On the basis of the presence of supercooled droplets, Wegener developed a theory explaining the growth of ice crystals. Atmospheric water vapor, as all gases do, exerts a pressure on the objects it surrounds. When all water vapor is at the same temperature, it exerts a vapor pressure proportional to its quantity in the air. Wegener discovered that vapor pressure near a frozen surface is less than the vapor pressure just a very small distance away. Since all substances move from areas of high pressure to areas of low pressure, the water vapor is drawn toward the frozen surface, where it condenses and then freezes. If the water vapor surrounding the frozen surface is supercooled, as soon as the water vapor is drawn toward the frozen surface it immediately becomes an ice crystal. When the process continues repeatedly, the ice crystals grow and hoarfrost results.

Although Wegener's discovery might not seem important, it was extremely valuable to airplane pilots. A light coating of frost on an airplane does not affect flight safety, but a large sheet of ice may cause the pilot to lose control. If a pilot continues to fly through supercooled clouds after the first crystals form, they will continue to grow. If the droplet size is very small and the temperature of the supercooled clouds is very low, rime ice results. If the droplet size is larger and the temperature of the supercooled clouds is "warm" (close to freezing), glaze ice forms.


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