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What caused the Dec. 1, 2015 Chennai downpour?Priority 1


Mains Paper 1: Important Geophysical phenomena, geographical features and their location- changes in critical geographical features

From UPSC perspective, the following things are important:

Prelims level: Cold Pool Theory

Mains level: This sought to be a better explanation for the floods in some unexpected regions.


IISc researcher answers this by linking the Eastern Ghats and rain-bearing clouds

  1. In December 2015 Chennai and its surrounding regions experienced an unprecedented, heavy rainfall.
  2. 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 in over 24 hours that day.
  3. This led to death of nearly 250 people, and Chennai was declared a ‘disaster zone’.

Cold Pool Theory

  1. 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
  2. The gusty cold wind that heralds an approaching thunderstorm is nothing but a cold pool, which plays a pivotal role in cloud dynamics.
  3. Unlike the Western Ghats, which run close to the west coast of India, the Eastern Ghats are nearly 200 km away from the coast.
  4. Therefore, the link between the mountains orography and the rainfall over the region is crucial.
  5. The cold pool was obstructed by the Eastern Ghats from flowing downward; hence it piled up and remained stationary over the Chennai region.
  6. 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.

Dust storms: Aandhis in North India are too caused by ‘Cold Pool’

  1. Cold pools are known to play an important role in the dust storms (Aandhi) that form in northern India.
  2. They form by the evaporation of raindrops.
  3. This process is more efficient in the drier and warmer environment as there is lot of scope of evaporation.
  4. So, the cold pools that form in these conditions, are deeper and more vigorous.
  5. 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.

The Way Forward

  1. For the first time, this study links cold pools and the mountain structure to explain rainfall over south India.
  2. Though the primary aim of the study is to explain the anomalous rainfall over Chennai on December 1, 2015, the understanding gained from this analysis can be useful for improving the general weather forecast over this region.

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