Pressure Gradient 2/22/23

Science with Richard Bleil

Two more balloons were shot down (as of the writing of this post, about ten days from it’s being published) flying at about 40,000 feet.  The big one, shot down a few days earlier after traversing the continental United States was flying at 60,000 feet.  The primary difference is that at 40,000 feet, the FAA claims these objects are a threat to airline flights, so downing them became more urgent as an issue of public safety.

If I’m not mistaken, it was Edgar Allen Poe who wrote about a balloon trip to the moon.  In the science fiction story, he discussed the discovery of a gas even lighter than helium and hydrogen, and lighter than the ethos, the invisible (dark matter, I suppose) gas that fills the void of space.  In the trip, the balloon reaches a point between the Earth and the moon in which the principle gravitational force switches, causing the balloon to reorient the basket from facing the earth to facing the moon. 

It’s an interesting concept, and raises the question of why, exactly, the smaller balloons were 20,000 feet lower than the big one.  Something that most of us are aware of is that there is a pressure gradient.  A “pressure gradient” is a very gradual change in pressure as a function of altitude.  You, my dear reader, are probably already aware of this, but I hope to touch on some more nuanced concepts that you may not have thought about.

You’re aware of the existence of the gradient (although maybe not the term) because you know that at high altitudes the air pressure is thinner than at low.  I’ve studied charts of air pressure (and temperature) produced by NASA, but even if you have not, you’re still aware of this thinning of the atmosphere.  In fact, it simply must exist as we are aware of the “vacuum of space”.  It’s not like there is a hard and fast ceiling in which the atmosphere simply disappears.  It’s a gradual fading of the atmosphere, or a gradient. 

This is why people climbing particularly high mountains will bring oxygen.  Some days, if they’re not careful to regulate their efforts, they need extra oxygen to recover from exerting themselves.  It’s because of lower air pressure that some food items, like boxed cake mixes, have different instructions for “high altitude” baking versus regular. 

What you might not have thought about is that you are feeling the effects of this pressure gradient every day.  If you hold your finger up in the air (no, not THAT finger…play nice!), the truth is that the air pressure is lower at the bottom of the finger than the top.  You can’t feel it, of course, but if you want to see it, all you need is a flame.

Fire is basically a gaseous mixture comprised of what chemists call “free radicals”, pieces of compounds with unpaired electrons seeking something, anything, with which to react (like your skin).  This gas is exceptionally low density (in part because of the temperature).  Because of the gradient of air pressure, the flame rises from heavier air at the bottom towards lighter air above.  In other words, the flame itself is created because of this pressure gradient.  On the space station, a simple experiment was created in which a match was lit.  The flame was actually spherical, moving in all directions, rather than just up.  The flame burned itself out.  Because on earth the flame burns up, oxygen gets to the burning material at the base of the flame (which is why it burns blue and hotter) keeping the flame alive.  Oxygen in the space station couldn’t get to the burning material, so the flame simply extinguished itself.

In a much less common example, partially depleted helium balloons have always been a fascinating example to me of this pressure gradient.  Fully inflated, the balloon floats to the ceiling, the highest it can get, which is fun, but not nearly as interesting.  When not inflated enough, it sinks until the string touches the ground.  Now it’s being held down by the weight of the attached string, which is fun to play with, but again, not as interesting, at least not in my mind.  But if you’re really lucky, you’ll catch the intermediate state, that point where the balloon is deflated just enough that it’s floating in the middle of the room, not touching the ground, nor the ceiling. 

At this point, the balloon has found its equilibrium.  In that gradient of air pressure that reaches from the heavier air near the floor to lighter near the ceiling, the balloon has found the density of air that is just perfect to keep it aloft, not sinking, and not rising.  That is the point that sparks my imagination. 


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