Graupel

Sleet is a form of precipitation, in which snow crystals clumped together by frozen- water droplets to small, up to 5 mm beads. For grain sizes with a diameter of less than one millimeter is also called Griesel.

Contrast to hail

Sleet are compared to hail significantly smaller and have only a maximum diameter of 1 to 5 millimeters on. Their density is less than that of hailstones, so they fall more slowly been due to the larger surface area and thus additionally the rougher structure. Therefore, they can hardly do any damage. In contrast to hail sleet falls mainly in winter at temperatures around 0 ° C.

Origin and species

Sleet is possible only if the clouds do not contain too much moisture, which is often the case in the spring. Frequently sleet occurs in dry polar air. If too moist clouds rather hail is formed.

There are various types of sleet:

• Frost sleet, which have a soft, opaque core that is covered by a transparent layer of ice. They are jumping on hard ground upon impact. They usually occur in conjunction with thunderstorms and rain. Frost sleet occur when the temperature falls in rain or storm clouds below -4 ° C and strong updrafts and downdrafts occur. Does the supercooled water droplets on a snow or ice crystal, so they clump together and creates the opaque core.
• Hail, which are completely opaque and soft. When hitting a hard surface they jump and break often. They usually occur along with snowfall. Hail is formed when snow thaw in high clouds and freezes again.
• Griesel or snow grains is a form of sleet with a diameter of about one millimeter or less. The grains are opaque aggregates of snow crystals and form exclusively at temperatures below 0 ° C. Griesel occurs only in layer clouds such as stratus and is therefore never be observed in conjunction with a shower.