Saturday, December 29, 2018

Wet vs dry winter: How temperature and humidity interact

The northern part of the country, including the national capital, is experiencing a severe cold wave.

Winter can be divided as either wet or dry winter.

Wet winter, or early winter, is characterized by fall in temperature along with high humidity. In cold weather, high humidity levels will make one feel colder.

The body cools itself by sweating. During summers, when the humidity is low, sweat evaporates easily and cools the body because the process of evaporation requires thermal (heat) energy. But, when humidity is high in summers, evaporation rate is reduced. The resulting limited evaporation in hot and humid conditions is not enough to cool the body. And, therefore, high humidity in hot weather makes one feel sticky and warmer. This combined effect of temperature and relative humidity is called heat index, which is experienced as the felt air temperature.

In winter, the cold air (low temperature) with high relative humidity "feels" colder. This is because the insulating effect of clothes decrease as the humidity rises. The moisture in the atmosphere is captured on the clothing, even though the clothes do not appear wet and results in greater heat loss from the body.

Another factor to be considered during winters is the sun. Cold days are more likely to be overcast, which reduces the amount of direct sunlight reaching the surface; whereas, on a dry sunny day, the body is warmed by radiant heating from the sun.

Fog and smog are common during wet winter.

Whenever the humidity is high, air movement is less and temperature is low, fog is the automatic result. It occurs when water droplets are suspended in the air.

When the level of pollutants is high in the atmosphere, the pollutants get mixed into the fog. This is called smog. Therefore, smog is the combination of smoke and fog and it further reduces the visibility. The smoke includes toxic emissions from vehicles and industries, dust and open burning of crops.

Dry winter, or late winter is characterized by absence of fog, smog and presence of chilly airy winds. Cold and dry air generally feels warmer than cold and humid air at the same temperature. 

Dr KK Aggarwal
Padma Shri Awardee
President Elect Confederation of Medical Associations in Asia and Oceania   (CMAAO)
Group Editor-in-Chief IJCP Publications
President Heart Care Foundation of India
Past National President IMA

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