Summer range in temperature is defined as the difference between the hottest and coldest temperature during the June 1 through August 31 time period each year.  Climatologists define the seasons on a monthly basis so Summer is defined as June 1-August 31, and NOT June 21 through September 21. 

The larger the number on the graph, the larger the range or difference in temperature during that season.  There are two time series graphs, with the 2nd one showing some recent trend lines.