Why does humidity make us feel hotter in the summer and colder in winter?

Originally published on November 10, 2018.

This week's question comes to us from Lucy Narbaitz from Ottawa, who asks:

Why does humidity in the air make us feel hotter in the summer but colder in the winter?

Dr. Deepak Chandon, a post-doctoral fellow in the Department of Physics at the University of Toronto has the answer.

In the summer, the human body tries to regulate heat and maintain an optimal temperature through the process of perspiration. The sweat then evaporates into the atmosphere, taking with it the excess heat it has eliminated from the body. But under humid conditions, the atmosphere cannot absorb any more water. This means sweat does not evaporate, and its function is thwarted, making us feel hotter.

In the winter, humidity does make us feel colder, but the reasons are less well understood. One reason could be that damp air in winter, the equivalent of humidity, causes the body to lose heat more quickly than dry air. It may also be that the condensation of water vapour on the fibres of our clothes in winter interferes with the ability of clothes to trap pockets of warm air. This reduces the insulation capability of clothing and makes us feel colder.