Everyone now knows to carry a bottle of water in hot weather to stay hydrated, but the risks of dehydration in cold weather are less appreciated. Cold-induced diuresis, or increased urine production, is a common condition in winter months. The exact mechanism is not known, but it is related to blood withdrawing from the extremities in response to cold. This decreases the volume of circulating blood, so more water is excreted by the kidneys. Cold-induced diuresis can double urine production in some people.

Cold, dry air also contributes to dehydration (PDF). Those steaming breaths you exhale are water vapour lost directly from your lungs, as cold air tends to have less moisture than warm air. Another factor is bulky cold-weather clothing. Your extremities may be chilly, but heavy clothing increases the effort of moving and you may actually sweat more on a cold day than a mild one.

These factors are exacerbated by the fact that people tend to drink less during cold weather. This is partly because of a decreased thirst response in the cold, and partly because a slug of chilled water is not so appealing in wintry weather. A scientific study found, unsurprisingly enough, that “voluntary dehydration” could be avoided by offering hot drinks instead.