Every Seattleite has been in this situation: On a cold, rainy December morning, you get your coffee to go from Vivace, Stumptown, or Starbucks, and then watch out the window for your bus. The bus, you know, might be a minute or two late, and you’ll have to wait a few minutes. You want to keep your coffee as hot as possible during your wait so that it’s still piping hot when you step out the door. You grab a lid for your cup, pausing at the cream. Should you add the cream to your coffee now, or will that only cool your drink faster? Maybe you should add your cream at the last minute, before you dash out the door.
The basic physics of heat provides the answer: you should go ahead and add the cream to your coffee now. Coffee with cream cools about 20 percent slower than black coffee, for three reasons:
Black coffee is darker. Dark colors absorb heat faster than light colors (just think about wearing a black T-shirt versus a white T-shirt on a hot, sunny day). But dark colors also emit heat faster than light colors — absorption and emission are essentially two sides of the same coin. So by lightening the color of your coffee, you slow its rate of heat loss slightly.
Stefan-Boltzmann says so. The Stefan-Boltzmann law says that hotter surfaces radiate heat faster — specifically, the power of emission is proportional to the temperature (in kelvin) raised to the fourth power. So let’s say you have two cups of coffee that start at the same temperature. You pour cream in cup number one and the coffee drops in temperature immediately. But the rate at which it loses heat also drops. Meanwhile, the hotter black coffee in cup number two cools so rapidly that within five minutes the two coffees are at about the same temperature. But you still haven’t added the cream to coffee number two! When you do, it cools even more; cup number one is now the hotter of the two.
Viscosity versus evaporation. This is the clincher. Adding cream thickens the coffee (adds viscosity), so it evaporates slower. You’d be surprised just how much heat evaporation carries away. Slow the rate of evaporation and you avoid a lot of that heat loss. (This is also one big reason that coffee stays warm longer with a lid on the cup.)
So, next time you’re caught in the rain, put the cream in your coffee right away. Your fingers will thank you.
Watch our high-speed video above of cream being poured into coffee at 2,000 frames per second.