The morning chill that Chennai and other parts of Tamil Nadu may have lost to the very severe cyclone ‘Thane' and associated moisture influx is set to return.
‘Thane' has left in its wake a band of steady moisture-laden easterly flows across the Bay of Bengal sustaining the humidity levels, but not enough for it to rain it down.
The air has been largely calm. Combined with clear skies and a bright sun, which are typical of the winter climes, the weather has been hot, muggy and sticky.
This is the legacy of ‘Thane,' which, after having pumped in incremental moisture into the south-eastern coast, has rendered it an extended wet season right into early January.
Forecasts now suggest that cooler north-easterly winds would start blowing from next week after helpful atmospheric parameters fall into place.
Some forecast said the minimum temperatures could drop down to 19 deg Celsius, 3 to 4 deg Celsius below normal.
But why is that it did not rain, even with humidity levels hovering above 90 per cent over the last few days?
According to experts, high humidity alone cannot trigger rains. For rain to happen, clouds are a must.
And clouds need enough particles in the air for water vapour to settle on (clouds are formed when water vapour condenses on particles in the air).
It may be just that these particles were not present in the required numbers, a seasonal pattern associated with winter. This could also be one reason why the heavily-endowed ‘Thane' did not rustle up the heavy to very heavy rain normally expected of such strong a system.
There are also other factors that would contribute to cloud formation such as temperature and wind speed.
The north-east monsoon is the main rainfall period for Tamil Nadu when nearly 47 per cent of the annual total of 91cm is recorded.
The coastal districts normally receive about 75 to 100 cm of rainfall during the period, constituting nearly 60 per cent of their annual total.
During winter, dry and cold air blowing from the northerly latitudes from a north-easterly direction prevails over the Indian region.
The rains generally occur over the western Himalayas, the extreme north-eastern parts, Tamil Nadu and Kerala.
Western disturbances and associated trough in westerlies are main rain-bearing systems in northern and eastern parts of the country (as is being witnessed concurrently).