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  • Writer's pictureKate

How to Keep Your Cool: Reduce summer overheating using water

Yep, it's another thing that people in hot countries worked out a long time ago- at least, in places that are relatively dry, for reasons that will be explained...

All those Near-Eastern images of courtyards in houses with fountains, beautiful trees, trellises (I'm not even going to attempt to draw them, imagination or Google are much better here)... Why would you put those in the middle of a house? OK, they look lovely too- but the really really useful thing is that the water and plants are fantastic at cooling things down. Helpfully, it's usually easy to use water in this country to help with keeping cool in summer too.

Here's the explanation bit- the uses bit is at the bottom and next week there will be activities linked to this way of staying cool- so stick with me!

When water changes from a liquid to a gas, it takes in quite a lot of heat energy to break the bonds between the water molecules. This happens even when water is much cooler than boiling temperature, as a few molecules at a time get enough energy to break away as gas and evaporate (otherwise your laundry would never dry unless you put it in the oven at above 100 degrees C- which is most definitely NOT a recommended idea). The evaporation of the water molecules leaves the surface left behind colder- as it's the molecules with lots of heat energy that evaporate.

The air at any temperature can only hold a certain amount of water before it doesn't have enough energy to hold any more. We measure this as humidity, and give it a percentage. The percentage tells you how much more water the air could hold compared with what's already in it. When the air feels hot and sticky, often like there may be a thunderstorm coming, that's high humidity. When it feels dry and baking, that's low humidity.

When the air already holds a lot of water it can't take in much more, so when some water evaporates taking heat away some other particles will condense, putting the heat right back again- meaning that in humid places (like a rainforest or swamp or a steamy bathroom with bad ventilation) evaporative cooling won't work very much - evaporative cooling won't work at all, if the humidity is at 100%.

Our bodies' main way to lose heat is by sweating- which works using exactly this principle of evaporative cooling- so the higher the humidity, the hotter the air feels, even when the temperature hasn't gone up. So 25 degrees with low humidity is fine for most people- 25 degrees with high humidity feels sticky and hot.

It's also why having a breeze- which helps remove humid air near your body and helps evaporate the sweat from your skin- keeps you feeling cooler.

This means that when you want to use water to stay cool, you have to keep the humidity as low as possible. So you need to ventilate as much as you can. Fans can help make a breeze but the humid air has to get out of the room- so open windows or extractors on is really, really necessary for this sort of cooling.

There are commercial evaporative coolers, sometimes called "swamp coolers" because they can put the humidity up if you don't ventilate enough- but just drying laundry in a well-ventilated room will make the temperature cooler.

The very simplest way to use evaporative cooling is just to put water on your skin and let it cool you down- the same way you get cold stepping out of the shower. You don't have to use cold water- warm water will make you lose almost as much heat as it evaporates (it just needs a little less energy adding to get to that point). A damp cloth wiped over your arms and face will cool you down really well- and any breeze from a window or fan will make evaporative cooling work much better.

Sleeping under a damp sheet (wring it out so it's almost dry- the aim is cool not horribly soggy) is also really effective, and some people recommend damping down curtains then letting the breeze blow through them: Using the scrap fabric curtains from earlier in the week could be a good idea?

But- once again- when you're using water to cool yourself down, remember that you need to keep letting the damp humid air out of your home- otherwise it will get really sticky- so make sure your ventilation is working as well as possible, and if you can, keep the windows open.

We'll be back on Monday with some more in-depth ideas about how to reduce overheating through harder-to-manage parts of the home such as the roof and walls, plus catching up on activities to experiment with how much cooling water can do and reducing the urban heat island in the long term using planting and campaigning for better cities

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