During summer when the surrounding temperature is high, we lose water in the form of sweat, direct evaporation considerably. Hence kidneys retain water for maintaining fluidity of the blood and less urine is formed. 

 But during winter, the surrounding temperature will be very low. There will not be any sweating, no evaporation losses. Moreover 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 out put and frequent urination under very cool conditions and in winter.