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SnowFlurry 17 2019/20 | Hazard pattern 9: Snow-covered sleet

What is sleet and how does it affect the avalanche situation?

by Stefanie Höpperger 03/21/2020
Due to the current situation caused by COVID 19 and the resulting curfew, we snow rummagers are also staying at home and using the time to share old, interesting profiles or technical knowledge with you. If you refresh and expand your snow and avalanche knowledge now, you will be all the better equipped for the next season - because it is sure to come.

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.

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Development of sleet

When supercooled water droplets in cumulus clouds hit the falling snow crystal, they freeze to the crystal and turn into sleet grains. Vertical up and down movements in shower cells can stir up the graupel grains again, whereby a new layer can freeze again and again, allowing the grain to grow. The graupel grains grow until they fall to the ground due to their increasing weight.

The vertical movements cause the graupel grains to pass through different temperature layers in the cloud, often with different wind strengths.

In colder temperatures, the water droplets freeze directly onto the ice or snow crystal. This gives the sleet grain its milky white color and makes the grain more brittle.

At slightly below freezing temperatures, the water droplet freezes more slowly and a liquid layer forms around the snow crystal or the already frozen droplet for a short time, which freezes again with a delay. This layer forms a clear, transparent surface around the milky white core of the sleet grain. The reason for this is the release of heat during the freezing process.

There are three types of sleet:

Frost sleet

This occurs when the temperature drops below -4 degrees Celsius in a shower or thunderstorm cloud and strong updrafts and downdrafts prevail.

Frost sleet has a soft, opaque, milky white core that is encased in a transparent layer of ice.

Sleet

Occurs when the updraft is so strong that the graupel grains can reach an altitude of 7-8 km, where temperatures of around -20°C to -30°C prevail. It usually falls to the ground together with snowflakes.

Grizzle

Is a form of sleet, but the grains are much smaller with a diameter of 1 millimeter or less. Drizzle forms in layered clouds (stratus) and not in cumulus clouds, as they occur in showers.

Difference to hail

In contrast to sleet, hail forms when water droplets (not snow crystals) are whirled into higher, colder layers by strong updrafts and freeze there to form ice. This process happens again and again and allows the hailstone to grow. Sleet is softer and opaque with a milky white core. Hailstones, on the other hand, are glassy and hard.

Hail mainly forms in summer, whereas sleet forms in the winter months and in spring. Sleet grains have a lower density than hailstones as they have more air pockets. They are also smaller, with a maximum size of around 5 mm, whereas hailstones can be the size of a tennis ball.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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