Answer:
The reaction is exothermic
Explanation:
The phenomenom described can be a result of an exothermic reaction.
Chemical reactions offen exchange energy with the enviroment, they can either:
Following the principles of Le Chatellier, the first ones will be benefited by low temperatures, because the reaction will try to reverse that and move the equilibrium to the products (releasing heat).
On the other side, endothermic reactions will be benefited by high temperatures because the reaction will try to reverse that and move the equilibrium to the products (absorbing heat).
Being all that said, the behavior described is the one of an exothermic reaction: having a higher yield in low temperatures (winter) than in high temperatures (summer).