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Cyclones and Thunderstorms

Cyclones and thunderstorms are two prominent weather phenomena that are commonly observed on the Earth. They are responsible for billions of dollars in damages every year around the globe.

Cyclones could be called air masses. These air masses are formed in the lower reaches of our atmosphere and consist of a centre and the encircling high-velocity winds. The centre of the cyclone is also called the eye of the cyclone. The centre has a very low atmospheric pressure, and all the inwardly spiralling winds encircle this low-pressure zone. Cyclones rotate in the counterclockwise direction in the northern hemisphere and in the clockwise direction in the southern hemisphere (as seen from above). Thus, a cyclone is a low-pressure system. Some larger low-pressure systems are polar vortices and extratropical cyclones. These are also called synoptic systems.

Thunderstorms are essentially precipitation accompanied by lightning and thunder. Thunder and lightning are components of the same phenomena. The only difference is that we see lightning, and we hear thunder. Weaker thunderstorms are termed ‘thundershowers’.

Thunderstorms occur in clouds known as cumulonimbus clouds. These are large clouds that appear higher than most clouds. Thunderstorms are usually also accompanied by strong winds and heavy rains. Sometimes snow, sleet or hail is also seen. Alternatively, some thunderstorms do not have any precipitations at all.

Cyclones

Some characteristics common to all the cyclones are centres with the lowest pressure in the area. The centre of the cyclone is also called the eye of the cyclone. These terms were coined by Henry Piddington around 1836. Piddington made his observations in Calcutta; today’s Kolkata. Cyclones are very frequent in the Bengal Bay area and, as you may know, occur every year.

For a cyclone to survive, its centre must be in perfect equilibrium. This is achieved by cancelling the pressure gradient force with the forces generated by the Coriolis effect. The pressure gradient force is the force generated by the pressure difference between the centre of the cyclone and the outer edge of the cyclone. The Coriolis effect is responsible for the counterclockwise and clockwise direction of the cyclone in the northern hemisphere and the southern hemisphere, respectively. This is the opposite for high-pressure systems. As a result, high-pressure systems rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

The process of formation of cyclones is termed cyclone genesis. In fact, the term ‘cyclone genesis’ can be used to describe any phenomenon that leads to the formation of cyclones of any kind. Cyclones can occur at various levels. They also occur at a microscopic level and at a macroscopic level. These larger phenomena are called synoptic phenomena.

Convection activities structurally cause cyclones. When these activities occur at faster rates, then these can lead to cyclone formation. The core of such cyclones is at a higher temperature than the atmosphere around it, which causes the wind to circle around this core and become cyclonic. When these warm cores are formed over the land, they are called mesocyclones. They can also lead to the formation of waterspouts.

Thunderstorms

Thunderstorms are formed by warm wind rising upwards very fast. The warm wind carries a lot of water vapour in it and is observed to move along fronts.

A front is a boundary between two winds that have different characteristics, like density, wind speed, temperature etc. These winds separate in the atmosphere due to these differences. Fronts are responsible for the formation of several weather-related phenomena.

Also, for thunderstorm formation, the warm wind must rise upwards at a very high speed. This can be achieved in various ways like fronts, shortwave troughs etc. When this warm, moist air moves upwards suddenly, it cools and condenses to form a large cumulonimbus cloud. Cumulonimbus clouds can reach as high as 20 km in the atmosphere. The water in these clouds approach the dew temperature and forms water vapours and ice. This leads to a sudden reduction in the pressure in the clouds. As a result, the water vapours and ice starts falling towards the surface of the Earth from a very high point in the atmosphere. Along the way, they also collide with other particles and become bigger. This causes a downdraft of the cold wind that was trapped up in the atmosphere. This cold wind gets pulled down with all the water vapours and ice that is falling down. Upon reaching the surface, this cold wind spreads across the surface and is responsible for the thunder and lightning that we see in a thunderstorm.

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