Cyclones: From Birth to Death

Cyclones are one of nature’s responses when there is a meeting between hot and cold air currents. These are circular shaped rotating storms which have huge diameters ranging from 150 to 1000 kilometres. Due to this high speed rotation, an intense low pressure is formed at the centre; this is called the “Eye of the storm”. Cyclones of high intensity are caused mostly in tropical areas. Cyclones are known to cause great devastation to life as well as property. They are characterized by their high wind speeds ranging from 60 KMPH to 280 KMPH and more in super cyclones. These high speed winds go about ripping apart homes and blowing vehicles off the roads.


How do cyclones form: Genesis of a Cyclone

First of all, as all matter has weight, so does air. Due to this weight it exerts pressure. This pressure may be different at different temperatures. This is due to the different densities possessed by air at different temperatures. Cold air has higher density and hot air has lower density.

 Air flow pattern from North to South Pole

The air flow pattern as we go from the North to South Pole varies according to the figure given above. This occurs because of the rotation of the earth. So when there is a low pressure area in the sub-tropical region, hot air and cold air rush in to fill this low pressure. Just like when you burst a balloon. The pressure outside the balloon is lower than the pressure inside it therefore the air rushes outward to balance the pressure. Therefore the hot and cold collide at this low pressure at certain angles. Now, the cold air goes downward and the hot air rises since they have different densities.

 Formation of cyclone


These air flows since coming in at an angle tend to cause a rotatory motion arising from downward motion of cold air and upward motion of hot air. Since the direction of attack is opposite in the northern and southern hemispheres, the direction of rotation will be opposite. There is anti-clockwise rotation in the northern hemisphere and clockwise rotation in the southern hemisphere. In the centre of this rotating air mass is an intensely low pressure area known as the Eye of the storm. The low pressure present at the eye of the storm tends to get filled by the rotating air mass. This causes the lateral motion of the cyclone. Hence the “Eye” of the storm continuously moves forward. Therefore the cyclone consists of both rotatory and lateral motion. The physics of the motion the cyclone can be calculated using vector notations.


As the air mass rotates, even the surrounding air is pulled which makes the cyclone grow in diameter. Typical cyclones have diameters ranging from 100 to 1000 kilometres. The diameter of the eye of the storm also ranges from 25 to 100 kilometres. The vertical height of a cyclone may range from 12 to 14 kilometres at its origin. Due to these enormous dimensions cyclones can be observed from space as well. Therefore cyclones are predicted by weather satellites in space.


Nomenclature of Cyclones

Most people are confused while distinguishing between cyclones, hurricanes, typhoons and tornadoes since all these words are used in a similar context. But there is a specific terminology used for the cyclone depending on its origin. Tropical cyclones originating in the Atlantic basin and the Eastern Pacific are known as “Hurricanes” (Mostly in southern states of USA and its neighbouring Islands). The types over the Western Pacific are known as “Typhoons” (Generally in Australia and New Zeeland). The systems originating on the Indian Ocean are termed basically as “Cyclones” (consisting of India, Bangladesh and Pakistan). Tornadoes on the other hand are an inverted cone shaped rotating air masses that lift the air from the ground level. A cyclone of a large enough magnitude could cause several tornadoes.


Categories of Cyclones

Cyclones are categorized into five types depending on their intensity and the damage that are capable of causing. Cyclones are categorized into five divisions based on their wind speeds.

Category 1 includes cyclones having wind speeds of up to 125 kilometres per hour. Their damage is limited to small farms and trees.

Category 2 cyclones show a wind speed of 126-169 kilometres per hour and can cause a significant amount of damage accounting to minor house damages and major crop damages.

Category 3 cyclones extend to a speed from 170-224 kilometres per hour. These can cause high levels of damage to infrastructure and also power failures.

Category 4 cyclones go from 225-279 kilometres per hour. These tear apart structures violently and can send debris flying into the air and widespread power failure.

Category 5 cyclones are the most dangerous having wind speeds above 280 kilometres per hour. This destroys everything in its path and can overturn vehicles. Chances of surviving a category 5 cyclone are very dim.


Destruction Caused by Severe Cyclones
Destruction Caused by Cyclone

Damage caused to Darwin, Australia from Cyclone Tracy

Cyclones that cross the path of cities cause major destruction to property as well as loss of life. A typical cyclone is characterised by heavy rainfall and strong winds. The winds of a cyclone are extremely powerful reaching average speeds of 150 kilometres per hour. These winds tear apart weak structures and some vehicles off the road. In certain areas lying next to water bodies, the water is carried onto the land causing flooding of that area. The heavy rainfall resulting from the cyclones cause many sub categorical problems like flash floods, soil erosion, power grid failures, crop destruction, water pollution, oil spills and many more. Areas lying next to higher terrains are affected by severe landslides caused by the rainfall.


One of history’s most violent cyclone was the “Bhola Cyclone” that occurred in 1970 off the coast of Bangladesh. Although the cyclone was of lower amplitude, the surges (floods) from the Bay of Bengal caused flooding of the low lying areas wiping out a few villages from the map. The death toll was estimated up to 5 lakhs. Other such cyclones of such high magnitude are the Cyclone “Nina” in China, Hurricane “Ike” which hit the USA, Cuba and Bahamas Islands, Hurricane Andrew and hurricane Katrina which struck USA and its surrounding islands.


Death of a Cyclone

A cyclone gradually loses its air speed due to resistance in its path and dies out. It may last from days to a week. Cyclones that rotate in the direction opposite to the actual direction that it is supposed to spin are called “Anti-Cyclones”. When Cyclones and Anti-Cyclones collide, they cancel out their rotations. A single cyclone can travel up to 1000 to 2000 kilometres in its life time. Even after the cyclone goes out, the floods that remain in the land cause the spreading of diseases and contaminating the water bodies.


Cyclones in Other Planets

Cyclone on MarsCyclones also occur in other planets since they also have thin atmospheric gases that collide with each other. Scientists continuously study the cyclones caused in different planets to get and outline of the weather patterns that exist in those planets. It is also helpful to determine the wind currents and latitudinal distribution of the other planets.