| Part of the nature series |
| Weather |
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Glossaries |
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A snow roller is a rare meteorological phenomenon in which large snowballs are formed naturally as chunks of snow are blown along the ground by wind, picking up material along the way, in much the same way that the large snowballs used in snowmen are made. They can be as small as a tennis ball, but they can also be bigger than a car. [1] Most snow rollers are a few inches/centimeters wide. [2]
A snowball is a spherical object made from snow, usually created by scooping snow with the hands, and compacting it into a roughly fist-sized ball. The snowball is often used to engage in games, such as snowball fights. Snowball fights are usually light-hearted and involve throwing snowballs at one's friends or family. The pressure exerted by the hands on the snow is a determinant for the final result. Reduced pressure leads to a light and soft snowball. Compacting humid or "packing" snow by applying a high pressure produces a harder snowball, sometimes called an ice ball, which can injure an opponent during a snowball fight.
Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects, an important one being its velocity ; another the density of the gas involved; another its energy content or wind energy. Wind is also a great source of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.
Alternative names for snow rollers include: snow bales, [3] snow donuts, snownuts and wind snowballs. [2] Unlike snowballs made by people, snow rollers are typically cylindrical in shape, and are often hollow since the inner layers, [2] which are the first layers to form, are weak and thin compared to the outer layers and can easily be blown away, leaving what looks like a doughnut or Swiss roll.
A doughnut or donut is a type of fried dough confection or dessert food. The doughnut is popular in many countries and prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and franchised specialty vendors.
A Swiss roll, jelly roll, roll cake, or cream roll is a type of sponge cake roll filled with whipped cream, jam, or icing.
The following conditions are needed for snow rollers to form:
- There must be a relatively thin surface layer of wet, loose snow, with a temperature near the melting point of ice. [2]
- Under this thin layer of wet snow there must be a substrate to which the thin surface layer of wet snow will not stick, such as ice or powder snow. [2]
- The wind must be strong enough to move the snow rollers, [2] but not strong enough to blow them apart.
- Alternatively, gravity can move the snow rollers as when a snowball, such as those that will fall from a tree or cliff, lands on a steep hill and begins to roll down the hill. [4]
The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at a standard pressure such as 1 atmosphere or 100 kPa.
Because of this last condition, snow rollers are more common in hilly areas. [1] However, the precise nature of the conditions required makes them a very rare phenomenon. [4] [5] [3]
The term snow roller also applies to a horse-drawn implement used in the mid to late 19th century (though tractor-drawn examples are still in limited use today, mainly at ski resorts) to pack snow.[ citation needed ] It was mainly used in areas with high snow accumulation for access for horses and horse-drawn sleighs or wagons equipped with skis or runners. The implement was essentially, a large, very wide, wooden wheel, weighed down with stone or concrete.
