There's this at the end:

> Temperature categories are determined by the normal distribution, so that a third of temperatures fall in each of the main three categories: hot, cold and normal for 1951 to 1980. Summer temperatures for each subsequent 11-year period are compared to the 1951 to 1980 baseline.

It's not so stupid. It essentially drives home the point that a day that in the 1951-1980 period would be considered "hot", today would fall inside the "normal" range.

That is, your normal distribution of temperatures has shifted a bit to the right.