To add a few things to auto's answer :
The SWDs cause clear ice, the nr 1 ice-related problem in flight.
They can exist from -40 to 0°C, being largest between -10 & 0°C
They have a size proportional to the vertical development of their cloud (ie small in shallow lyr cloud, large in CB & thick NS).
For physics freaks, it has been demonstrated that about 1/80th of each SWD freezes on impact for each 1°C below 0°C, eg :
* at -4°C : 4/80th of the SWD will freeze on impact & 76/80th will flow back => the clear ice amount is a major factor
* at -34°C : 34/80th will freeze on impact & 46/80th will flow back => less clear ice => safer flying conditions
MF