The temperature of....the water!
Important when trying to establish feeding temperature and depth of fish. Also important for rate of retrieve. Cold water, slow retrieve, also deeper fish. Very cold water forms ICE which can really spoil a good days fishing although watching a trout try to take a DRY FLY through ice can be entertaining. In warmer summer temperatures the fish may also be deeper due to increased oxygen levels in cooler, deeper water (cold water dissolves more oxygen than warm water)

Courtesy of Carl