Lateral Thinking – What could be the reason for this rather bizarre behaviour?

Carey Queen works on the 35th floor of an office tower. Most mornings Carey will ride the elevator as far as the 25th floor, then climb the stairs to the 35th. On mornings when it’s raining, however, he will always ride the elevator to the 35th floor. Since Carey is neither fond of exercise nor superstitious, what could be the reason for this rather bizarre behaviour?