Cyclone Dana (Prelims & Mains- Geography)

Why in news?

The Indian Meteorological Department recently issued a cyclone warning for residents of the northwest and central parts of the Bay of Bengal.

About Cyclone Dana:

• Cyclone Dana is a severe cyclonic storm (Tropical storm) forming over the Bay of Bengal.
• What does ‘Dana’ mean?

Dana is an Arabic word that means ‘generosity.’ ‘Dana’ or ‘Danah’ holds cultural significance in Arabic culture, signifying ‘the most perfectly sized, valuable, and beautiful pearl.’

• Which country has named Cyclone Dana?

According to the World Meteorological Organisation (WMO), Qatar suggested Dana’s name.

• The severe cyclonic storm, which weakened into a cyclonic storm, will continue to cause heavy to very heavy rainfall over Odisha, West Bengal, and Jharkhand. The cyclonic storm Dana is likely to move northwestwards across north Odisha and weaken gradually into a deep depression during the next six hours.

How a cyclone is named?

• Various warning centres name tropical cyclones and subtropical cyclones to simplify communication between forecasters and the general public regarding forecasts, watches, and warnings. The names are intended to reduce confusion in the event of concurrent storms in the same basin.
• The selection of names is based on their familiarity with the people in each region, aiming to aid in the understanding and remembrance of cyclones. The naming procedures can vary, with some regions using alphabetical order and others using the alphabetical order of country names.
• Once storms develop sustained wind speeds of more than 33 knots (61 km/h; 38 mph), names are generally assigned to them from predetermined lists, depending on the basin in which they originate.
•  Some tropical depressions are named in the Western Pacific, while tropical cyclones must contain a significant amount of gale-force winds before they are named in the Southern Hemisphere.

About Tropical Cyclone

• Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall and storm surges. 
• They occur between the Tropics of Cancer and Capricorn.
• The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of moisture from the sea, the storm is further strengthened. 
• A mature tropical cyclone is characterised by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. 
• They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy-willies in the Western Australia.

Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are: 

(i) Large sea surface with a temperature higher than 27° C; 

(ii) Presence of the Coriolis force; 

(iii) Small variations in the vertical wind speed; 

(iv) A pre-existing weak low-pressure area or low-level-cyclonic circulation; 

(v) Upper divergence above the sea level system.

Eye and landfall

The eye is a region of calm with subsiding air. Around the eye is the eye wall, where there is a strong spiralling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here. From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region. The diameter of the storm over the Bay of Bengal, Arabian sea and Indian ocean is between 600 – 1200 km. The system moves slowly about 300 – 500 km per day.

On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the cyclone. While landfalls are often the most devastating moments of cyclones, they also mark the beginning of their end.