The bottom dropped out for temperatures over northern Iowa on Tuesday morning thanks to ample new snow cover, a surface high nearby allowing for clear skies and calm winds. Waterloo was able to reach -21 degrees and the Osborne Nature Center south of Elkader was able to reach -25! Having calm winds certainly helps to increase the cooling of the air near the surface as warmer air aloft is not mixed downward. The featured chart presents the frequency of having a 5+ knot (~6 MPH) wind speed by air temperature during January for Waterloo. The relationship is rather clear as colder temperatures have less frequent winds at the given speed. Note that for values below -20, the dataset population rapidly decreases and thus gets a bit noisy.

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