These data are the normal average January minimum temperature 
in degrees Fahrenheit, the latitude, and the longitude for 56 
U.S. cities. The temperatures were computed in two stages: 

(i) for each year 1931 to 1960, take the average of each 
January's  31 daily minimums 

(ii) take the average of the averages (from part i) across 
the 30 years. 

From the orientation of the earth to the sun, we know that 
latitude strongly affects climate across the globe and 
therefore January temperatures. We are interested in the effect 
of longitude after the effect of latitude is removed.