The featured chart presents an analysis of duration and strength of the first fall periods at or below freezing for Des Moines based on one minute interval temperature observations since fall 2000. Each event represents a sequential fall date during which the low temperature reached 32 degrees or colder. For example, event three would be the third day during the fall season at or below 32. The time duration is the period between when the temperature hit 32 degrees and rose back above 32. The maximum duration period for each date was used in cases of multiple periods on a given date. The box and whisker plots represent the spread of duration. A strength metric is also calculated and is simply the temperature difference from 32 multiplied by time. So what is the moral of the story here? The initial dips below freezing are brief (1.5 hours on average) and shallow. Both the duration and "depth" increase with sequential event with more noise than signal appearing after the fourth or fifth event.

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Tags:   freeze   fall