Random Riddle: The Elevator

A man who lived on the top floor of a twenty story building had to go up and down daily for work, and of course, for food and the other necessities. On most days he could only ride the elevator to the fifteenth story, and he would have to walk the rest of the way. When it rained, however, he could ride all the way up to the twentieth story.

Why?
 

Random Riddle: The Elevator