Late Season Snowfall in Spring
Learn How Snowflakes Fall in Above Freezing Temperatures
Mar 25, 2008
Late Season Snowfall
As the special weather reports issued from the National Weather Service this week warn, snow is predicted for the Pacific Northwest and the Rockies. The NWS predicts snowfall as low as the valley floors and foothills during this last week of March. Late season snow in the Pacific Northwest is unusual but not as rare as some may believe. The effects of La Nina hit the area this year with a wetter than usual winter, bringing more snow. When comparing climate data records over the past 50 years, numerous snowfall data entries have been recorded during the month of March, and even into April.
How Often Does it Snow in Spring?
Although spring is here and the calendar says it's March, late season snow does occur. As climate data records are regionally specific, it can be difficult to compare large sets of data over a long period of time. State climatologists compare snowfall data for certain regions and cities, figuring predictability of snow during spring months. The National Snow and Ice Data Center keeps snow and ice records for regions across the United States.
Spring brings a state of transition as unstable air consumes the weather world. A mix of precipitation, sun and thunderstorms and tornadoes all have a part in spring weather extremes. As the ocean temperatures warm, cold air from the north continues to dip across the United States. As warm and cold fronts clash and collide, unstable conditions persist.
Above Freezing Snowfall
Although it's a fact that snowflakes need atmospheric temperatures at or below freezing to form, it's also true that with the right conditions, snowflakes can reach the ground even if temperatures are above freezing. These snowflakes fall to the Earth's surface without completely melting, witnessed by observers in wonder. This weather splendor depends a great deal on the humidity level of the warmer air near the ground.
Snowflakes form within clouds, as water vapor condenses onto tiny dust particles forming ice crystals. Freezing or below freezing temperatures are required for snowflake formation. As the snowflakes fall, ideal conditions for snow accumulation require uniform freezing temperatures from the atmosphere above to the surface below. However, snowflakes can persist through above freezing temperatures near the ground.
As the snowflakes fall, they begin to evaporate. In result, the warming of the snowflake, causes the air surrounding it to become cooler. This happens because of conduction, as energy transfers. As the snowflakes evaporate in the warmer air below, they actually cool. The air near the ground must be dry, or contain a minimum amount of moisture. Dry air is needed in order for evaporation to occur, adding to the snowflake's longevity.
Strong downdrafts make snow fall faster. Hail is a good example of ice falling at a rapid speed, so fast it does not melt as it falls, even during the summer thunderstorms. This lessens the effects of melting, and actually can make the air near the ground colder. This explains how snowflakes fall to the ground during spring, when the air may be warmer.
References:
The Weather Channel
National Snow and Ice Data Center
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