Updated: Jan 20
There are many opinions in the aviation community that flying through snow is not only an icing hazard, but also against Federal Aviation Administration (FAA) regulations for aircraft without a certified ice protection system. You can read one of them here from a recent post by the good folks at boldmethod. Keep in mind that each weather system is unique and there are many exceptions to the general discussion presented here and by boldmethod. Let's discuss some of the many factors associated with flying through snow.
Snow falling out of the base of a cloud means that there are fairly deep saturated conditions aloft. To produce snow typically requires that the cloud top temperature (CTT) be sufficiently cold. That usually means a CTT of -12°C or colder - colder is often better. In this situation, ice crystals can develop and lead to the development of snowflakes in the cloud aloft. If you are flying through snow below the cloud base, does that imply icing conditions exist? Just to be clear, this is not a discussion of flying in the clouds that are producing the snow, but below the cloud base. For the hazard of flying through clouds producing snow, read this post.
Snow is visible moisture. It can be mixed with other precipitation types that may include rain, freezing rain or ice pellets. In general, snow falling from the base of a cloud doesn't represent a significant airframe icing hazard unless it is mixed with other types of precipitation such as freezing rain. It can be an issue with induction icing, but not airframe icing. In the unlikely case that snow does adhere to the airframe, an exit plan should be executed.
Outside of a mixed precipitation scenario, snow is usually classified as wet or dry. Wet snow occurs when the static air temperature is at or above 0°C. That is, the snow falls into an atmosphere that is warmer than freezing andbegins a melting process. Although liquid water doesn't necessarily freeze at a temperature below 0°C, it is the case that snow must begin to melt at a temperature warmer than 0°C. If the temperature is warm enough, it will completely melt the snowflake into a raindrop before reaching the surface. You may have experienced this in your car. You'll see the wet snowflake splat on your windshield and quickly melt. Wet snow can begin to accumulate on grassy surfaces or other vegetation, but usually melts quickly on other surfaces.
Moreover, because you are flying at an airspeed where kinetic heating occurs on the leading edge, even at a temperature of 0°C, snow will typically not accrete on the leading edges of the wings and horizontal stabilizer due to this kinetic heating primarily due to adiabatic compression. This is typically referred to as ram air rise. And certainly with a static air temperature above 0°C, it is very unlikely to accrete with the additional ram air rise. In fact, even at a static air temperature of -1°C or -2°C, accreting ice is typically difficult at best. Once the static air temperature gets colder than -3°C, then you are no longer dealing with wet snow since now melting is occurring.
Certainly, wet snow can be problematic while taxiing. It may collect on some surfaces and begin to accumulate over time. It is recommended that you never depart with any of the aircraft surfaces contaminated to include wings and the horizontal stabilizer. Doing so, may cause the aircraft not to develop the lift necessary to takeoff and climb creating a risk of impact with terrain. Always takeoff with uncontaminated surfaces.
Another metric to use is the Current Icing Product (CIP) found on the Aviation Weather Center website here. CIP utilizes a recent three hour forecast from the Rapid Refresh (RAP) model for parameters such temperature, moisture aloft, supercooled liquid water content and other useful model data. This is mainly to "seed" the forecast for these items given that observational data is rather sparse throughout the atmosphere for these important parameters. Nevertheless, it combines this with surface observations, ground-based radar, pilot weather reports, satellite imagery and lightning to produce an hourly analysis of icing probability, icing severity and supercooled large drop icing potential from the surface through 30,000 feet.
CIP looks for information about the presence or absence of six precipitation types that include freezing rain (FZRA), freezing drizzle (FZDZ), ice pellets (PL), rain (RA), drizzle (DZ), and snow (SN). A report of any of the first five precipitation types means that altitudes below cloud base need to be considered for possible icing and SLD, because subfreezing liquid precipitation may be present. However, an observation in which only snow is reported at the surface, ice crystals are clearly present beneath and within the lowest cloud layer and those are not considered an icing threat especially below the lowest cloud base.
For example, if an airport is reporting an overcast sky at 2,500 feet and only snow is being reported, the algorithm in CIP will remove any possible occurrence of icing from the cloud base down to the surface regardless of what other sources may say. This because snow that is not mixed with other precipitation types such as freezing rain is not seen as an icing hazard...even wet snow.
Is it legal to fly through snow in an aircraft without a certified ice protection system? First you may want to read this letter from the FAA's Office of the Chief Counsel. As indicated in this post, the essence of this letter is...
"The formation of structural icing requires two elements: 1) the presence of visible moisture, and 2) an aircraft surface temperature at or below zero degrees Celsius. The FAA does not necessarily consider the mere presence of clouds (which may only contain ice crystals) or other forms of visible moisture at temperatures at or below freezing to be conducive to the formation of known ice or to constitute known icing conditions. There are many variables that influence whether ice will actually be detected or observed, or will form on and adhere to an aircraft. The size of the water droplets, shape of the airfoil, and the speed of the aircraft, among other factors, can make a critical difference in the initiation and growth of structural ice."
Yes, snow is visible moisture, but will it adhere to the airframe? Dry snow is not going to adhere to the airframe. Wet snow as mentioned above is more of an induction icing or ground icing concern than airframe icing while in flight.
Sometimes it's not about airframe or induction icing. Flying through falling snow can also be very disorienting at times especially when the snowfall is moderate or greater. It will often lower flight visibility to 3 statute miles or less and can make runways extremely slick. Landing while it is snowing on a snow-covered runway can lead to a flare at an altitude higher than normal making for a hard landing.
One last point. Often when snow falls into a dry layer below the cloud base it can sublimate. This usually occurs with the onset of precipitation as a weather system approaches. Evaporation and sublimation are both cooling processes and that will lower the temperature in the dryer air. What may be an atmosphere that is a few degrees above freezing that can lead to melting wet snow, cool temperatures created through this process can quickly move the temperatures to below freezing allowing for snow to reach the surface instead of melting.
If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me book that is available in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist