On the 11th and 12th of February in 2003, the people of central Illinois witnessed a rare phenomenon that occurred due to near perfect weather conditions. During the morning of February 11th, between one and four inches of snow fell across central Illinois. In the evening, 40 to 60 mile per hour wind gusts were recorded in many areas, propelling a cold front through the region. The following morning residents noticed that fields, lawns and other open areas were covered with log-shaped snowballs known as snow rollers.
Snow rollers have been said to resemble the bottom of a snowman, but really they are more elongated, often with a hollow space in the middle, making them look more like a donut or jelly roll. The size of a snow roller depends on the cohesiveness and weight of the snow, the slope of the terrain and the strength of the wind. They can be as small as a golf ball or as large as a 30 gallon drum. Typically, the diameter of a snow roller is 10 to 12 inches and diameters of up to 3 feet have been recorded. Although they are more common in mountainous or hilly areas the phenomenon that residents of central Illinois witnessed is one that most will go their entire lives without seeing.
The reason snow rollers are so rare is because of the near perfect weather conditions required to form them. First, the ground must be covered in a crusty, icy snow, which falling snow will not stick to. An inch or so of loose, wet snow must then accumulate on top of the previous snow. This top layer of snow needs to be near 32 degrees Fahrenheit. The slope of the hill must be flat enough that it will allow enough wind to propel the snow roller, while being steep enough that gravity can also assist in fueling it. Snow rollers are started when a gust of wind scoops up a ‘seed’ from the upper layer of snow. Wind and gravity propel this ‘seed’ and it accumulates more snow as it rolls. In perfect conditions, this will result in a snow roller with a hollow spot in the center and will leave a trail behind.
Despite their rare nature snow rollers have been spotted in Idaho, Kansas, Nebraska, Vermont and Illinois among other locations. So, if you notice or live in an area that experiences these weather conditions, keep your eyes open. You may just be lucky enough to see one of these rare phenomenons for yourself.
