We’ve all been there: you take the fast food chain apple pie out of its case, bite into it, and immediately regret every decision that ever took you to the point where you somehow forgot that these things inevitably feel hotter than the actual sun.
Panicking, you blow desperately at the snack to cool it down – but are you actually doing anything useful? Does blowing on hot food really help it lose heat? And if so, why? We have the data – and the theory – to explain what’s going on.
What the data says
Before we can say why blowing on hot food cools it down, we’d better check that it actually does. After all, as common sense as it may seem, there are potentially other explanations for something getting colder after being huffed upon.
It could be, for instance, that the amount of time we spend blowing on our food is enough for the temperature to drop regardless of whether we blow or not. Perhaps how we blow has more to do with the effect – maybe we’re more likely to cut our food up before blowing on individual smaller portions, thus increasing the surface area and allowing faster cooling.
Heck, maybe it’s just psychological – we “know” that blowing on your food cools it down, and so our mind just tells us the food is cooler after we blow on it.
Clearly, some hard data is needed. Here’s the problem, though: it seems like none of the big international scientific journals are all that interested in food-blowing. And trust us, we really did look.
That said, it’s not like nobody has investigated this question. Peer-reviewed it ain’t, but YouTubers Nate Bonham and Calli Gade did carry out a couple of experiments to check the claim back in 2021, on the YouTube channel TKOR.
“Everyone believes that blowing on your food is going to help cool it down,” Bonham says in the video, “and while I love that in theory, I’ve never tested out how well it actually works in practice.”
“Obviously things are going to cool down as long as the atmosphere around them is cooler,” he adds, “but how much are you speeding that process up by blowing on the food?”
The results were… well, we’re not going to lie: they were pretty much exactly what you would expect. After two minutes, a plate of instant mashed potatoes that had been blown on had a surface temperature 8.33 to 10 degrees Celsius (15 to 18 degrees Fahrenheit) cooler than the same potatoes without the blow treatment. That difference increased noticeably when a smaller amount – like, a single bite size – of potato was taken.
Meanwhile, blowing on a spoonful of tomato soup resulted in even more dramatic cooling the pair found – something like 3.33 to 4.89 degrees Celsius (6 or 7 degrees Fahrenheit).
“It was a significant reduction in temperature,” Bonham reported. “Like, it went from like ‘oh can I eat it?’ to like ‘oh, that’s nice’.”
Similar results were obtained for hot pockets and pizza pockets, with the smaller pizza pockets losing more heat from blowing than hot pockets – perhaps because of their smaller size, the pair suggested.
So, it looks like blowing on your food really does help cool it down, at least for the surface temperature. Which leaves us with the question: why?
What’s going on?
People may not have expended much time confirming experimentally that blowing on hot stuff cools it down – but when it comes to the actual math and physics of it all, we’re pretty confident about what’s happening.
There are three fundamental ways that heat can transfer from one location or object to another: conduction, convection, and radiation. All three occur when you blow on hot food – though we’ll be honest, the radiation and conduction aren’t really doing much of the heavy lifting.
So, what do each of these terms mean? Well, let’s start with radiation since it’s both the most ubiquitous and the quickest to explain: this is when heat is lost due to, well, radiation.
Key to radiative heat transfer is the fact that it doesn’t require a medium – for example, it’s the process by which heat from the sun reaches us here on Earth despite hundreds of millions of kilometers of vacuum being in the way. It’s actually the quickest form of heat transfer – but it’s also, arguably, the least important one for our current purposes.
Why is that? Because blowing on something hot doesn’t increase how much it radiates thermal energy. In fact, it does precisely the opposite: the rate of heat transfer by radiation is determined by the Stefan-Boltzmann law of radiation, which depends only on the object’s surface area, emissivity, and, to a much, much higher degree, its temperature.
But blowing on a solid piece of food can only change one of those things – the temperature – and it acts to reduce it. In other words: the more you blow, the less radiation you get.
So much for radiation – what about conductive heat transfer? This one’s a bit easier to understand: it’s when heat is transferred between two objects through the random collisions of their atoms and molecules.
“Thermal energy causes molecules to move,” explained chemist and science writer Anne Marie Helmenstine in a 2019 article for ThoughtCo. “This energy can be transferred to other molecules, reducing the movement of the first molecule and increasing the movement of the second molecule. The process continues until all the molecules have the same energy.”
You’re probably pretty familiar with conduction – it’s why metal, a great conductor of heat, usually feels cold to the touch, while something like fiberglass is used to dramatically slow heat transfer. Like radiation, though, it’s not super important when it comes to blowing on food, for one simple reason: it depends on colliding atoms, and in gases, those atoms don’t collide all that often.
This just leaves convection – the process by which thermal energy gets moved around in everything from your tea kettle to global atmospheric phenomena. At its core, it’s pretty simple: basically, it’s when a hotter fluid (which in this case includes gases, physicists are weird, sorry) is shoved out of the way by a cooler fluid.
Just as you might expect, the cooler the replacement fluid, the faster the convection happens, since the formula governing the rate of convective heat transfer depends on three things: the convection coefficient – basically a measure of how good the particular fluid you’re dealing with is at heat transfer; the exposed surface area; and the difference between the temperatures of the two fluids.
In other words: “When you blow on food[,] you move your relatively cooler breath where the heated air used to be,” Helmenstine wrote. “This increases the energy difference between the food and its surroundings and allows the food to cool more quickly than it would otherwise.”
Bonus cooling: hot soup
Let’s go back to that soup from the beginning – why would it be cooled so much better than solid food?
Bonham and Gade had an idea: “I think because [the soup] was so shallow and even able to move around a little bit,” Bonham suggested, “it did a much better job of cooling off.”
It might have just been a hunch, but it was a good one. You may have noticed in the explanations above that surface area plays a part in how well heat is transferred – well, unlike solids, blowing on liquids actually can increase the surface area, thanks to creating ripples. That means that any cooling effect will be slightly larger. But is it the whole story?
Not by a long shot, actually. “When you blow on a hot drink or a food containing a lot of moisture, most of the cooling effect is due to evaporative cooling,” Helmenstine wrote. “Evaporative cooling is so powerful, it can even lower the surface temperature below room temperature.”
Technically, this isn’t an example of either conduction, convection, or radiation – it’s an energy transfer due to a phase change. “Water molecules in hot foods and drinks have enough energy to escape into the air, changing from liquid water to gaseous water (water vapor),” Helmenstine explained. “The phase change absorbs energy, so when it occurs, it lowers the energy of the remaining food, cooling it.”
Blowing on the soup – or tea, or custard, or whatever – then is not primarily a way to cool the air near the food, but a way to remove the water vapor above it. That lowers the vapor pressure, allowing further water from the food to evaporate.
So, what’s the takeaway? It’s a whole lot of physics, but it all boils down to one conclusion: yes, blowing on your hot food will make it cooler – particularly if it’s a liquid.
If that doesn’t work fast enough, you can always try shredding it into pieces. You know – to fully maximize that surface area.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
Source Link: Does Blowing On Hot Food Actually Do Anything, And If So... Then How?
