Frost is not snow. Frost is a layer of ice that forms on cold surfaces via a direct conversion of water vapor to solid ice. It occurs when the temperature of Earth’s surface falls below freezing. Frost resembles dew and occurs on cold, still nights.
Hoar frost is the most common type. It appears as a cover of feathery ice crystals that form when dew freezes. It can also be a combination of dew and water vapor that freezes before it has a chance to become liquid. Rime frost, a less common occurrence, takes place when supercooled water droplets in fog touch a sub-freezing surface and turn to ice, forming deposits of rough ice crystals.
Frost occurs everywhere on the globe except in tropical climates where it can only at high altitudes, such as mountaintops.
While not as slippery as ice, frost still makes its presence known on roads. It also inflicts damage to fruit trees and vegetation, which is why gardeners stay abreast of the frost forecast.
Sleet, like frost, is only possible at or below 32 degrees Fahrenheit, but rather than forming on the ground, sleet precipitates like snow or rain in the form of ice pellets up to a fifth of an inch in diameter. It can also occur when raindrops descend through a layer of sub-freezing air, becoming “supercooled water”—meaning a liquid that drops below freezing without becoming a solid.
If snow comes down through air temperatures above 32 degrees, it will melt, but heat always travels to where there is less heat (a phenomenon called “thermal expansion”). So, while warmer air melts the snow, the cold from the snow itself absorbs the heat, which can cause the air to drop below 32 degrees, refreezing the melted snow. The result is sleet.
Here in the Northland, where we get an average of 80 inches of snow between late autumn and early spring, you might have heard it said that “it’s too cold to snow.” This isn’t strictly true. While the likelihood of snow decreases with temperature, snow is possible even on severely cold days, given the right conditions, but it is unlikely.
This is because snow requires a combination of saturated air, enough wind to lift the saturated air, and temperatures that allow snow to reach the surface. As temperature decreases, the air’s capacity for saturation decreases. So, it’s not that it’s too cold to snow, but that the cold makes it too dry to snow.
When it does snow on very cold days, the flakes are smaller (less than a twenty-fifth of an inch in diameter) and accumulation is lower. When surface temperatures are warmer, ice crystals tend to stick together, creating large flakes (up to three inches in diameter) and resulting in heavier accumulations.
Snow is a great insulator, which slows down the freezing process within the Earth’s surface, so to some extent, snow is both a cause and an effect of warmer winter temperatures.
This is also why blizzards—snowstorms with sustained winds of at least 35 mph—are more common in the early and late winter, when temperatures are shifting from warm to cold or cold to warm.
The worst blizzard on record struck the eastern United States on March 11, 1888. New York City was paralyzed under 30 inches of snow and more than 50 inches came down in other areas. Wind speeds reached 80 mph, and temps dropped to nearly zero (blizzards tend to bring cold air in their wake). Due to the effect on America’s largest city, electric and communication lines were placed underground, and the storm fueled efforts to install a subway system.
The “Storm of the Century” impacted a wider area on March 12-15, 1993. A cyclonic storm formed around a low pressure system in the Gulf of Mexico, dumping up to 44 inches of snow across 25 states from southern Florida to eastern Kentucky and up into Canada before dissipating over the North Atlantic. Every airport from Atlanta to Maine closed at some point, grounding about a quarter of US flights.
However, thanks to advances in meteorology, this massive storm did not strike without warning. As a result, the death toll was 270, as opposed to the 400 lives lost in 1888, despite the fact that many more people were affected by the 1993 storm.
Look for average snowfall and temps this December. A storm may blow through shortly before Christmas, resulting in mild conditions for the holidays.