Freezing Rain and Ice Pellets
Ice pellets (also referred to as sleet by the United States National Weather Service) are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.
Ice pellets form when a layer of above-freezing air is located between 1500 meters (approximately 5,000 feet) and 3000 meters (approximately 10,000 feet) above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze, and freezing rain will be the result at the surface. A temperature profile showing a warm layer above the ground is most likely to be found in advance of a warm front during the cold season , but can occasionally be found behind a passing cold front.
Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet (or meters), just above the surface, and then freeze upon impact with any object they encounter. The resulting ice, called glaze, can accumulate to a thickness of several centimeters. The METAR code for freezing rain is FZRA.
Liquid, Freezing, and Frozen Precipitation
Precipitation forming and remaining liquid falls as rain or drizzle. Sublimation forms snowflakes, and they reach the ground as snow if temperatures aloft remain below freezing.
Precipitation can change its state as the temperature of its environment changes. Falling snow may melt in warmer layers of air at lower altitudes to form rain. Rain falling through colder air may become supercooled, freezing on impact as freezing rain; or it may freeze during its descent, failing as ice pellets. Ice pellets always indicate freezing rain at higher altitude.
Sometimes strong upward currents sustain large super-cooled water drops until some freeze; subsequently, other drops freeze to them, forming hailstones.
Exam Question Tips:
If you have to fly through a warm front when freezing level is at 10000 feet in the warm air and at 2000 feet in the cold air, at which altitude is the probability of freezing rain the lowest?
a) 9000 feet
b) 12000 feet
c) 5000 feet
d) 3000 feet
Answer is 12000 feet.
Draw a Z. The sloping bit represents the frontal surface viewed from the side, the top of the Z is the freezing level in the warm sector, 10,000ft in this instance, and the bottom of the Z is the freezing level in the cold air. Freezing rain exists where rain falls, initially above zero, from the air in the warm sector, the top of the Z. As it passes through the front it falls into an area where the temperature is below freezing, it is now freezing rain. When it gets below 2000ft in the cold air, the bottom of the Z, it has now warmed to be above zero again and therefore is no longer freezing rain.
This means freezing rain could exist between 10,000ft and 2,000ft. The only answer not in this range is 12,000ft.
Freezing fog exists if fog droplets are super cooled