When you walk on the snow and it squeaks, it's too cold.
When hard liquor starts getting thick, it's too cold.
When you can't make a snowball stick together, it's too cold.
When you have to heat up a key to get a lock to work, it's too cold.
When it takes 20 minutes to put on your clothes, it's too cold.
Lastly, when you ask "How cold is too cold?", it's too cold.
BTW, don't let the Florida locale fool you, I grew up in Wisconsin.
