Rare snow phenomena

Rare snow phenomena are different events other than the usual snow that almost everyone, especially in the Northern Hemisphere, has seen at least once in their lives. For many residents of northern countries such as Finland it is a familiar weather event for most of the year. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what rare snow phenomena is and how does it work. In particular, we are going to deal with the natural snowmen, penitentes, and ice volcanoes.

Natural snowmen

Most people know how to make a snowman, or have seen one being made, at least in a video. But sometimes, a big snowball can form on its own, without any human intervention. This extremely rare phenomenon is called snow rollers.

Snow rollers can vary dramatically in size, and can get as big as a car, but normally don't exceed 2 cm (a bit under an inch) in diameter.

They are formed when gravity, or wind (or both), make pieces off the superficial layer of snow roll downhill in a carpet-like fashion. Which is why, unlike human-made snowballs for snowmen, snow rollers have cylindrical or donut-like shapes (when the wind blows out the center).

This phenomenon is considered rare because, for a roller to form, several conditions need to coincide: the temperature should be just a little bit above freezing, and there should be a layer of wet but loose snow that doesn't stick to the layer underneath, for example, ice. Aside from that, the wind should be strong enough to roll the rollers, but not so strong that it destroys them; about 50 km/h (31 mph) is just right.

It is not easy to find rollers, but there is some good news for those hoping to see them: when all the conditions do coincide, rollers appear in massive numbers, sometimes in the thousands. They are most easily found in hilly areas, where it's easier for gravity to do its job.

Natural snowmen always leave a mysterious trails. Photo: Brian Bayliss via Bbc.com

Penitentes

High up in the mountains in tropical and subtropical regions, you can find long thin blades of hardened snow or ice. These formations are called penitentes, or nieves penitentes (Spanish for "penitent snows"), as they remind you of a crowd of kneeling people doing penance, as they were seen by those who first discovered them. The formation evokes the tall, pointed hoods and habits worn by brothers of religious orders in the Processions of Penance during Spanish Holy Week. In particular, the brothers' hats are tall, narrow, and white, with a pointed top.

The formation and growth process of penitentes is complex, and not yet fully understood. It seems that they start with a relatively flat snow cover that sublimates, i.e.changes from a solid state to a gas without melting, in thin air.

But because of different air saturation levels, the snow sublimates with different intensities. In other words, if the air is more humid, the snow evaporates more slowly, as there is less free space in the air for the water molecules. In this way, burrows are formed in the snow, which temporarily "lock" sunlight between their reflective walls, and thus absorb more heat.

The blade shape is caused by the combination between snow sublimation and sunlight absorption, models demonstrate. Most penitentes are found in the Andes and in the Himalayas, as a rule, above four thousand meters.

Penitentes in Chile. Photo: European Southern Observatory (ESO)

Ice volcanoes

To be honest, currently scientists are not observing any true cryovolcanism on Earth, when water, ammonia and methane erupt from a volcano instead of hot magma. But there is one phenomenon that looks so similar to volcanism that it got unofficially baptized as an ice volcano.

We are talking about mounds at the borders of ice shelves near the shores of large lakes and seas, from which streams of water and snow "erupt".

This is because of the waves that can pierce holes beneath the ice shelf, if they are more than a meter high, and the wind is at least 40 km/h (almost 25 mph). The water rushes up through the hole and freezes as it falls, creating walls around its "throat".

For an ice volcano to emerge, it needs some depth, so reefs and shallows don't slow down the wave before it hits the ice shelf. When that happens, the energy of the wave is concentrated in one spot because of surface irregularities in the ice shelf.

Such "volcanoes" form quickly, in just a couple of hours, and can be destroyed pretty easily too, especially if there is warm weather or a strong storm. This phenomenon is most often observed at the shores of Lakes Erie, Ontario and Huron. While being quite spectacular and much less dangerous than magma volcanoes, ice volcanoes are still not advisable to approach, because of the high risk of falling through the shelf into ice-cold water.