Warm air can hold more moisture than cool air, causing high temperatures to increase humidity's ability to thwart temperature regulation.

A 1-degree Fahrenheit (0.55-degree Celsius) rise in temperature equals as much as a 4% increase in atmospheric water vapour. This is why humidity feels worse in summer.

The "heaviness" we feel is not due to the high water vapour content. The water vapour pushes out small amounts of nitrogen and oxygen, meaning there's less oxygen to breathe. Our bodies are already taxed by being overheated, so it feels like it takes more work to breathe.

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