Some of the coolest air this season has settled into Iowa overnight with temperatures stepping below previous lows for the fall season. The featured chart presents some diagnostics of these step downs in temperature with the 'step down' representing the setting of a new (since 1 July) cold temperature for the year. The average number of these events is around 19 for Des Moines, which is roughly one out of every ten days for the half year period. The second and bottom charts present histograms of the temperature levels and day of year whereby these steps occur. The increased frequencies at the start of July is an artifact of the algorithm, but there is a clear jump around mid September, which is about the time cooler air masses start arriving. There is also increased frequencies in the 50s, which is a temperature a bit lower than our supporting/typical 60 degree dew points in the summertime. There is also another jump just below freezing. These are levels that it takes a bit of time to push through as dew point and non-frozen soils offer resistance to cooling below those levels.

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Tags:   fall   spring