The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Clouds
Fronts
Visibility and Fog
Aircraft Icing
Thunderstorms
Altimetry
Weather Reports and Forecasts
Practice Exam

Hoar Frost

When moist, humid air comes into contact with a surface that is below the dew point, condensation occurs on the surface (just like the condensation that forms on the outside of a cold drink in warm weather). If temperatures are below 0°C, frost will form. The typical frost that you find on your car on a winter morning will be found on aircraft parked outside too.

Frost on the surface of an aircraft disrupts the flow of air and degrades performance, sometimes to the point that the aircraft will not be able to take off!

Significant hoar frost has formed on this aircraft wing overnight. We can see it beginning to melt in the morning sun.

Hoar frost can form on our aircraft in two ways:

First, an aircraft parked outside overnight will cool as temperatures drop. If the air is moist and the airframe surface is below the dew point, condensation occurs. This will freeze and form frost if the temperature is below 0°C.
Secondly, if an aircraft cruises for an extended period at an altitude where the temperature is below 0°C, the airframe will slowly reduce below 0°C as well. It can take some time for the airframe to warm above 0°C again after descending and if warmer, humid air is encountered during the descent, a layer of hoar frost can form on the sub-zero airframe.