Why do you blow on coffee to cool it, but on your hands to warm them? Plus: if veggies have no fat, why is veggie oil all fat?

Dear Cecil:

Two questions. (1) Why do you blow on hot coffee to cool it, but you blow on your hands in winter to warm them up? (2) How come vegetables have no fat, but vegetable oil is 100 percent fat?

Cecil replies:

You broadcast guys are such a pain. But when you’re a person such as myself who is singlehandedly keeping the book industry afloat, you do what you gotta to promote the product.

(1) I suppose we could get into the physics of heat transfer. Then again, try this: your breath (98 degrees F or thereabouts) is cooler than the coffee, so the coffee cools. It’s warmer than your near-frostbitten hands (perilously close to 32 F), so your hands get warm. Any other questions?

(2) Vegetables are low fat, but they’re not no fat. Any living cell has fat in its cell membranes and elsewhere. In vegetables the fat content typically is less than one percent, but it’s higher in the seeds, from which most vegetable oils are made. Some vegetables are pretty fatty even if we ignore the seeds — olives and avocadoes, for example. Olive oil is one of the few types of vegetable oil that is made from the flesh of the plant, as opposed to the seed. They’d probably make avocado oil from the flesh, too, if there were any market for avocado oil, which there isn’t. Be that as it may, the fat storage bodies in avocado flesh are what makes it smooth and creamy.


Dear Cecil:

In reference to your 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.

Dear Cecil:

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-Thompson 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.]

Cecil replies:

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 ‘net, 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 dryer 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 under seven minutes. Then I repeated the process with the dryer at the “hot” setting (180 F). The water dropped to 100 F in under 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 dryer on the “cool” setting and under 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 a mistake, and I will. Get on up there, little Ed, and tell them it was your fault.

Send questions to Cecil via cecil@straightdope.com.

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