Even while quite cold, it was nice for most of the state to see a bit of sunshine yesterday and get rid of the low clouds and fog. The featured chart looks into the frequency of horizontal visibility as reported by the Des Moines airport weather sensor by week of the year. The data range partitions are somewhat tricky as observation techniques and reporting standards have changed over the period of record, but the annual signal is rather clear. The middle of December does appear to have the highest frequencies of lowest visibilities by a slight margin over weeks during the rest of winter. The months of June and July have the lowest overall frequencies of reduced visibilities as solar heating is the strongest and often works to make quick work of early morning fog.

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