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!
Hoar frost can form on our aircraft in two ways: