When small water droplets below 0°C hit the aircraft, they freeze almost instantly. This causes some air to be trapped inside the ice, which makes it look milky, opaque, and rough. This is known as Rime Ice.
Rime ice is fairly brittle because of the air trapped inside, making it easier for it to break away from the aircraft.
These small water droplets are most commonly found in clouds, which means rime ice typically forms on the leading edges of aircraft as you fly through clouds with temperatures below 0°C.
Below are pictures from an icing experiment in an aircraft manufacturer’s testing lab. Large quantities of rime ice can be seen built up along the leading edge in the ‘after’ photo.