Which Uses More Energy, Exercising Hot Or Cold?

This question occurred to me as I was dutifully on my nightly run, now that summer’s sliding into autumn and the evenings are getting nippy. Assuming that all other factors such as speed and distance are constant, which burns up more energy, exercising in hot or in cold weather? My run is about 10 km, which conveniently takes about an hour: so which uses more joules, running 10 km/h for one hour in 20 degree heat or running the same speed and time in 10 degree heat?

I dunno, but Cecil did have this to say (emphasis added):

If I had to guess, I’d say working out in 10 degrees would burn more calories than 20 degrees, but 30 degrees might burn more, too. I don’t know the actual numbers.