In reference to your May 13 column about blowing on coffee to cool it: You blow on coffee to cool it not because your breath is cooler than the coffee but rather to induce convection cooling. Your blowing on it removes the hot air from directly above the coffee and replaces it with cooler air from the environment, thus speeding up the cooling process in the same way a convection oven speeds up the heating process. –Samuel Pullara, Chicago
Your explanation of the blow hot/blow cold question is, dare I say it, full of hot air. It’s all in how you blow. Here’s why. When you warm your hands, you blow steadily with your mouth open. This allows a greater volume of warm air to reach your cold hands. But to cool a hot cup of coffee, you first pucker. This causes the air to do interesting things before it reaches your cup: (1) The narrower opening reduces the volume of gas that escapes. (2) The increased pressure compresses the air. (3) The velocity of the escaping air increases. According to Boyle’s law . . . at a constant temperature the volume of a definite mass of gas is inversely proportional to the pressure [blah, blah]. When a gas is allowed to expand adiabatically through a porous plug the temperature of the gas changes. This rate of change is known as the Joule-Thomson differential [blah, blah]. As the rapidly expanding air leaves your mouth, it sweeps along neighboring molecules by adhesion (van der Waals force) [blah, blah]. Thus we . . . see that air that normally blows hot ceases to do so whenever it is compressed and allowed to expand. Isn’t science fun? [Two single-spaced pages, two equations, one poem deleted.] –Michael Godfrey, Cupertino, California
Science is fun, Mike, and whatever it is you’re doing, that’s fun too. I know this because when I showed your comments to the Straight Dope Science Advisory Board they laughed. Suffice it to say no one believes you can pressurize air significantly by puckering your lips.
Pullara’s objection is more serious. I’ve gotten several similar notes. Here’s another: “You are forgetting that the latent heat of vaporization of water is very high. When you blow on your coffee, you replace the vapor-laden air above the coffee with dry air, allowing evaporation to proceed at the maximum possible rate. The temperature of your breath is not important. The coffee is not cooling by conduction of heat directly to the air but by shedding heat in the steam which comes off the coffee.”
Cecil was skeptical that the temperature of your breath was unimportant, as indeed he was obliged to be, since it was the heart and soul of his answer. So, having consulted on the Internet, I retired to the Straight Dope Laboratory and Kitchen of Tomorrow and performed an experiment. I heated a cup of water to boiling, stuck in a candy thermometer, started the timer, and noted the temperature every 60 seconds. It took 25 minutes for the water to cool down to 100 degrees F. (I noted the whole thing on graph paper. I can be very professional when I want to be.) Then I did the same thing again, only this time I set up Mrs. Adams’s hair drier so that it directed a stream of air over the water. With the dryer on the “cool” setting (85 F), low speed, the boiled water dropped to 100 F in less than seven minutes. Then I repeated the process with the drier at the “hot” setting (180 F). The water dropped to 100 F in less than eight minutes. Conclusion: the temperature of the airstream (e.g., your breath) isn’t irrelevant, but it’s pretty damn close. To confirm, I did the experiment yet again, only this time I directed the airstream directly at the water, as opposed to over it. Contrary to what my theory compelled me to predict, the water cooled off even faster, reaching 100 F in less than four minutes with the drier on the “cool” setting and less than five minutes on “hot.”
In sum, your breath does not cool the coffee because it’s cooler than the coffee; it cools it because it increases the coffee’s rate of evaporation. It takes a big man to admit he was wrong, and I will. It was my assistant Ed Zotti’s fault.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.