The graph above plots the number of years from the closest month where the USHCN high temperatures averaged over 100 degrees in Meeker, Oklahoma. Prior to 1960, Meeker would have an extremely hot month about once every 2-3 years. The 30 year spell from 1981 to 2010 had no months over 100 degrees.
Contrary to widely reported claims (normally made by people who have done no research) heatwave frequency has declined – dramatically.
Here are all the months which averaged over 100 degrees :
1896 - August 1901 - July 1902 - August 1913 - July, August 1916 - August 1918 - August 1934 - July, August 1936 - July, August 1943 - July, August 1954 - July 1956 - August 1980 - July, August (2011 - July)