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Snow at 40 Degrees

Introduction

Snow reaching the ground with temperatures near the surface in the upper 30s to lower 40s oF is not too uncommon. Snow at even warmer temperatures has been observed. Such an event can seem a little strange when you stand outside with snow falling and the air just doesn't feel cold enough. Some of the reasons why snow can reach the ground at warm temperatures are less obvious than others. The following is a list of contributing factors.

Ice crystal size

The larger the ice crystal or ice crystal aggregate, the more melting that has to occur to prevent part of the snowflake from reaching the ground.

Ice crystal fall rates

The speed at which ice crystals fall varies upon how aerodynamic they are. Dendritic ice crystals are less aerodynamic than other crystal shapes due to their feathery tree branch structure and tend to resist falling as fast. Heavily rimed ice crystals or snow pellets are smoother and more round so they fall faster. The faster the ice can fall through the warm air, the less time for melting before it reaches the ground. Some dendrites can get very large however and still take a longer time to melt completely.

Steep lapse rates

If the temperature at the surface is 40 oF, a strong drop in the temperature with height, such as close to adiabatic, versus a slower decrease with height, results in only a thin layer of warm air that the snow has to survive to reach the ground. Stronger lapse rates are more likely late in the snow season, such as March into May, and on days when the sun is shining between intermittent snow showers.

Cooling near the surface of the ice crystals

Sublimation, melting, and evaporation are all cooling mechanisms where water takes heat from the air to change from a lower energy state to a higher energy state. Sublimation is a term not commonly used in layman discussions of weather but is simply the process of water changing from ice to water vapor without first melting. Evaporation is generally used as a catchall term to include both evaporation and sublimation.

In the presence of dry air, the sublimation of water molecules from the ice crystals cools the air in contact with the surface of the crystals. As conduction and mixing attempt to warm the air, sublimation balances off the warming. As some of the ice melts, the melting process cools the air. The water that melts then evaporates, also cooling the air to slow the melting rate. The consideration here is that the absorption of heat keeps the air near the ice crystals cold enough to survive the trip through the warm air without completely melting. The drier the air and the deeper the layer of dry air, the better the chance that ice crystals can reach the ground. Of course, the danger here is that the snow completely dissipates in the dry air. An interesting article on the ScienceBits website called "Snow at above freezing temperatures" discusses some aspects of the cooling process in high detail.

References

Snow at above freezing temperatures - ScienceBits (www.sciencebits.com)