GREAT FALLS — We are taught in grade school that water freezes at 32 degrees. Have you ever wondered why it can still snow at temperatures above the freezing point? Under the right circumstances, snow can actually fall with a temperature at the surface as high as 50 degrees. There was even a report of snow falling at 53 degrees in Jacksonville, Florida!
In Montana, it is actually fairly common to see warm temperature snow compared to other parts of the country. This has to due with the availability of very dry air and also very cold air. The two really go hand in hand. There is no shortage of cold-enough air above the surface for snow to form nearby, essentially throughout the entire year. Showdown Ski Area saw snow on Independence Day in July 2018!
As a snowflake falls to the ground, the temperatures and humidity it will encounter during its decent ultimately determines the precipitation type seen on the ground. A consistent, warm layer leads to rain. A snowflake that melts in an above freezing layer and refreezes once it reaches a shallow layer of below freezing air near the surface reaches the ground as freezing rain. A deeper layer of that below freezing air near the surface is what causes sleet to form. Of course, a consistent below freezing layer is when the snowflake makes it all the way to the ground! But, how does humidity factor into the precipitation type?
Dry air at the surface actually correlates to snow falling at higher temperatures. As a snowflake falls through a warm layer of air, conduction causes the snowflake to melt. If the airmass is dry enough, it will evaporate any moisture on the fringes of the flake. Once the moisture evaporates, it leads to a localized area of cooled air surrounding the flake. This is a process known as evaporative cooling. The air must be extremely dry for this to occur. For example, for snow to fall at 50 degrees the relative humidity would need to be less than 8%. That level of dryness is already fairly rare, especially with precipitation falling.
The snowflake can survive if the cooling due to evaporation outweighs or balances the warming due to conduction from the surrounding, warmer air. Generally, the snow is rather light in these situations otherwise the entire airmass cools off and temperatures at the surface begin to fall. It is more common in Montana than in other areas of the country due to the availability of dry air.
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