Frost vs. ice in the sky

With winter well underway, anyone flying at higher latitudes and altitudes at this time of year will likely encounter frost or ice. However, not everyone understands the similarities and differences between frost and ice and when either can form. Hence, a question about the technical difference between aircraft icing and frost that was recently posted on the Ask a Flight Instructor site along with the answers given are well worth reading. The poster had also asked:

-In order for aircraft icing to occur, there needs to be visible moisture present.  But what about temperatures?  Do temperatures HAVE to be at or below freezing for icing to occur?  What OTHER conditions are necessary for ice to occur?

-As for frost…The collecting surface must be at or below the dewpoint of the air..and the dewpoint has to be below freezing.

Paul Tocknell responded by pointing out that frost is a form of ice and both are considered to be frozen contaminants. He also noted that:

Frost will occur on aircraft surfaces if the aircraft’s surface temperature is below the ambient dew point and the surface temperature is below freezing.  Frost can occur even when the surrounding air temp is above freezing.  For instance, an airplane parked overnight will radiate heat and become cooler than the ambient air.  If it cools to it’s dewpoint (and it’s dewpoint is below freezing) then frost will occur.

Paul then pointed out that according to the FAA’s AC 91-74, nearly all icing will occur in supercooled clouds where liquid droplets are present in temperatures below 0° C while at temperatures near 0°, the cloud may actually consist entirely of such droplets with few or no ice particles present. In addition, he noted that the right type and size of water droplet will also matter as large droplets will tend to lead to in flight icing conditions.

In addition, Wesley Beard responded by noting that the Aeronautical Information Manual states that icing is most prevalent in temperature ranges from +2deg C to -10deg C and in most of the corporate jets he flies, the manufacturer considers icing conditions to exist when inside visible moisture and the temperature is between +10deg C and -40deg C.

However, Wesley also pointed out that:

There are a lot more variables than just visible moisture and temperature in determining whether or not ice will form.  The size of the droplets also matter.  Droplets smaller than 40 mu meters don’t have a chance on sticking they are simply too small.  Conversely, droplets that are too big and are not supercooled probably will not form ice either.

In other words, there can be many unpredictable variables with the formation of frost and ice that pilots need to consider and be aware of at this time of year.

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