Most of the airport weather stations produce an estimated pressure that is extrapolated to a mean sea level surface (MSLP), which effectively attempts to remove the elevation above sea level of the site. For those of you that geek out on weather maps, this pressure is what you see analyzed on surface observation maps as the removal of elevation allows for more meaningful horizontal comparisons of pressure. Today's featured chart presents the frequency of MSLP by week of the year based on hourly reports from the Des Moines Airport. On Wednesday, the MSLP reading at Des Moines topped 1030 millibars. Such high pressure values are somewhat rare overall for the year with a frequency of just 5%, but the frequency is over 10% for the winter months of December through February. The increased winter time frequency is often attributed to colder air being more dense and thus having a higher pressure, but that isn't the full story and the chart shows would not explain the increase in the lowest pressure frequencies during the cold months as well. The full answer is a complex interplay between air temperature, weight of water vapor, and strength of air movements in the atmosphere.

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