The weather certainly is character building, isn't it? Making plans, changing plans, adjusting plans, shuffling one obligation over another obligation... Sure, not all of these trivialities are dependant on the weather - but what if you had planned to go to the bank or run indoor errands on one of the few days where it's bright, sunny, and breezy all at once? OR if you had planned to avoid doing lawn-mowing so you prayed for rain - on the ONLY day it DOESN'T rain? What about those plans you'd made in lieu of the mowing?
So, Fair-Weather friends on occasion mean that they are those that only come to be your friend when they need something. But in other cases - they could in fact be friends that you find yourself hanging with when the weather actually is FAIR. People who you have fun with romping outdoors and playing frizbee with, or going to the water park, or throwing water-balloons, or even riding bikes and such.
Some people are just better enjoyed outside.