It's mainly psychological but, there are some studies that show diminished abilities in colder temperatures, even slight changes can cause them. When your body perceived that it's cold, it focuses your blood towards your bodies core. This helps maintain your vital organs but, causes your extremities, Arms and Legs to tire easily do to oxygen atrophy. Further more, cold air is heavier then warm air and causes the ball to be heavier and not fly as far. Being wet emphasizes this effect as well as overcast skies.
So if someone is used to playing in such conditions, they'll have an advantage.