Answer: Corn "Sweat"
If you are not from the Midwestern United States, there is a very good chance that you believe we are pulling yours Leg, but we assure you, we are not. During the height of summer heat in the Midwest, corn is a major contributor to the humid air humidity. Corn, well, sweats.
When it gets really hot outside, corn loses moisture through evaporation ̵
Along with the sheer volume of maize in the Midwest, there are 96 million hectares of corn planted in the United States, and most of them in the Midwest – actually increasing the humidity in the area significantly. The phenomenon is called "corn sweat" and can make miserable days in corn-covered areas of the Midwest in July and August, as highlighted in the USDA corn production map shown here.