Are tropical cyclones always hazardous?

by K.R. Abhayasingha

A low pressure area developed in the Bay of Bengal, east of Sri Lanka land mass, on May 8 was developed into a tropical cyclone on May 12. Cyclones developed in tropical oceans are called 'Tropical Cyclones' (TC).

The TC developed in the Bay of Bengal on May 12 was named 'Mahasen' but it was withdrawn later due to some reasons. As any other cyclone developing over the Indian Ocean, this TC too affected the weather over Sri Lanka resulting in a series of natural disasters such as flood, landslides, tornadoes and lightning. Even though some of us forget the influences of natural events causing a significant amount of natural hazards during all the climate seasons, the disasters occurred in last May were normal climatic behaviour under the influence of a close by TC.

Aftermath of a cyclone

But if all of us were keen to take precautions in time to manage the natural disasters, the situation could have been averted.

Tropical cyclones, TC, form over all the three oceans, Indian, Pacific and Atlantic every year, but the number may fluctuate seasonally or yearly. More than 50 TCs develop all over the tropical oceans in a year and five to six TCs develop in the Indian ocean, on average.

There were times in the history where the accurate atmospheric observations were not possible and as a result, no one knew if a low pressure area, depression or a cyclone was active over the ocean and therefore those who tried to cross over water bodies occasionally met with fatal disasters. As the facts were not known for such disastrous events, the gods or ghosts were believed to have created such hazardous environments. Not only of TCs, other natural events such as lightning, tornadoes, water spouts and tsunami were also believed to have been created suddenly by unseen, super natural powers. You can read the literature in websites about gods responsible for lightning, rain, storms or many events that are well understood now.

Tropical cyclones do have some major benefits. Although the storms cause damage, injury and death at times, they are a necessary evil because the storms balance the earth's atmosphere by taking huge amounts of heat out of the tropical oceans and transporting it to the high latitudes and polar regions of the globe. They also provide beneficial rains to many areas that would otherwise be too dry. Hurricanes and tropical storms have been known to break droughts which have saved farmers and water supplies in many communities. Most of the time, we know when they are coming, so we can get out of harm's way.

As far as property damage is concerned, we should build stronger structures to withstand the storms in cyclone prone areas.

Before understanding the facts, advantages and disadvantages of a TC, we need to know a little about our earth and its atmosphere and the movement of the earth and our main energy source, the sun.

The sun, being a hot body with average surface temperature of about 6000 degrees of Kelvin, is supposed to be fixed at a focus of an ellipse along which our earth completes one journey round during 265.242 days.

The earth vibrates once round around its own axis in 23 hours 56 minutes and 4.1 seconds.

During all the movements, the axis of the earth makes an angle 23.5 degrees with the normal (perpendicular direction) to the plane of motion of the earth.

Due to the astronomical characteristics, the environmental events show significant fluctuations or variations every moment. Such variations are clearly observed in seasonal frames.

Sub Solar Point

During the 12 months of the year, the sun rises overhead only over the belt on the earth between latitudes 23.5 north and 23.5 south. The sun's overhead position is named Sub Solar Point (SSP). The date with the overhead sun on 23.5 north latitude is June 21 or 22 while that with the overhead sun on 23.5 south is December 22 or 23. The sun rises overhead to the equator on March 21/22 and September 22/23 during the sun's northward and southward transitions. As a result, the majority of the solar energy incident on the earth is absorbed by the belt between latitudes 23.5 north and 23.5 south. It is obvious that the maximum energy absorption occurs in areas near the equator.

Generally, the tropical belt or zone around the globe receives the max portion of the solar radiation.

The areas beyond 23.5 north and 23.5 south receive a very small percentage of solar radiation or some extreme parts do not receive any solar radiation directly.

As the great scientists Stefan and Boltzmann showed that any surface radiates heat if its temperature is above zero degrees Kelvin at a rate which is proportional to the power of its temperature.

Accordingly, any location on the earth surface radiates energy outward but the outward radiation is insignificant even over tropical areas when compared with the incoming solar radiation. On the other hand, the areas in high latitudes radiate more energy than they receive from the sun. The average surface temperature of the earth and the sun are 288 and 6,000 degrees of Kelvin.

Solar radiation

It is easy to understand what will be the ultimate environmental situation if the tropical areas receive the enormous amount of solar radiation while distant parts lose energy through continuous radiation. Tropical areas get warmed and some parts in high latitudes cool intermittently and therefore the whole globe will become a planet with no living organism in a few days or months time.

But it has never happened and also it will never happen because there are a number of natural mechanisms to distribute the energy over the globe in a useful manner and to maintain the heat budget in all locations of the world.

You will be wonder amazed to hear that tropical cyclones are the main agents over the globe that take the responsibility of fetching the extra energy from hotter to cooler regions of the world.

As the tropical oceans receive solar radiation, water gets warmed. The important parameter used by scientists to know whether an ocean is warm or cold is the Sea Surface Temperature (SST).

When the solar energy heats up a sea surface and the SST exceeds 26.7.degrees Celsius, the local atmospheric pressure becomes very low compared to the surrounding, and the atmospheric air starts to rotate around the location with low pressure.

It turns out that the vast volume of the heat released in the condensation process is used to cause rising motions in the thunderstorms and only a small portion drives the storm's horizontal winds.

An average TC, called a hurricane in some areas, is supposed to release energy of about 5.2 x 1019 Joules/day or 6.0 x 1014 Watts which is equivalent to 200 times the worldwide electrical generating capacity - an incredible amount of energy.

The total kinetic energy (wind energy) generated has been estimated to be about 1.3 x 1017 Joules/day or 1.5 x 1012 Watts which is equivalent to about half the worldwide electrical generating capacity - also an amazing amount of energy.

The above figures may not be 100 percent accurate, but they can be quoted as approximates when as far as TCs in Bay of Bengal are concerned. In comparison, a TC is supposed to be a heat engine that has energy of more than 350 nuclear bombs.

What we have to understand is that a single TC transports an amount of energy equivalent to that of more than 350 nuclear bombs from tropical areas to the sub tropics, sub polar and polar areas of the world through a number of natural mechanisms.

More than 50 tropical cyclones develop over the globe in a year and one can guess how much energy is taken from tropics and transported to distant cold regions.

Also it is up to all of us to imagine what will be the situation if this energy transfer does not take place in the world.

At the same time one can evaluate the service rendered by a tropical cyclone to the world.

Any query about the facts in the article can be emailed to [email protected].

The writer is a former Director of Meteorology.