A Bunch of Hot Air

Over 2500 years ago, a series of Greek philosophers attempted to understand and describe the reality of the world around us. One of the very first was a man named Thales of Miletus. Thales applied his intellect to a great deal more than just philosophy.  He was a respected advisor to the Ionians, and he is said to have been the first man to successfully predict a solar eclipse.  He also tried his hand as a tactician, once helping an army cross a river by ordering a crescent-shaped bypass to be shoveled around behind them, thus reducing the flow of the river ahead so it could be forded and passed. But for the purposes of this already digressing blog entry, Thales’ biggest contribution was his attempt to answer the question, “What is everything made of?” Thales proposed that all things in the universe are made from water.

That sounds like a quaint notion now, but consider just how revolutionary his proposition was.  Until Thales made that bold statement, the Mediterranean was awash in mythology from East to West.  All of the happenings on Earth and in the sky were attributed to one or more gods, with no further explanation deemed prudent or necessary.  For Thales to come forth and suggest that the world was made of something other than the labor of deities, whether that something was water or beer, was nothing short of a giant leap for humankind, on equal footing with the first steps of Neil Armstrong on the Moon.

When you consider where Thales lived, it does not seem silly at all that he selected water as the fundamental substance of all things.  He was a man of the sea, living in an island and coastal culture, surrounded by water in its liquid form, so much that it seemed the very Earth on which he stood was merely floating in the midst of an endless ocean. Water was critical to survival then as now. We know today that water and carbon form the basis for our entire biology, so to elevate water to the status proposed by Thales so many centuries ago is most certainly a forgivable crime. This is a central tenet of science: the people who have been wrong play just as important a role as the people who have been right. Science transcends ego. What matters is not whether Thales was right or wrong, but that he even asked the question.

Following on the thread pulled by Thales, another philosopher named Anaximenes (also of Miletus) proposed that all things are made of air. This is just as remarkable as Thales’ idea, given that in most circumstances air cannot be seen. People knew of air on some level because it can be felt – hot air, cold air, and most importantly, moving air (more lovingly known as wind). As a life giving substance, air is just as precious as water for us – but we also now know that we are not made solely of either, and that both are made of atoms. You might imagine it took centuries or even millennia after Thales and Anaximenes to figure that out, but in fact it took mere decades for the philosopher Democritus to suggest it and even give atoms their name (a name meaning “that which cannot be cut”). It then took a couple of millennia for us to rediscover atoms, and another couple of centuries to actually see them with electron microscopes. Imagine the level of abstract thought to have come up with the notion in the fifth century BC, when no microscopes of any kind were available.

Back to water and air: it’s fairly common knowledge that water is made of molecules, each of which contains two atoms of hydrogen and one atom of oxygen – H2O. Air is also made of molecules, but with quite a bit more variety, leading to a mixture of invisible gases that surround us and our planet. The most common molecule in the air is nitrogen, making up 78 percent of our skies. Only 21 percent is oxygen, the gas we all need to breathe. Almost 1 percent is argon, and the rest is a smattering of other molecules – such as methane, carbon dioxide, ozone, nitrous oxides, and an extremely variable amount of water vapor.

Besides turn hot, turn cold, move, and keep us alive, what does air do? Well, getting back to the hot and cold part, the air at any given place and any given time has a temperature. There can’t be any question you read the last post, right? Ok, I’ll give you a few minutes. Let me know when you’re ready.

Welcome back. So, anything made of atoms or molecules with a temperature higher than absolute zero radiates energy in the form of photons. The air, being made of atoms and molecules and typically having a temperature higher than absolute zero, therefore radiates energy in the form of photons. Science is relentless. The air can also absorb the photons radiated by other things, just like other things can absorb the energy radiated by air and other other things. How much is absorbed depends on what molecules are involved. And that’s where things get even more interesting.

You know from experience that different substances absorb sunlight differently. A white shirt reflects a lot more light than a black shirt, and so you feel cooler and look brighter in white than in black. Most substances also absorb certain colors of light differently than others. The typical healthy plant contains a fair amount of chlorophyll, which absorbs more blue and red light than green light, and therefore reflects more green, some of which strikes our curious eyes, behind which our brains recall, oh yeah, most plants are green.

This concept extends into kinds of light and radiation that we cannot see – like ultraviolet and infrared. At the temperature of most things on Earth – including you, those things are emitting infrared radiation more than anything else. Some of those photons travel upward into the atmosphere, where they hit the different molecules in the air, and those molecules in turn might scatter or absorb the photons, depending on the molecule. On balance, the molecules that absorb the most photons in our atmosphere are water vapor, carbon dioxide, and methane.

So now you’ve got this rather complicated situation. The Sun, thousands of degrees hot at its surface, from 93 million miles away, radiates photons of visible light in all directions, and some of those photons plunge into the top of our atmosphere. Some are absorbed, some are scattered, and some make it down to the surface, where they hit all manner of things, including us. This causes us to heat up and emit more of our own infrared photons back into the atmosphere, which then absorbs some and scatters some, and the photons it absorbs causes it to heat up and emit more of its own infrared photons in all directions, including back toward the surface, where they hit all manner of things, including us. This causes us to heat up some more and… you get the picture. You may have heard of this effect before – it’s called the greenhouse effect…

…which is a misleading name, as is the idea of a blanket. Greenhouses and blankets keep things warm because they don’t let the warm air escape. The atmosphere stays warm because it lets a lot of photons in but it doesn’t let all of those photons escape back into space. Eventually, even if getting absorbed and re-radiated many times along the way, some photons do eventually make it all the way up to the top of the atmosphere and trickle into space, but the rest contribute to warming our Earth and atmosphere. And so we end up arriving at some sort of equilibrium, which brings with it a general distribution of temperatures throughout the atmosphere and across the Earth. Forget greenhouses and blankets: because of the things that photons do, a planet or moon with an atmosphere is generally warmer than one without. That’s why the night side of our Moon, essentially the same distance from the Sun, is always unimaginably cold, while the night side of our Earth can often be perfectly comfortable. More generally speaking, it’s one reason our planet can support life as we know it. Usually, a bunch of hot air is couched as a negative thing. In this case, we owe it our existence.

What goes up must usually come back down.

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