Clear ice forms when larger water droplets begin freezing once they hit the aircraft. However, because the droplets are larger they do not freeze instantly. Instead, they have time to spread back across the aircraft as they freeze, creating a layer of clear ice that builds up along the leading edge and spreads further back along the wing.
The result is a spread out, more transparent layer of ice that has very little air trapped inside. With no trapped air, clear ice is heavier and much harder to remove from the aircraft.
As the droplets remain liquid for a period of time before turning into clear ice, the water has time to seep into areas that provide much better adhesion – making it harder to remove. Areas around rivet heads and small chips in propeller blades make perfect surface areas for clear ice to latch onto.
When a combination of small and large water droplets is present, both rime and clear ice will form on an aircraft. This is known as cloudy or mixed ice.