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 nonclassical. The classic situation is what most pilots are taught during their primary training. That is, the precipitation starts out high in the cloud as snowflakes. These snowflakes fall through a melting layer that’s warmer than 0°C. If the melting layer is sufficiently warm and/or deep enough, it will melt those snowflakes turning them entirely into raindrops. That rain falls into a subfreezing layer and becomes freezing rain creating a significant airframe icing hazard.
The nonclassical case is a bit more complex to explain, but essentially the entire process remains liquid. In other words, the precipitation high in the cloud doesn’t involve snow. This occurs when the weather system isn’t terribly deep and the top of the moist layer is at a temperature warmer than -12°C. Warmer subfreezing temperatures at the tops tend to prefer a liquid process over the production of ice crystals. In the non-classical case, the entire temperature profile aloft may be below freezing or may also have a melting layer. Regardless of the actual profile, the non-classical case is strictly an all-liquid process. In most situations, you’ll see a lot of tiny drops that produce a nasty freezing drizzle environment. Surprisingly, 92 percent of the cases are nonclassical based on a study done by the National Center for Atmospheric Research (NCAR).
Ice pellets (PL) are similar to the classical freezing rain case mentioned above, except that the melting layer is very shallow. This doesn’t entirely melt the snowflake, and the drop retains a slushy inner core. These slushy drops quickly refreeze as they fall through a deep layer of subfreezing air near the surface, and eventually reach the ground as hard little nuggets that bounce on impact.
Keep in mind that ice pellets indicate the presence of supercooled large drop (SLD) icing aloft. While the frozen pellets will bounce right off of your aircraft while in flight (taking a bit of paint with it), they are often mixed with other forms of freezing precipitation including freezing rain especially at altitudes right below the shallow melting layer. It's worth noting that may people call this precipitation type hail. It is not; hail is produced within deep, moist convection which has an entirely different temperature profile.
Here’s a little bit of ice pellet trivia. The abbreviation for ice pellets used to be PE. However, when rain and ice pellets occurred together with rain being the dominant precipitation type, the surface observation includes the term RAPE. This was deemed to be politically incorrect in English speaking countries and the abbreviation for ice pellets was then modified to PL.
So the next time you venture out this cold season, pay attention not only to the precipitation types that are being reported or forecast but also will provide you with a sense of the temperature profile aloft.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer CFI & former NWS research meteorologist