Why do I find I want to urinate more frequently and more copiously in very cold weather?
• It’s for the same reason that you feel thirsty after going for a swim. When you go out in the cold, or get into cold water, your body restricts the flow of blood to your peripheries and the skin by constricting peripheral blood vessels. This allows it to preserve heat and maintain core temperature.
One consequence of this is that the total volume of your blood vessels is reduced, which initially means your blood pressure increases. In response, your kidneys excrete more fluid and thus reduce blood volume, restoring your blood pressure to its previous level.
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When you come into the warm again, the mechanism works in reverse and you have to either drink more or more water has to be taken up from your gut to compensate.
In warm conditions we transpire to keep cool and in consequence excrete less urine.
Terence Hollingworth, Blagnac, France
On a related theme, we have an answer to the second part of the following question but not the first. Why should running water have this effect? – Ed