Graupel is a spherical form of precipitation. It forms at temperatures around 0°C or slightly below zero when water droplets clump together with snow crystals in the cloud. The globules can be up to 5mm in size. If the grain size is less than 0.1 centimeters, it is referred to as sleet. Sleet showers typically occur in late winter and spring during thundery showers.
As long as sleet and surface frost are not covered by snow or drifting snow, they do not pose a problem. But if they do, extreme caution is required! It is not for nothing that sleet is referred to as a weak layer that acts like a ball bearing. The concept of a ball bearing is not quite correct, because the snow slab comes off because the weakly bonded layer of sleet breaks, not because it slips on the graupel grains. But the catchphrase "ball bearing" also makes the danger clear for all those who do not deal in detail with different types of breakage.
In addition, snow-covered sleet is one of the most difficult hazard patterns to recognize, as it can only be seen by looking into the snowpack. But even that is difficult, because a few meters to the left, right, above or below, the layer may no longer be there, or it may already be there.
Luckily, sleet is usually only deposited on a small scale in depressions, hollows, etc. This kind of weak visibility is rarely found over a large, contiguous area. Furthermore, these events primarily occur in spring, when the temperatures and radiation are intense enough for rapid settling and bonding with the graupel layer to occur, which in turn leads to a rapid calming of the avalanche situation. This means that snow-covered sleet is usually only a problem for a few days.
In order to recognize a possible danger from an existing layer of sleet, very intensive and precise weather observation and basic meteorological knowledge are required.
