The most common precipitation types are rain, snow and hail. However, there are other types of frozen precipitation which we are going to talk about: snow and ice pellets.
It’s common knowledge that water turns into ice at temperatures below 0 degrees Celsius. In practice, water needs more than just sub-zero temperatures to freeze.
When a drop of clean water travels through the freezing air, it does not turn into ice until it collides with a fine particle (which might, for example, be a particle of dust, pollen or smoke) and then starts to freeze around this. The random particle thus becomes the core of a future snowflake, the base around which the ice crystal grows.
Heavy precipitations fall from mixed clouds which contain both ice crystals and supercooled water. Cumulonimbus, stratus and altostratus clouds all have these features and their structure is also where the most water vapor is accumulated. That is why snow falls from such clouds.
In some cases, usually at temperatures close to zero in temperate climates, it may both snow and rain at the same time. In such conditions, a cloud can have layers saturated with supercooled droplets where they continue to collide and freeze into the snowflake.
An ordinary snowflake (left) collects supercooled water and becomes a snow pellet (right). Photo: 9NEWS
Then a ball of frost known as rime begins to form around the beautiful and symmetrical ice crystal. The snowflake grows thicker, loses its transparency and can form white uneven particles up to 2-5mm in size, which are known as graupel or snow pellets.
Snow pellets are common in autumn and spring, and they usually fall before it starts to rain or snow. It may start raining immediately after snow pellets have fallen, as pellets may melt before reaching the ground.
Although you can hear snow pellets “hitting” the ground, they are not dangerous — they are fragile and break up easily once they’ve touched the ground. In the mountains, however, thick layers of dense snow pellets (20-30cm) present a high risk of avalanches.
Sometimes snowflakes and pellets melt and freeze again before reaching the ground. This happens when a layer of above-freezing air is located beneath the cold “snow” cloud.
In this case, when a completely or partially melted snowflake or pellet falls back into a sufficiently thick layer of sub-freezing air closer to the surface, it can turn into another form of precipitation — ice pellets.
Ice pellets have the same dimensions as snow pellets (up to 2-5mm) but differ in shape and structure. Snow pellets are white and very fragile, while ice pellets are a translucent ball of ice with a white core, which make a noticeable “tap” when they hit the surface and don’t break but bounce off the ground.
Ice pellets are most likely to be found in autumn or spring, usually in advance of a warm weather front. Occasionally, however, they can be formed in other weather conditions, for instance, behind a cold front or with a stationary front.
If the warm layer is too thin and its lower boundary is close to the ground, the ice pellets will not have time to re-freeze before hitting the surface.
In this case, we are talking about freezing rain. Freezing raindrops freeze as soon as they touch the ground, creating a coating of slippery ice on surfaces. Freezing rain poses significant hazards for cars and pedestrians and often creates large power outages.
Text: Windy.app team
Cover photo: Philip-Bunkens / Unplash