Updated: May 1, 2019
The big freeze is coming! So it’s that time of the year to review one important aspect of airframe icing, namely, precipitation type. The three basic wintry precipitation types include freezing rain (and freezing drizzle), snow and ice pellets (colloquially known as sleet). It's important to understand that METARs and TAFs typically report or forecast these precipitation types based solely on what’s reaching or expected to reach the surface. Therefore, if the surface temperature is expected to be +1°C, you will see a forecast for rain (RA) or drizzle (DZ) in the TAF instead of freezing rain (FZRA) or freezing drizzle (FZDZ). However, just 500 to 1000 feet above the ground a serious icing hazard may be lurking in the form of freezing rain. So let’s take a look at the three primary precipitation types and examine the temperature profile aloft that’s typically common for each.
For snow (SN) to reach the surface, there needs to be a deep moist layer that is, for the most part, entirely below freezing. The key to getting snow versus other precipitation types is that the top of this moist layer must be sufficiently cold to produce ice crystals at the expense of supercooled liquid water. While there is no definitive temperature, ice crystals begin to dominate the cloud when the top of this moist layer is colder than -12°C. Precipitation continues to fall as snow when the temperature remains at or 0°C from the cloud base to the ground. You can also see wet snow if the temperature near the surface is at most 3 or 4°C.
There are two processes in the atmosphere that can produce freezing rain (FZRA), namely, classical and nonclas