Used to be, a big chunk of the American population, more than 40 percent, lived in cold places. Not Montana or Minnesota or Maine cold, but Detroit, Pittsburgh and Cleveland cold. The great industrial cities have winter. It snows a lot. You wear mittens.
But it is not that cold everywhere in the United States. Some places require you to heat your home a lot less because the temperature outside is much closer to the temperature inside. Energy people have a measure for this climactic condition: the heating degree day. They take a fixed inside temperature, say 65 degrees, and then compare that to the average outdoor air temperature. So, it's an average of 35 degrees outside for a week? That'd be 210 degree days. Do all that math over a winter and you've got your number of heating degree days.
The relationship is obvious: cold places rack up more heating degree days.
And that's what makes the following graph so interesting. It shows all the US states in heating-degree-day tranches. Look at the truly population level change in the climates where people now live. Cold states are losing population while the warmest states keep gaining it.
Fascinatingly, though, the really, really cold places haven't lost that many people (though they are declining slightly). It might be that there are about 10 percent of Americans who love cold weather, or at least bear it better than the rest of us. Or that the economics of the resources located in those cold places keeps that many people there.
From this map, it's not possible to tell if the great move south is a win for energy efficiency in the country. Many of the places gaining population have very high cooling degree day numbers, which means lots of air conditioning.
Alexis C. Madrigal is a staff writer at The Atlantic and the author of Powering the Dream: The History and Promise of Green Technology.