During winter, the surrounding temperature will be very low. There will not be any sweating and no evaporation losses. To keep the body temperature constant, the blood capillaries in the skin get constricted and the blood pressure is increased. The result is that our body gets warmed up. In order to maintain the blood pressure normal, the excess water along with the wastes is excreted mostly in the form of urine. This is the reason for more urine output and frequent urination in winter.