Some snow visited the state again on Tuesday, but nothing to get excited about. It was a cold and dreary day with highs only in the low 40s and low clouds. The featured chart presents the frequency of the height of the overcast cloud layer as reported by the Des Moines Airport. This is referred to as the ceiling. There are caveats galore with this plot including changes in instrumentation and reporting algorithms. These caveats explain some of the bright bands in the plot at certain levels. Putting those issues aside, you can see a clear annual signal with low ceilings much less common during the warm half of the season and the very low ceilings confined to winter. The reason being that lower atmosphere temperatures are warmer in the summer with strong surface heating, this makes it more difficult for the air to remain saturated as heating lowers relative humidities. In the winter time, fog and snow are more persistent leading to lowered reported ceilings.

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Tags:   ceilings