• On December 1, 2015 Chennai and its surrounding regions experienced an unprecedented, heavy rainfall.
  • In a region where the average rainfall during the season is expected to be 8-10 mm per day, one of the rain gauges in the city recorded an abnormally high, 494 mm, rainfall over 24 hours that day.
  • This led to death of nearly 250 people, and Chennai was declared a ‘disaster zone’.
  • There have been attempts to explain this phenomenon of how clouds remained stationary over this region, continuously giving rain over 24 hours.

Tell us about the Cold pool

  • When clouds give out water droplets, the droplets evaporate mid-air, as they fall down.
  • This cools the surrounding air, forming a cold pool of air which sinks down and flows horizontally.
  • The gusty cold wind that heralds an approaching thunderstorm is nothing but a cold pool, which plays a pivotal role in cloud dynamics.
  • Unlike the Western Ghats, which run close to the west coast of India, the Eastern Ghats are nearly 200 km away from the coast.
  • Therefore, the link between the mountains’ orography and the rainfall over the region is not obvious, and this is the first study to link the two.
  • The interaction between mountains, clouds and cold pools became clear after performing the model experiments.
  • The cold pool was obstructed by the Eastern Ghats from flowing downward.
  • Hence it piled up and remained stationary over the Chennai region.
  • The reason for the clouds remaining stationary was that there was a balance between the piling of cold pool along the mountain and the winds from the bay.
  • This does not happen in all heavy rainfall incidences over Chennai.

What about the Dust storms?

  • Cold pools are known to play an important role in the dust storms (Aandhi) that form in northern India. They form by the evaporation of raindrops.
  • This process is more efficient in the drier and warmer environment as there is lot of scope of evaporation.
  • So, the cold pools that form in these conditions, are deeper and more vigorous.
  • As pre-monsoon conditions in north India are very dry and warm, cold pools that accompany the pre-monsoon thunderstorms there are far more destructive, causing widespread damages.
  • For the first time, this study links cold pools and the mountain structure to explain rainfall over south India.