Climate is a slowly moving target, and that's why every 30 years the National Oceanic and Atmospheric Administration publishes a new set of "normals" for U.S. measuring stations.
"Normal" is an imprecise term that can generate confusion, so we'll leave aside its weaknesses and look at the numbers.
Across the country, the new normal temperatures based on analysis of the 1981-2010 data will be 0.5 degrees Fahrenheit higher than they were in the previous 30-year period.
That's juiced considerably by the general warmth of the first decade of the millenium, when temperatures nationwide were 1.5 degrees higher than they were in the 1970s.
The new numbers go into effect July 1, and we don't yet know precisely what they will say about Philadelphia.
We can say that summers during the last 30 years generally have been warmer in Philadelphia, increasing about 0.6 degrees Fahrenheit higher, compared with the previous period.
In the 1971-2000 period, Philadelphia's average high temperature in summer was 84.7, compared with 85 for the more-recent 30 years. The minimums for the period ending 2010 averaged 66.9, compared with 66.2.
Later, we'll examine what's wrong with "normal."