When flying an aircraft through an inversion (where most supercooled water can be found) or any other low-temperature conditions, one must be very wary of freezing rain and aircraft icing. The ice coats the wings of the aircraft, adding so much weight and ruining the aerodynamics of the aircraft to such an extent that flying can become impossible. In extreme circumstances the controls can freeze over, but this doesn't usually occur (as the aircraft is most often already in serious trouble before that could happen).

There are two types of ice that result from freezing rain and aircraft icing; Rime Ice and Clear Ice (sometimes referred to as Sheet Ice). Rime ice is easier to remove if the aircraft is equipped with the necessary tools, and it also is more aerodynamically effecient. Clear ice is more difficult to remove and can bring an aircraft down in a matter of minutes.

Freezing rain most often occurs in an inversion (where the air is warmer higher up, but is colder closer to the ground). As the fancy graph in the writeup above shows, a water droplet formed in the "above freezing" section will travel into the cold air mass and become supercooled. An inversion can occur for many reasons, but the most common is when a warm front collides with a cold front. Because warm air rises, a warm front will "ride up" over the cold front (or an approaching cold front will "wedge under" a warm front). As the warm air rises, it begins to cool and cannot contain as much moisture, and it then rains.

\ .  . .           WARM AIRMASS
 \. ..
   \              (= Warm air travelling this way
  ~ ~ \  
     ~ \ 
COLD    \
---------------------------- (ground)
In my fancy-wancy diagram, you can see the rain (.) starting in the warm airmass, passing through the temperature layer (\) and entering the cold airmass. As it does so, it becomes supercooled water (~).