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Keeping your space cool - a short lesson in thermodynamics
Selectively gleaned from the mind of a real engineer...translated so that anybody can understand

What make the playa so hot?
The sun produces radiant energy that pours down out of the sky. Radiant energy is made up of a broad spectrum of electromagnetic radiation.
Spectrum = collection of wavelengths
Electromagnetic radiation = http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

One part of this spectrum is visible light, but sunlight contains many wavelengths from high-frequency ultraviolet light down to low-frequency radio waves. Our atmosphere filters out most of the ultraviolet light, thanks to the ozone layer, which is good thing; otherwise we would all be really toasted.

It’s important to note that all wavelengths of sunlight, visible or otherwise, have the potential to ‘heat stuff up’. Electromagnetic radiation will heat any mass that absorbs that radiation. That is, any mass that is able to absorb any wavelength of radiant energy will heat up. For instance, window glass absorbs almost all the ultraviolet light, but allows most visible light to pass through. The glass heats up because it absorbs the ultraviolet light.

On the playa, we all know that creating ‘shade’ helps to create a cooler space. And it isn't just sunlight that 'heats stuff up'. The coolness of your space is dependent upon the reflection of the radiant energy, so that your shelter and its contents do not absorb it. And then you must provide some means for any existing heat to escape.

For instance, if you build a shelter with fabric that is non-reflective, lightweight and translucent, some of the radiant energy will be absorbed by the fabric…and the fabric will heat up. And quite a bit of radiant energy will pass through the fabric, heating up whatever absorbs it inside your shelter…like your body. Meanwhile, the heated fabric will re-radiate radiant energy as infrared radiation, heating up the air next to the fabric. (When you hold up your hand to something, and it feels hot, what you are feeling is the infrared radiation being ‘given off’ by the object.) If this shelter is an enclosed space with no air movement, like a little tent, you end up in an oven.

On the other hand, if you make a shelter using those semi-reflective, heavy, opaque gray tarps, some of the radiant energy will be reflected and none of it will pass through the tarps. But much of the radiant energy will be absorbed by the tarps, causing the tarps to heat up. In fact, in full sun, these tarps get too hot to touch! The hot tarps radiate their ‘absorbed heat’, and heat up the air next to their surface. This hot air needs to be vented back out of your shelter, or you end up in oven again…even though this oven is dark inside.

If your shelter has a *highly* reflective surface, it will reflect much of the radiant energy. Coverings with an aluminized surface such as Reflectix or "ply foil insulation" and wildland firefighter’s emergency fire shelters do an excellent job of reflecting this radiant energy. Both of these coverings work well as an outer layer on top of your tarp or tent. If you create a ‘moving’ air space (as opposed to a ‘dead’ air space) between the reflective layer and your inside layer, any heated air will move out and not heat up your shelter. These two layers of protection, with a moving air space in between, is one of the keys to a cool space.

Another wonderful product is a reflective knit fabric called Aluminet shadecloth: the “70% shade” version reflects 70% of the radiant energy, but allows 30% to pass through. Be sure to get the grommets too. This product is also beautiful and easy to work with: you can create a fantastic tension structure or a simple lean-to. And air can move through it, which can move heat out and create a cooling breeze. On the other hand, this fabric traps dust from dust storms, which reduces its reflectivity and, once it is laden with dust, a stiff breeze or bump will shower you with dust. This is a minor annoyance compared to its excellent reflective properties. Reflectivity is another key to a cool space.

Whatever the shape or covering of your shelter, air movement is important. Moving air makes you feel cooler for a couple of reasons. If the air isn’t moving, the air right next to your skin (that has been warmed by the heat from your body) won’t move away. A breeze blows this warmed air away from your skin…that is, as long as the moving air is cooler than the air next to your skin. Moving air also accelerates evaporation from your skin, cooling you. You can increase this effect by spritzing your body and clothes with water.

If possible, create your shelter with air movement you can control. You will want to be able to create openings on the windward and leeward sides of your shelter when you need a cooling breeze to flow through, and you will want to shut those openings if a dust storm comes up...or if the air coming in from the playa is much hotter than the air inside your shelter. Try to avoid openings on the sunny side of your shelter. Since the direction of the wind and sun will vary, ideally the position of the openings should vary too. Fans and turban vents are active methods for moving air. Air movement is another key to a cool space.

Why does spritzing your body with water make you so much cooler?
If you spritz yourself with water that is cooler than your body, the water will feel cool against your skin. But it is the evaporation of this water where you get the most cooling. In the dry air of the playa, water can evaporate quickly. This evaporation removes heat from your body: the water molecules absorb large amounts of energy during the process of ‘state change’ from liquid to gas. This state change is a peculiar event: it takes a lot of energy to cause a substance to ‘change state’. In this case, the heat from your body causes the water to change state: the water vapor molecules absorb this heat. Then the heated water vapor blows away from your body and over to the person holding a glass of cold beer. The water vapor molecules condense on the outside of the glass, changing state back into water, and warming up the glass, which, in turn, warms up the beer. It might be best to keep that last part a secret from the person with the warm beer. And so, evaporation is another key to a cool space.

Uh, what are the keys to a cool space?

• Two layers of protection from the sun, with moving air in between that can escape from your shelter

• Reflective outside surface, especially on the sunniest sides of your shelter

• Controlling air movement inside your shelter through openings, vents, and fans

• Evaporation

• Really cool decorations and lights that make anybody go "Ooooo, ahhhhh!"

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