Why does blowing on food make it cooler?

March 30, 2006 10:23 AM

Why does blowing on food make it cooler?

I was eating some beef stew last night and it occured to me I had no real understanding of why blowing on something makes it cool down. I suppose I should start with an understanding of what it is for something to be hot.

I understand heat as being a function of the motion of molecules. Something that is hot has more energy. This is about all I know.

posted by macinchik to science & nature (16 comments total)

Convection.
posted by devilsbrigade at 10:28 AM on March 30, 2006

We also blow on things to make them hot -- like your hands in the winter. Aesop wrote a fable about this (both the fable and scientific explanation are here...)
posted by Faze at 10:32 AM on March 30, 2006

Put your hand over (don't touch!) something hot. Sense the hot air there?

Heat transfers are proportional (actually exponential, I think) to the temperature _difference_. Dipping something in cold water will cool it much quicker than dipping it in warm water, for example. That's why you put stuff on the refrigerator: the air there is colder, so things you put in there will cool quicker than stuff on the outside air (unless you live in Canada :))

The thing is: the air around your hot food becomes hot. So, it isn't really good in taking the heat off your food. When you blow it, you move the hot air off your food, and circulate _cold_ (actually warm), humid (yeah, tiny speckles of saliva, yikes!) air around your food. Cold, humid, air absorbs heat quicker than the hot air.
posted by qvantamon at 10:33 AM on March 30, 2006

Also, most food contains water (because we do) and blowing air over the food increases the evaporation rate, which takes heat out of the food. But that's a smaller factor than simple convection.
posted by Steven C. Den Beste at 10:37 AM on March 30, 2006

Basically, heat is how fast a bunch of molecules are moving (how much energy each one has). It is the sum. Temperature is the average of the heat energy of each molecule (so it doesn't change with mass). Applying heat (ie, energy) to a substance will make it raise in temperature according to its specific heat capacity. Specific heats are very, very important things to know for materials.

For gasses, there's a whole lot of stuff around gas laws - the ideal gas law says that PV=nRT, which means pressure * volume = # of moles * R (a constant) * Temperature. This has many, many implications for the relationship between those. This is ideal though; many gasses deviate from that substantially.

This page gives a good overview of heat transfer in general. It'll basically be some combination of conduction (direct contact, most efficient), convection (movement of gasses), or radiation (non-direct contact. think the sun).

Looking at the phase diagram of a substance will give you an idea of what that substance does under different temperatures and pressures. Keep in mind that most pressures we see on the surface of the earth are very tame - they barely fluctuate unless artificially induced. Therefore most matter has one state on the surface, except for those who's boundary between solid/liquid or liquid/gas is within a reasonable range for temperature to rise sufficiently to change (again, either naturally or artificially). Water is an obvious example of this. You don't see iron or steel melting on a hot day very often ;)

Then there's a lot of stuff with supercooling, superheating, and supersaturation, but that gets complicated, and you can read about it on the Wikipedia site.

Hope that was mildly useful ;)
posted by devilsbrigade at 10:39 AM on March 30, 2006

here is how i understand it.

the food releases energy in the form of heat into the surrounding area, which then heats up the air around it, as the air becomes warmer it causes less transfer of energy between the air and the food. Blowing on the food (or better...OVER the food) causes the warm air surrounding the food to be replaced with cooler air, allowing a higher rate of energy transfer between the food and air.

see Newtons Law of Cooling and
entropy
posted by I_am_jesus at 10:41 AM on March 30, 2006

Convection and evaporation are linked. When you blow on the food, you are driving hotter and wetter air away. This causes both additional heat transfer into the now-cooler air, and additional evaporation into the now-dryer air.
posted by grimmelm at 10:46 AM on March 30, 2006

We also blow on things to make them hot -- like your hands in the winter.

Well, sorta. We "huff" on our hands to make them warmer... which I think is a bit different from "blowing".
posted by Witty at 11:09 AM on March 30, 2006

We also blow on things to make them hot -- like your hands in the winter.
Well, sorta. We "huff" on our hands to make them warmer... which I think is a bit different from "blowing".

In fact, we blow into our cupped hands because the air in our bodies is warmer than the outside air -- we are trapping a pocket of hot air and using that to transfer heat to our cold fingers, rather than letting it get blown away. It's the same principle macinchik is asking bout, we just use it in different ways for different things. Science! Hurray!
posted by Hildago at 11:19 AM on March 30, 2006

When you blow on liquids (soup or coffee, for example), you stir them, bringing heat to the surface where convection will displace it, and you increase the surface area by making wavelets, which speeds cooling.
posted by weapons-grade pandemonium at 12:11 PM on March 30, 2006

Very simply, blowing pushes away the cushion of warm air that is insulating the food, replacing it with cooler air. The cool air will absorb the heat faster. This is the same as the windchill effect when you walk around on a windy winter day.
posted by knave at 1:45 PM on March 30, 2006

air is a lot more tightly packed than you might think. an air molecule typically travels about 0.1um (0.004 thousandths of an inch) before bumping into another molecule. that's a tiny amount.

the water molecules at the surface of hot, moist food "fly off". that's because, as otehrs have said, hotness is just a measure of how much things are vibrating/shaking/moving (you can see this in water if you boil it - at boiling point the water shakes/vibrates so much it tips itself apartl that's what boiling is).

but when a hot water molecule flies off, it doesn't get very far - it hits another gas molecule in the air and bounces back!

the net result of molecules flying off and bouncing back is a kind of equilibrium, where the food is covered in a thin layer of air that is saturated with hot water molecules. it soon reaches the point where for every hot water molecule that leaves the food, another one returns from the "cloud".

but when you blow on the food you move that cloud of hot water molecules (steam, more or less) away, clearing the way for more to leave (and build a new layer, which you can blow away again...)

that's probably enough explanation, but there's one more detail - typically the hottest molecules leave most easily (because they are vibrating/moving/shaking around the most). so you are left with the cooler molecules. so the temperature of the food is less (which is what you already expected, no doubt).
posted by andrew cooke at 1:47 PM on March 30, 2006

The Straight Dope: Why do you blow on coffee to cool it?
posted by Sharcho at 1:51 PM on March 30, 2006

Awesome! Everyone's answer was very helpful, thank you very much. When I reheat the stew tonight for dinner I will think of all of you as I mindfully utilize convection to keep from burning my tongue!
posted by macinchik at 2:41 PM on March 30, 2006

This is the same reason that just about every PC has a little fan in it blowing on the CPU -- to help dissipate the generated heat. You can think of it as a little guy trying to cool down the CPU's beef stew if you want.
posted by Rhomboid at 2:51 PM on March 30, 2006

devilsbrigade and Rhombold are right. It's all about convection cooling.

Basically, the heat from CPUs as well as your food has to go somewhere, so it goes into the air. But air can only take some amount of heat and then it's hot itself - and when CPU/food and air have the same temperature, nothing happens.

By blowing you increase the amount of air that passes your food/CPU, so there is more fresh, cool air that can take up some heat.
posted by bloo at 3:05 AM on March 31, 2006