moisture is water in liquid form & there is moisture on the surface of the airplane ->  there is water on the surface of the airplane  an airplane is climbing in the sky & climbing increases altitude ->  the airplane is increasing in altitude  the airplane is increasing in altitude & as altitude increases , the temperature of the air will decrease in the troposphere ->  the temperature of the air decreases when the altitude of the airplane increases  there is water on the surface of the airplane & the temperature of the air decreases when the altitude of the airplane increases ->  the water on the surface of the airplane will decrease in temperature  the water on the surface of the airplane will decrease in temperature & water is a kind of substance & freezing means a substance changes from a liquid into a solid by decreasing heat energy & water is in the solid state , called ice -> the water on the surface of the airplane will freeze into ice