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Droplets of supercooled water often exist in stratus and cumulus clouds. An aircraft flying through such a cloud sees an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes, unless the aircraft is equipped with an appropriate ice protection system.
Hail growth becomes vanishingly small when air temperatures fall below −30 °C (−22 °F), as supercooled water droplets become rare at these temperatures. [19] Around thunderstorms, hail is most likely within the cloud at elevations above 20,000 ft (6,100 m).
As tiny water particles that make up the cloud group together to form droplets of rain, they are pulled down to earth by the force of gravity. The droplets would normally evaporate below the condensation level, but strong updrafts buffer the falling droplets, and can keep them aloft much longer than they would otherwise.
Icing conditions exist when the air contains droplets of supercooled water. They freeze on contact with a potential nucleation site, which in this case is the parts of the aircraft, causing icing. Icing conditions are characterized quantitatively by the average droplet size, the liquid water content and the air temperature.
As the hailstone ascends it passes into areas of the cloud where the concentration of humidity and supercooled water droplets varies. The hailstone's growth rate changes depending on the variation in humidity and supercooled water droplets that it encounters. The accretion rate of these water droplets is another factor in the hailstone's growth.
The formation of ice particles in supercooled clouds allows those particles to grow at the expense of liquid droplets. If sufficient growth takes place, the particles become heavy enough to fall as precipitation from clouds that otherwise would produce no precipitation. This process is known as "static" seeding. [citation needed]
Water does not always freeze at 0 °C (32 °F). Water that persists in liquid state below this temperature is said to be supercooled, and supercooled water droplets cause icing on aircraft. Below −20 °C (−4 °F), icing is rare because clouds at these temperatures usually consist of ice particles rather than supercooled water droplets.
In clouds warmer than about −37 °C where liquid water can persist in a supercooled state, ice nuclei can trigger droplets to freeze. [ 1 ] Contact nucleation can occur if an ice nucleus collides with a supercooled droplet, but the more important mechanism of freezing is when an ice nucleus becomes immersed in a supercooled water droplet and ...