Cyclone Biparjoy made landfall in Gujarat State, close to the border with Pakistan, on the evening of Thursday 15 June 2023, bringing with it sustained wind speeds of 85 km per hour (a sustained wind speed is a wind speed maintained for a minute or longer) and gusts of up to 105 km per hour. Two people are known to have died following the event, described as a father and son who entered a flooded gully in an attempt to rescue some Goats, with 22 people reported injured in India and Pakistan. A much larger number of casualties was avoided by the evacuation of about 175 000 people from coastal areas in both countries, something made possible by satellite-tracking of tropical storms, which formerly killed thousands of people in South Asia every year.
Tropical storms, called Cyclones in the Indian Ocean and South Pacific, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.
Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.
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