There are lots of things to consider here.
30-45 is generally considered the best to race a marathon in.
Perhaps you like it a bit warmer for a 5K.
Milers like it warmer yet.
Sprinters probably like it hot man!
As for me it is easier to get out the door when it is 80 than when it is 55. But I can probably
run longer in 55 degree temperatures.
Running at 15 (How about -10 like I ran in last night baby?) it takes motivation to get out there.
Once I am out there generally I can run for a long time...if I dressed right. Dressing right can be
tricky.
Next lets talk about snow...then ice the next thread!!!
Traffic?
Bugs?
Just do it~