Wind, within a narrow range. are doughnut-like formations of rolled snow that appear when moisture, snow, wind and temperature arrive in exactly the right combination. The snow must be a , sticky enough to adhere to itself but lying on a surface it will not cling to. The wind has to be strong enough to curl it and no stronger.
That window is why nobody sees them often. Too little wind and the snow stays put; too much and the roll is blown apart before it forms its . A slope will substitute for wind, with the snow rolling gently downhill into the same shape. Either way the National Weather Service calls them rare, distinctive finds that are impossible to replicate.
Sometimes a combination of mundane ingredients produces something unexpected, and the recipe for snow rollers is a good example. They are little doughnut-like formations of rolled snow, and they need just the right mix of moisture, snow, wind and temperature arriving at the same time.
The snow is the fussiest ingredient. It has to be a light dusting, sticky enough to adhere to itself, but lying on a surface it will not cling to. Both conditions at once is a narrow state: snow that sticks to everything will not roll, and snow that sticks to nothing will not hold together once it does.
The wind then has to thread its own needle. It must be strong enough to encourage the mounds of snow to curl up and form their signature loop, but not so strong that the whole thing gets blown apart. There is a second route to the same shape, which removes the wind from the problem: snow on a hill can gently roll and arrive at the same formation.
Either way, the National Weather Service describes snow rollers as rare, distinctive finds in nature that are impossible to replicate, and suggests appreciating the conditions that came together if you ever see one. A photographed field of them in the southern Idaho Panhandle had rollers about eighteen inches tall or more. One caution: this is a public education page, so the ingredients are given qualitatively and no thresholds are attached to any of them.
wat. A field of snow doughnuts means the wind got everything right at once.
wat wat
It is a case where the rarity is the information. Finding one tells you several variables were simultaneously inside narrow bounds on that particular morning.
How do we know?
The National Weather Service’s guide to unusual weather sets out the conditions snow rollers require, the alternative route down a slope, and a photographed field of them in the southern Idaho Panhandle.
What is still unknown
A public education page rather than research. It gives the ingredients qualitatively, with no thresholds for wind speed, temperature or snow depth, so what counts as the right mix is left to the reader.
Learn something?
Where to next
Does cloud seeding actually work?weather
What can a lichen tell you about the air coming off an oil refinery?wind
How can a cloud hang motionless over a mountain in a gale?wind