Lenses fogging up usually isn’t a big problem for me here in the UK. But it is a problem I’ve faced while shooting in warmer climates. You go from a nice cool air conditioned building to the hot and humid outdoors and there it goes. Within 10 seconds, your lens is virtually impossible to see through.
If your lens is weather sealed, it’s just a waiting game for it to clear up. If it’s not, you can face long term consequences. This two minute tips video from David Bergman offers several ways to prevent fog on your lenses when you change environments.