Why do we urinate more often in winter compaired to summer?
There is no 100 % sure answer for this as, obviously, nobody is that much interested in studying about urination. But there are some theories, of which the most popular and accepted one is cold diuresis.


  Cold-induced diuresis, or cold diuresis, is a phenomenon that occurs in humans after exposure to a hypothermic environment, usually during mild to moderate hypothermia.


According to this theory, what exactly happens is this (I'll explain in steps for better understanding):


When your body senses cold or low temperature of environment, it constricts the blood vessels near your skin and external organs and restricts your blood to the inner core of your body to maintain its temperature.
Thus, it causes the blood pressure to increase as there is same amount of blood in lesser volume.
This induces negative feedback on ADH (a vasoconstrictor) i.e. its secretion is reduced.
Now when blood vessels in kidney get proper amount of blood (as blood vessels aren't constricted now), they sense high blood pressure and try to reduce it as soon as possible.
So they transfer more amount of water from blood to bladder which fills the bladder more quickly and causes an urge to urinate.
Thus, we urinate more in colder climate.


There are some more theories related to it too, but they aren't as popular. According to another theory, when we are exposed to cold weather, aquaporins are inhibited around the body, making it impossible for water to be taken in by cells, leaving a lot more of it in the blood. Again, the body will try to balance that pressure, pulling water from the blood and storing it in the bladder. You can get a more frank and interactive explanation here.