vicinity.
  • When driving in highland areas under torrential rain, watch out for landslips.
  • Avoid going out to sea when a tropical storm or typhoon approaches.
  • If a tropical storm is expected to hit your area, secure all loose objects that could be blown away by strong winds.
  • Prepare to evacuate if you live in coastal areas or near river banks.