Why The Comfortable July Humidity?

Weather enthusiasts probably have noticed how low dewpoints have gotten the past couple of afternoons over northeastern Illinois and even non-weather enthusiasts probably noticed how amazing comfortable humidity levels have felt. Interestingly, a look at a map of surface dewpoints this afternoon shows a rather distinct difference between dewpoints (an absolute measure of the amount of moisture in the air) over most of the Chicago metro area versus northwest Indiana.

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map of dewpoints temperatures at noon Friday July 8th

Map of dewpoint temperatures at noon Friday July 8th.

 


So why is there such a big difference in dewpoints between northwest Indiana and northeast Illinois? A look at recent rainfall trends likely offers a bit of insight into this. Notice that during the past 14 days much of northern Illinois has been abnormally dry with rainfall less than 25% of average, and in many cases no rainfall at all. This is in stark contrast to northwest Indiana where rainfall is running as much as 200% of normal. Notice the very strong correlation between the areas with very low July dewpoints and the areas that have much below average rainfall during the past 2 weeks.

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map showing percentage of normal rainfall during the past 2 week
Map showing percentage of normal rainfall during the past 2 weeks.

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Map showing the actual observed rainfall totals during the past 2 weeks.

 
Rainfall adds moisture to the ground and allows vegetation to green up, both of these result in increased evaporation and evapotranspiration which add moisture to the air. Interestingly, this cycle can have a tendency to feed upon itself, as the dry ground lessens the amount of moisture in the air, which is needed to produce rainfall. This is why major droughts can become so persistent. While the area is not in a drought or even a developing drought yet, much of northern Illinois is experiencing a period of abnormally dry weather during a time of year which can often be quite wet.


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