Freezing rain
Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural icing, freezing necessary componants. Most helicopters and small airplanes lack the necessary deicing equipment to fly in freezing rain of any intensity, and heavy freezing rain can overwhelm even the most sophisticated deicing systems on large airplanes. Icing can increase an aircraft's weight but not typically enough to cause a hazard. The main danger comes from the ice changing the shape of its airfoils.