Wednesday, May 29, 2013

Physics on a Manure Heap

Chapter 18 of Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics by Craig Bohren
One of the more important individuals in my academic development was Craig Bohren, a meteorology professor at Penn State University who was my undergraduate advisor and physical meteorology instructor.  Craig was a lunatic.  When they banned smoking on campus, Craig came into class smoking a cigarette and blowing smoke rings to teach us about vorticity.  When students asked him if they could bring a one page cheat sheet to the exam he responded, "you can bring whatever the hell you want, it won't help you one bit because I don't give those kind of tests."  His tests were indeed nasty.  If you got something like a 45 out of 100 on an exam, you were probably on track for an A.

Craig was a lunatic, but he was brilliant and inspirational (it took a few years past graduation to realize this).  He always carried a camera.  Whatever he saw on his long walks became the topic for his next class (this was in the pre-digital days, so he probably spent a fortune at one-hour photos).  Steam on a pile of manure?  That was a class in atmospheric physics covering mixing clouds, forward scattering, and backward scattering that eventually became a chapter in his book Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics.

I thought of Craig's Physics on a Manure Heap lecture yesterday afternoon as I walked home from the U and observed a mixing cloud over the asphalt.  It wasn't a manure heap, but it will suffice.

The processes responsible for forming this cloud are similar to that responsible for forming clouds around geysers or near the spout of a tea pot.  Many people call those clouds steam, but that is technically incorrect.  Steam is water vapor that is at a temperature that is equal to or greater than the boiling point.  Water vapor is invisible.  What you see instead is the mist of water droplets that form as the steam mixes with cooler environmental air.

In the photo above, the asphalt is likely warmer (probably much warmer) than the overlying airmass, which has been cooled rapidly by precipitation.  Relatively warm, water vapor enriched air near the asphalt is mixing with cooler environmental air, leading to the formation of a mixing cloud.  The concept is the same as that observed over a hot spring.

Perhaps on a future trip to Yellowstone I can find a steamy buffalo "deposit" next to a hot spring.  Add in someone's breath condensing and a contrail in the sky and you have the ultimate mixing cloud physics lecture.

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