Seeing fog this time of the year during the early morning hours is certainly not uncommon. The featured chart looks at the frequency of fog reports by the automated weather station in Des Moines. The chart is a 2-D histogram showing the observation frequencies by hour of the day and week of the year. The highest frequencies are in the early morning hours, when the best conditions for fog exist (longest duration of overnight cooling, highest humidities, calmest winds and least vertical mixing). The summer months show a clear minimum in the mid afternoon and overnight periods. These are periods influenced by heating, which lowers humidities and well mixes the lower atmosphere.

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Tags:   fog