BY – Ananya Singh

TROPICAL CYCLONE

• Tropical cyclones are violent storms that originate over oceans in tropical areas and move to coastal areas bringing violent winds, very heavy rainfall & storm surge.
• They’re irregular wind movements involving closed circulation of air around a low-pressure centre which arises due to rapid upward movement of hot air which is subjected to Coriolis force.
• Direction: anti-clockwise in the northern hemisphere and clockwise in southern hemisphere (due to Coriolis force)

The conditions favourable for the tropical storms formation and intensification are:

• Large sea surface with temperature above 27° C;
• Presence of the Coriolis force;
• Small variations in the vertical wind speed;
• A pre-existing weak low-pressure area or low-level-cyclonic circulation
• Upper divergence above the sea level system.

They are known as Cyclones within the Indian Ocean, Hurricanes within the Atlantic, Typhoons within the Western Pacific and South China Sea, and in Western Australia as Willy-willies.

Positive effects

• Easterly jet stream steers ‘Tropical Depression’ into India and plays a significant role in the distribution of monsoon rainfall over the Indian subcontinent.

Negative effects

• The tropical cyclones are not associated with surface anticyclones and thus having greater destructive capacity.
• Tropical cyclones cause greater destruction due to winds, storm surges and heavy rains such as the uprooting of trees, of crops, destruction of houses, buildings, vehicles etc.
• Erosion of beaches

Thunderstorm

• A thunderstorm is an intense local storm related to a tall cumulonimbus cloud during which there are strong updrafts of air.
• Thunderstorm produces both hail and cloud-to-ground lightning.

Tornado

• A tornado occurs during a small but intense vortex during which air spirals at tremendous speed. 
• It’s linked with thunderstorms spawned by fronts within the mid-latitudes of North America.
• Tornadoes also occur inside tropical cyclones (hurricanes) and produce both hail and cloud-to-ground lightning.

TEMPERATE OR FRONTAL CYCLONE

The cyclonic activity associated with polar fronts & limited to 35°&65° latitude in both hemispheres is known as mid-latitude cyclones or extra tropical cyclones or temperate cyclones or frontal cyclones.

Seasonal Occurrence of Temperate Cyclones

The temperate cyclones occur largely during winter, late autumn and spring. During summer, temperate cyclones shift towards north and there are only few temperate cyclones above sub-tropics.

Positive Effects of temperate cyclone

• Temperate cyclone is a common weather phenomena in India known as ‘western disturbance’ they are the cause of the most winter and pre-monsoon season rainfall across North-West India and are good for the standing rabi crops like wheat etc.
• In a temperate cyclone, rainfall is slow and continues for many days, sometimes even weeks thus provide relief from drought conditions

Negative effects

• These cause less destruction due to wind but cause more destruction due to unusual rains and flooding.
• The temperate cyclones show a variety of cloud development at various elevations and bring dense fog over Indo-Gangetic plains.

Influence of tropical and temperate cyclones on India:

Tropical cyclone attacks on both the coasts although it does not last for than a week but causes greater destruction.

Temperate cyclone bring rains to north-west India “Western Disturbances”.