Blowing Hot and Cold
Mastering evaporation (safely)
In the latest of our shortlisted entries to the 2011 Wellcome Trust Science Writing Prize, Imogen House explains the science behind her cold hands.
They say that there is no such thing as bad weather, only inappropriate clothing. However, as a carefree girl-about-town (or, impoverished student with small handbag, whichever you prefer) I happened to be ill prepared one day when the weather took a turn for the worse. As luck would have it on this drizzly day I had cycled into university. In order to retain my right to scoff at ‘fair weather cyclists’ I felt obliged to brave the precipitation, coat or no coat. For some time I was perplexed as to why my hands were getting so nippy. They were disproportionately cold compared to the rest of me but I couldn’t work out why. It turns out that there really is a very simple explanation – it all comes down to evaporation.
There are two main reasons for this ‘chilling’ effect. The first is the varying energies of different molecular states, the second is the law of diffusion.
Molecules (with a few exceptions) are found in one of three states – solids, liquids or gases. A gas molecule has more energy than a liquid molecule, which in turn has more energy than a solid. This energy takes the form of tiny molecular vibrations – generally measured as heat. As temperature increases the individual molecules gain more energy and start to vibrate more vigorously. As the vibrations increase the molecules gradually break the bonds attaching them to their neighbours. This is why a substance becomes less rigid and more free flowing as it goes from solid to liquid to gas.
Molecules always diffuse from an area of high concentration to an area of low concentration. This is a fundamental rule of physics, which explains why the smell of burnt toast manages to permeate every room in the house. If you spill a glass of water, the liquid on the table is of a higher concentration than the water vapour suspended in the air. Eventually the table will dry as all the water evaporates in an attempt to even out the concentrations. As a result of the evaporation the surface will also be left cooler. Moving air increases the rate of evaporation, as the concentration of molecules in the air is kept much lower than at the surface.
In order for a water molecule to move from being a liquid to being a gas it needs to gain some energy. But how does this relate to my cold hands? As the tiny water droplets evaporated from my skin they used some heat energy, making my skin feel cool. My body then had to burn stored energy (from food) to generate more heat, bringing my skin back up to temperature. I wouldn’t recommend hypothermia as an effective weight-loss method though!
You already know about this aspect of physics – perhaps without even realising. It’s why you blow on your soup or cup of tea when it’s hot. By blowing on the liquid you increase the rate of evaporation. The greater the number of particles that escape the surface the quicker the liquid loses heat energy, making it less likely to burn your tongue.
We can also see this effect in action during the summer. A fan in a closed room will not actually cool the space since it’s only moving air around, not changing the moisture concentration in the air (if anything, it’s heating the room due to the friction between its moving parts). However, if it’s blowing air at you, you will feel cooler as the sweat on your brow evaporates.
All this explains why my hands were so very chilly in the wind and rain. What I really needed was a good pair of waterproof gloves to keep me warm. In fact I own some, but I’d forgotten them. Maybe what I really need is a better memory – unfortunately neurology isn’t my speciality.
Imogen House
This is an edited version of Imogen’s original essay. Views expressed are the author’s own.
Find out more about the Wellcome Trust Science Writing Prize in association with the Guardian and the Observer and read our ‘How I write about science‘ series of tips for aspiring science writers.
Over the coming months, we’re publishing the shortlisted essays in this year’s inaugural competition.
