Water at temperatures far below freezing point, is very unstable and when disturbed by an aircraft, rapidly changes into clear ice. Air Force research has shown that this rate of ice buildup on an airfoil can be much more rapid than previously thought. In a number of cases, two to three inches of ice developed in less than five minutes. As little as one half inch (1.27cm) of ice can reduce aircraft lift by 50%. As Captain Airclues already stated, the strong convective currents of a cumulo-nimbus cloud develop the large super cooled droplets at those very low temperatures. While an icing layer in the winter may be only about 3,000 or 4,000 feet thick, in cumulo-nimbus the layers may be 20,000 feet in thickness. The freezing level of these convective clouds will be about 15,000 to 18,000 feet AGL and if the cloud top is near 40,000 feet, there is a freezing layer of 20,000 feet or greater.
From an article published in Professional Pilot by Norman Schulyer.