I think cool weather or dry air. Mine runs when I'm running even if its 90+ out. My sister was asking the other day why I always wear short sleeves when I run (rather than tank tops)...because it's easier to just turn my head and wipe my nose on my shoulder! In cool weather, arm warmers work well, too.

Fatcyclist has coined the word "snotulum" for his problems with a runny nose while riding. Mine never accumulate to that amount, just get drippy. It will eventually make my nose a little raw so I sometimes apply chapstick to my nose as well while exercising.