Based on hourly temperature reports from the Des Moines Airport, the featured chart presents the frequency of a given hour reporting the first fall temperature below 32 degrees. Note, all events are presented in Central Daylight Time to rectify the dataset. The midnight to 8 AM period is the obvious preferred time as these events are almost always driven by accumulated radiational cooling and the time for such cooling is when sunshine is not present and a number of hours after the sun set for the previous day. An interesting outlier is highlighted that happened just last year! The first event for 2020 happened with the 11 AM observation on 19 October. You may recall the wild weather we had then with a major early season snowfall impacting the state. So far this morning, Des Moines has remained above freezing but there is still time before the sun rises and we'll take another run at freezing Saturday morning.

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