Temperature of the middle seasons

Chart Link

 I have created a spreadsheet with the data from this chart here by looking at the average low and high temperatures on each 1st, 11th, and 21st of each month.  Those correspond to the middle seasons.  

On this spreadsheet, I added a couple columns to show how the high and low temperatures CHANGE from middle season to middle season. That's the "delta" on the graph.  It's represented by the angle of incline or decline on the blue or red lines.  

Sometimes, temperatures don't change much from one middle season to the next.  But other times, the temperature change is radical-- 5, 6, or even 7 degrees change in just ten days.  October and November see huge changes.  Also interesting is that it always changes (with a very few number of instances.)  Spring and Fall are times of rapid change.  At the same time, the middle of summer and the late winter are times of small change -- doldrums of temperatures at least.