For July, the combination of air temperature and dew point readings on Tuesday made for a pleasant day. The featured plot attempts to put the dew point values into a bit of climatological context for July for Des Moines. At each air temperature, a dew point climatology is constructed with the purple area representing the 25th to 75th percentile, the green area between the 5th and 95th and the blue line is a simple average. The black dots are the observations from Tuesday. The black line computes the relative humidity value for the combination of air temperature and average dew point. For yesterday, you can see that the values were generally below average for the given temperature. The chart nicely shows the decrease in dew point as temperatures start to exceed the mid 90s. Typically, such temperatures are driven by drier air masses originating from areas outside of the Gulf of Mexico and even drying due to down-sloping from the Rocky Mountains. Additionally, drier air masses are required to partition more of the solar energy into heating the ground than water vapor in the air so to drive higher temperatures. The warmest of air masses are also likely well mixed, so near surface humidity is mixed vertically with drier layers of air.

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