US summer afternoon temperatures have not increased over the last 60 years, but nighttime temperatures have.
The increase correlates with the increase in corn yields in the US.
Corn yields in the United States, 1866 to 2014
Cornfields increase humidity which produces higher nighttime temperatures and lower afternoon temperatures.
How does corn affect Midwest weather?: University of Illinois Extension
How cornfields make Midwest unbearably humid | The Gazette
‘Sweaty’ Cornfields Cause U.S. Heat Wave | GearJunkie
The average daily temperature range during summer has declined sharply in the US since the 1930s drought.
