Many pilots are taught during primary training if they encounter freezing rain while in flight, the best choice is to climb. Why? Well, they are taught that freezing rain is a sign of warm air aloft. That is, if you are experiencing freezing rain, then it is because of snow falling into a layer that is warmer than 0°C that effectively melts the snow turning it into rain which then falls into a subfreezing layer creating freezing rain. So, the goal to climb is to get into that warm layer above you.
What is described above is called classical SLD. SLD stands for supercooled LARGE drop icing. This describes the condition where the precipitation starts out as snow, melts into rain and then falls into that subfreezing layer below. They key here is that it starts with snow. Turns out that only 8% of the freezing rain temperature profiles start out as snow. The remaining 92% are due to an all liquid process called nonclassical SLD. Moreover, that subfreezing layer usually is very close to the surface. That's why we say that freezing rain is generally a ground hugging event. Most of the time you will experience this on departure or arrival, but not en route.
If you look at a terminal forecast and it doesn't show the potential for freezing rain, does that mean there's no chance of encountering freezing rain? Nope. In fact, the TAF simply tells you what precipitation type is expected at the surface. Here's a TAF for Charlotte Douglas Airport (KCLT).
KCLT 131405Z 1314/1418 02007KT 5SM -RA BR OVC007
TEMPO 1314/1318 4SM -RA BR OVC004
FM 131800 02006KT 4SM -RA BR OVC005
FM 140200 03006KT 5SM BR OVC006 PROB30 1402/1408 3SM -RA BR OVC003
FM 140800 04006KT 5SM BR OVC005 PROB30 1408/1414 3SM -RA BR OVC003
FM 141600 04007KT 4SM -RA BR OVC004 AMD
Notice that the TAF simply shows -RA (rain) and not FZRA (freezing rain). That's because the forecaster believes the temperature a the surface will be above freezing. But is there a chance of encountering freezing rain aloft? In this case, yes.
Notice that on the sounding analysis below valid at 14Z for KCLT, the temperature at the surface is slightly above freezing. However, the temperature quickly drops to below freezing just 1000 feet above the surface. Given this classical SLD structure, there is a very shallow, but potentially deadly region of freezing rain from 1,000 feet MSL to 2,500 feet MSL.
While flight at 5,000 feet MSL is free of ice, a descent into KCLT for an ILS approach (given the potential for a ceiling of 400 ft), will undoubtedly produce a significant risk of SLD on the approach. Just keep in mind that it is very unlikely you will be flying along at 10,000 feet above the terrain and run into freezing rain. In most cases, it will be on approach or on departure where you will experience it.
Most pilots are weatherwise, but some are otherwise™
Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist
