Tropical Cyclones - (Part - 2) | Additional Study Material for UPSC PDF Download

• The “eye” is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. 
• There is little or no precipitation and sometimes blue sky or stars can be seen. 
• The eye is the region of lowest surface pressure and warmest temperatures aloft (in the upper levels) – the eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone. 
• Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.

• The eye is surrounded by the “eye wall”, the roughly circular ring of deep convection, which is the area of highest surface winds in the tropical cyclone. Eye Wall region also sees the maximum sustained winds i.e. fastest winds in a cyclone occur along the eye wall region. 
• The eye is composed of air that is slowly sinking and the eye wall has a net upward flow as a result of many moderate – occasionally strong – updrafts and downdrafts [Explained in ‘Thunderstorms’]. 
• The eye’s warm temperatures are due to compressional warming (adiabatic) of the subsiding air. 
• Most soundings taken within the eye show a low-level layer, which is relatively moist, with an inversion above – suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km of the surface.

• Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eye wall convection. 
• Convection in tropical cyclones is organized into long, narrow rain bands which are oriented in the same direction as the horizontal wind. 
• Because these bands seem to spiral into the center of a tropical cyclone, they are called “spiral bands”. 
• Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. 
• A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. 
• Subsidence is distributed over a wide area on the outside of the rain band but is concentrated in the small inside area. 
• As the air subsides, adiabatic warming takes place, and the air dries. 
• Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. 
• Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eye wall form. 
• Thus, the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eye wall and to a forced descent caused by the moist convection of the eye wall.

• The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm. 
• The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place. The outflow layer lies above 7 km. 
• The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.

➢ Size and Shape 

• Tropical cyclones have symmetrical elliptical shapes (2 : 3 ratio of length and breadth) with steep pressure gradients. They have a compact size—80 km near center, which may develop up to 300 km to 1500 km. 

➢ Wind Velocity and Strength 

• Wind velocity, in a tropical cyclone, is more in poleward margins than at center and is more over oceans than over landmasses, which are scattered with physical barriers. The wind velocity may range from nil to 1200 km per hour.