With morning temperatures in the 40s, one may not expect to see much snow with a temperature well above freezing. Dew point temperatures were in the 20s at the time. So how did locations like Des Moines and Audubon receive snow Tuesday morning? The reason has to do with a process called the wet bulb effect. When snow showers moved over the area, the drier air near the surface cooled and moistened as evaporation happened. Eventually the temperature cools to low enough temperatures to support snow! The featured chart compares air and dew point temperatures for three sites on Tuesday. Both Audubon and Des Moines received snowfall, Marshalltown did not. The bottom panel shows the difference between the air and dew point, known as dew point depression. You can see the wet bulb effect as the snow fell late morning and the return to pre-snowfall levels after the precipitation departed. As an aside, thunder snow was reported with this event!

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Tags:   wetbulb