Definition of tropical cyclones

Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about the large scale of destruction caused by violent winds, very heavy rainfall and storm surges. This is known as one of the most devastating natural disasters in which the wind speeds exceed 120km per hour.

Cyclones are the centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that air blows inward in anticlockwise on the northern hemisphere and clockwise in the southern hemisphere. They vary in shape from circular, elliptical to V shape. Cyclones are most common in areas where the low-pressure condition is well pronounced and this always encourages violent wind storm. Anticyclones on the hand areareas of high pressure surrounded by low pressure or areas of sinking air, they are referred to as high.

Cyclones, hurricane and typhoon are the same weather phenomenon, the only difference is the geographical location where they occur. For instance, in the Indian Ocean, they are known as Cyclones, Hurricanes in the Atlantic, Typhoons in the Western Pacific and Southern China Sea, and Willy-willies in Western Australia. However, cyclones that occur in the regions lying between the tropics of Capricorn and Cancer are known as Tropical Cyclones, these are not regular and uniform like extratropical or temperate cyclones. Cyclones vary in shape, speed, size and weather conditions. The weather conditions of low latitudes mainly rainfall regions are mainly caused by Tropical Cyclones.

Tropical cyclones are confined to a particular period of the year usually during summer season), they vary considerably in size, their diameters range between 80km and 300 km averagely. Weak cyclones move at the speed of about 32 km per hour while hurricanes attain the velocity of 120 km per hour or more. Tropical cyclones become more turbulent over the oceans but tend to weaken and feeble while blowing over the land areas. This is the reason why these cyclones affect only the coastal areas, for example, the West Bengal coasts of India, Tamil Nadu and Orissa. The middle of the cyclone is associated with extremely low pressure. Tropical cyclones are not characterized by temperature variations in their different parts because they do not have different fronts. There are no different rainfall cells hence each part of the cyclones yields rainfall. Tropical cyclones are not always mobile. They usually move from east to west under the influence of trade winds.

Tropical cyclones are distributed over six major regions which include:

1. West Indies, Gulf of Mexico and the Caribbean Sea
2. Western North Pacific Ocean including Philippines, Islands, China Sea, and Japanese Islands.
3. Western South Pacific Ocean, in the region of Samoa and Fiji Island and the east and north coasts of Australia.
4. Eastern Pacific coastal region of Mexico and Central America
5. Arabian Sea and Bay of Bengal
6. South Indian Ocean of Madagascar (Malagasy),

Tropical cyclone Formation/ Causes

All cyclones take place in the same manner and revolve around that low-pressure eye. When hot air rises, the water temperature of about 80⁰F is needed to sustain the rising air in the development of a tropical depression.

The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With a continuous supply of moisture from the sea, the storm is further energized. On reaching the land the moisture supply is cut off and the storm vanishes. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone. The cyclones, which cross 20⁰ N latitude generally, recurve and they are more destructive.

This is the eye of the storm and the spiralling rising air it produces creates a huge column of cumulonimbus clouds. When the air inside the cyclone reaches its highest level, it flows outwards from the eye, producing a broad canopy of cirrus cloud. The air cools and falls back to sea level, where it is sucked back into the centre of the storm. The Coriolis force causes the air that is sucked into the bottom of the hurricane to spin into the storm in a clockwise direction, while the air escaping at the top spins out in an anticlockwise direction (note that this is the opposite in the southern hemisphere).

The conditions favourable for the formation and intensification of tropical cyclones are:

• Where there is a continuous supply of abundant warm and moist air. Tropical cyclones originate over warm oceans with a surface temperature higher than 27C (80⁰F)
• The presence of Coriolis force is required for the origin of these cyclones.
• Cyclones are associated with inter-tropical convergence (lTC), which extends from 50-300N latitudes during the summer season
• Where there is anti-cyclonic circulation at the height of 9000 to 15000m above the surface disturbance
• Where there are small variations in the vertical wind speed
• A pre-existing weak low- pressure area or low-level-cyclonic circulation

The eye

A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air with no cloud, here the air tends to sink rather than rising. The eye is normally circular in shape

The eyewall

Around the eye is the eyewall, where there is a strong spiralling ascent of air to greater height reaching the tropopause. This is the most dangerous and destructive part of a tropical cyclone. The wind speeds are mostly found in this region, reaching as high as 250 km per hour. Torrential rain occurs here. The clouds also are at their highest altitude, and precipitation is very heavy as well. The heaviest wind damage occurs where a tropical cyclone’s eyewall passes over land.

Rainband

There are usually secondary cells arranged in bands around the center, these bands are called rainbands, spiral into the center of the storm. In some cases the rainbands are stationary relative to the center of the moving storm, and in other cases they seem to rotate around the center. This is where you find thunderstorms and rain showers that spiral inward toward the eye wall. Rainbands can extend out for many miles and increase as the heat engine feeds the storm. Hurricanes lose their energy when they move over land because of the lack of heat generation. Once on land, the storm system breaks down. Rainfall and winds can continue, but with decreased intensity.

From the eyewall rainbands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region. The diameter of the storm over the Bay of Bengal, Arabian sea and Indian ocean is between 600 – 1200 km. The system moves slowly about 300 – 500 km per day. The cyclone creates storm surges and they bombard the coastal low lands. The storm gradually becomes weaker on the land.

Types of Tropical Cyclones

1. Tropical disturbances or easterly waves- these are weak troughs of low pressure that develop from an unorganized cluster of thunderstorms.
2. Tropical depressions- these are tropical cyclones with maximum sustained winds of38 mph (33 knots) or less.
3. Tropical storms- these are tropical cyclones with maximum sustained winds of39 to 73 mph (34 to 63 knots).
4. Hurricanes or typhoons- they are tropical cyclone with maximum sustained winds of74 mph (64 knots) or higher.In the western North Pacific, hurricanes are called typhoons; similar storms in the Indian Ocean and South Pacific Ocean are called cyclones.

Effects of Tropical Cyclone

1. Tropical cyclones occurring during summer season from July-August causes heavy loss of crops, households, lives and property. These storms are seen frequently at the far end of this summer.
2. Tropical cyclones when occur can drastically alter the physical and cultural landscape if it reaches the land.
3. When a hurricane makes landfall (comes ashore), the storm surge causes extensive flooding hence, high winds and storm surges have caused serious flooding and damage to the human environment.
4. In 1970, the Bholo cyclone hit the coast of Bangladesh, resulting in the death of between three hundred thousand and one million people.
5. Tropical cyclones are very severe disastrous natural hazards which inflict heavy loss to human lives and property in places where they occur.
6. High velocity of winds have disrupted many communication system ,transport systems, water and power supply systems.
7. Torrential rains whichoccur in the towering cumulonimbus clouds inundate the low-lying areas, cause floods and landslides resulting in great loss of life and property damage.
8. Strom waves of great heights are great hazard to shipping. These are called storm surge, itmay reach up to 20 meters in height. If cyclone wave combines with the spring tide, the result is disastrous.
9. Even though these weather patterns can bring huge devastation to the landscape, they also redistribute moisture in the form of rain and help regulate global temperatures.
10. The winds also causesevere destruction of standing agricultural crops, domestic and wild animals and natural vegetation.