Thin Air

Every morning, I bring water to a boil so that I can start my day with oatmeal. Here, the temperature will reach about 212 degrees Fahrenheit—100 degrees Celsius—before the water molecules will become sufficiently excited as to jump around beyond the electromagnetic bonds that normally hold them in place.

The heat is just enough at this point so that these molecules jiggle free, turning from a liquid into a gas. We call this process boiling.

If you live in Denver, water boils at 203°F/95°C. Why should this be?

Recently, I wrote about the pressure at the deepest depths of the ocean, and how life abounds anyway. GF Writes reader (and fellow Substack writer)

Sam Rake

pointed out that that sort of deep dive virtually demanded a piece about the inverse, about how the pressure changes as you go up.

I tend to agree!

Imagining the entire atmosphere is challenging. It’s everywhere around us, and it’s invisible, so we don’t tend to think much about it during our daily lives. However, you might be able to imagine a person-wide column of air instead.

This imaginary column is perfectly cylindrical in shape, and it goes all the way up to the edge of space, an arbitrary line that is always up for debate, but for our purposes, we’ll use the Kármán line—roughly 50 miles above the surface.

That column above us is constantly pressing down, smashing downward and all around us with pressure due to gravity’s cumulative effect. In other words, the closer to the surface of the Earth you get, the more of that column is above your head, keeping our bodies intact and properly pressurized.

If you climb up a mountain or go up in an airplane, the constant pressure pushing in from all directions goes down. There’s just not so much of the atmosphere here, in that imaginary column above you. That’s why your ears will pop: a sensitive cavity in there called the middle ear will expand when the pressure outside goes down, so a little tube that connects your ears to your throat opens up, helping to relieve the pressure on your eardrum.

That’s why yawning or chewing gum can help relieve inner ear pressure due to changes in elevation. If you have ever used a pressure-cooker, you are already familiar with this principle—you can bring water to a boil much faster with higher pressure.

Other animals have evolved to deal with these changes in pressure, and even to thrive under very different conditions than we’re used to. In addition to the lower pressure, llamas in present-day Peru and Bolivia have evolved to have much larger lungs and amazing circulatory systems designed to make the limited oxygen stretch longer.

Some geese can fly over the tallest mountains, up there where the air is so thin that they need a specialized hemoglobin molecule, just to extract more oxygen.

Plants, too, have to grow and behave very differently at very high altitudes. For one thing, there are fewer resources up here, so they tend to grow much slower than their ground-dwelling cousins. Another thing is the powerful UV radiation up here, where the atmosphere’s protective layer is far too thin to be as effective. Plants develop darker pigments as a result.

It seems that life insists on existing everywhere, even all the way up above mountains.

If this piece leaves you wanting a deeper understanding of atmospheric pressure, you might enjoy the deep dive that inspired this conversation. Or, you might appreciate the story of Jean-François Pilâtre de Rozier and François Laurent d'Arlandes, the first human beings to fly overhead. They certainly knew a lot about atmospheric pressure:

Alternatively, tell me about times when your ears have popped! What’s the highest elevation you’ve visited? Let’s share stories today.