"But doesnt it have to be cold to have ice crystals? Miami isnt the first place I think of when I think chilly."
Yes, but the ice crystals form miles above ground level. The air up there is always cold... According to one web site I looked at, the temperature at 30,000 feet up is around -50 C. (That's about cruising altitude for big jets.)
For more information on temperature variation as a function of altitude, check out
http://lidar.ssec.wisc.edu/papers/pp_thes/node22.htm
Fun and games with physics!
- Pie