Rime ice forms and accumulates on the leading edge of exposed parts, producing rough, irregular formations. FROST. —Frost is formed through the process of sublimation; ice crystals form when water vapor contacts a cold surface. Frost may form in the air and on the ground. When aircraft descend through a cold layer of air into a layer of warm, moist air, frost formation is possible. It may also form at flight level when aircraft pass from a sub-freezing air mass into a moist and slightly warmer air mass. On the ground, frost may form during a clear night with subfreezing temperatures.