US August 4th afternoon temperatures peaked in 1930, and have been declining ever since.
On August 4th, 1930 and 1947 huge swaths of the US were over 100 degrees.
05 Aug 1947, 1 – Chicago Tribune at Newspapers.com
04 Aug 1930, Page 1 – Moberly Monitor-Index at Newspapers.com
One hundred years ago today, it was 113 degrees in Iowa, 112 degrees in Illinois Oklahoma, and Arkansas, and 111 degrees in Missouri and Kansas.