Sleet is a form of precipitation in which snow crystals clump together into small balls up to 5 mm in size due to frozen water droplets. Particle sizes of less than a millimeter in diameter are also called Griesel .
Difference from hail
Granules of sleet are significantly smaller than hail and only have a maximum diameter of 1 to 5 millimeters. Their density is lower than that of hailstones and they have a rougher structure. As a result, they fall more slowly and can hardly cause any damage. In contrast to hail, sleet falls mainly in winter at temperatures around 0 ° C.
Origin and types
Sleet only occurs when the clouds do not contain too much moisture , which is often the case in spring . Sleet often occurs in dry polar air . If the clouds are too moist , hail is more likely to occur.
There are several types of sleet:
- Frozen barrels have a soft, opaque core that is covered by a transparent layer of ice. They jump up when hitting hard ground. They usually occur in connection with thunderstorms and rain. Freezing barrels occur when the temperature in shower or thunderclouds falls below −4 ° C and there are strong updrafts and downdrafts . If the supercooled water droplet hits a snow or ice crystal, they clump together and the opaque core is created.
- Hoar barrels are completely opaque and soft. When they hit hard ground, they jump up and often break. They usually occur together with snowfall . Hoar frost occurs when snow thaws in high clouds and then freezes again.
- Griesel or snow gravel is a form of sleet with a diameter of around one millimeter or less. The granules are opaque aggregates of snow crystals and only form at temperatures below 0 ° C. Griesel occurs only in stratified clouds like Stratus and is therefore never seen in connection with a shower .