Part of metabolism is simply to keep body temperature stable.
As it gets warmer, not as much energy is required to maintain body temperature.
In the cold, our metabolism may increase as our bodies need to work harder to maintain our temperature.
Although there may be slight changes throughout the seasons – It is important to know that dehydration (which can happen it warm or cold weather) actually has a greater effect on your metabolism.
Of course – this is more likely to happen in warm weather.
Basically when your cells are deprived water, they shrink slightly. The body senses the change and uses it as a signal to slow metabolism, inevitably causing you to burn less calories at rest.