DISCUSSION: Have you ever been impacted by a classic mid-Winter snowstorm and found that there was snow falling over your region according to Doppler radar imagery, but there was nothing falling at the ground? Most people get confused about when this situation unfolds with an approaching snowstorm. However, there is a relatively simple explanation for why this type of situation can be realized in association with any given snowstorm. That simple explanation comes with the meteorological term which is known as virga. Virga is simply snowfall or any other type of precipitation which is unable to reach the ground.
As a classic inland or coastal winter storm is approaching a given region, there is often a particularly cold air mass in place. The exceptionally cold air which happens to be in place across the specific region will typically have the characteristics of being a drier air mass. As a result of there being a drier air mass present over the given region, this will consequently lead to there being a larger dew point depression. A larger dew point depression is simply the measurement which is defined by the difference between the air temperature and the dew point temperature at a given location. The larger the dew point depression happens to be at a given location, the longer it will take snowfall to evaporate the respective layers of the atmosphere from cloud level and on down to the surface of the Earth.
As the snowfall continues to evaporate through the depth of the drier low levels of the troposphere, this consequently leads to a decrease in the regional dew point depression values which ultimately leads to an increasingly moister near-surface layer. Thus, as the near-surface to sub-cloud layers become increasingly moister with time, this will gradually allow snowfall to finally reach the ground and accumulate on the ground if the surface is sufficiently cold. Hence, if you ever find yourself looking at a news station’s regional radar imagery or your local National Weather Service forecast office radar imagery and see snowfall over your region (but not quite reaching the surface yet), you will now have more insights as to why this is happening.
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© 2018 Meteorologist Jordan Rabinowitz