Tuesday, October 22, 2013

For Spacious Skies --part 2

Two posts ago, we began looking at the processes that form clouds at the molecular level.   A quick review, and then we'll get to something you can see more easily.  Molecules of both liquid water and gaseous water (water vapor) are in motion.  If the molecules of liquid water move fast enough, they evaporate (become gas), while if water vapor molecules slow enough, they condense (become liquid).  The "dynamic" part of this business is that both of these processes happen simultaneously pretty much wherever water exists.

Molecules of liquid water becoming water vapor is called evaporation, while the opposite process of molecules of gaseous water clinging to form a drop of liquid is called condensation.

Evaporation happens when molecule jostling in a liquid gain enough speed (really kinetic energy) to overcome the electrical forces that make them cling, so that they come loose and leave the water to become water vapor (gas) molecules.  Condensation happens when gas molecules zinging off each other slow down enough (lose kinetic energy) so that, instead of bouncing off each other, they cling to form liquid water.

(A nice, succinct explanation of most of today's topic can be found at: http://www.ems.psu.edu/~fraser/Bad/BadClouds.html  Even if you read on here, you would be well advise to look at this page.)

Now it's time to get concrete for a bit.

Put a bucket of water into a closet.  Put a window in this closet so you can closely watch what is going on.  Shut the door.  (Do not try this at home--your imagination will work just fine and be quicker.)  On a dry day, water molecules on the surface of the water in the bucket will be zinging off into the air as chance collisions with other water molecules give them enough energy to break loose from neighboring molecules to which they cling.  Over a period of time, more and more water molecules will be in the air as water vapor.  Some of these water molecules, in colliding with each other, will lose enough energy to cling back to the water in the bucket.  In the beginning, water will be evaporating from the bucket much faster than they condense from the air.  As you observe the bucket, the water level will be sloowwly dropping.  But as water vapor accumulates in the closet, the condensation rate will increase--simply because there are more molecules in the air that, by chance, slow enough to stick back to the liquid.  (If you could sample that air, you would find it more humid than when you began.)  At some point, these two processes will balance out--water evaporating out of the bucket and condensing back into it--and it will seem as if everything has stopped.  But you know the truth: both evaporation and condensation are still happening, but at equal rates.

We don't have a cloud yet (though we're getting closer); what we have is a dynamic process of evaporation and condensation in conditions which are not changing.  Now we change the temperature.  Remember that temperature is directly related to the average speed of a group of molecules: the higher the temperature, the faster they go.  It turns out that changing the temperature in the closet will not much alter the rate water vapor condenses back to liquid, but it WILL change the rate of evaporation from the bucket.  The warmer the water in the bucket, the faster the molecules move, the faster the rate of evaporation.  If we warm the closet, the result is more water evaporates until there is enough water vapor in the air to rebalance the rates.  At that point, the level in the bucket has dropped a bit again, and the air is more humid.*

What if we cool the closet?  You guessed it: the evaporation rate slows much more than the condensation rate does, and water condenses back into the bucket until there is less remaining water vapor so the condensation rate drops back into balance with the new evaporation rate of the cooler bucket.  The bucket has regained some of its water, and the air is drier.

By now, it might have occurred to you that the real world doesn't behave quite like this. For one thing, water condensing out of the air won't just condense inside the bucket, but on any surface--the walls, floor, ceiling, etc.  This often happens outdoors when the humidity is high enough and surfaces cool off during the night: water accumulates on these surfaces because the chilled water on them doesn't evaporate fast enough to equal the condensation rate.  We call this DEW.  It's fascinating to go out in the morning and observe the kinds of surfaces that do and do not collect dew.  (We'll save that discussion for a later post.)

It's important to note that condensation requires a surface to condense upon.  But clouds are in the air!  It turns out that there are all kinds of surfaces available in normal outdoor air: dust particles, soot, salt crystals, and even bacteria drifting in the air can serve as "condensation nuclei," so that "cloud droplets" form around them. 

We haven't quite gotten to clouds yet, but we know how to form them in principle: provide enough water vapor, surfaces for droplets to condense on, and then cool the environment.  For any given level of water vapor in the air, there is a temperature at which condensation will begin to win out over evaporation.  This temperature is called the DEW POINT.  The higher the level of water vapor present (called "absolute humidity") the higher the condensation rate and so the higher the dew point temperature.  When you feel damp, sweaty, and miserable, you might hear the weather forecaster name a dew point that is only a few degrees below the air temperature: the air is so humid that only a tiny temperature drop will cause net condensation.  Or the forecaster might say the relative humidity is nearly 100%--with 100% RH being the balance point between evaporation and condensation in that damp, uncomfortable air.

 By the way, the reason humid air is so uncomfortable is that our body loses excess heat by sweating: the evaporating sweat absorbs heat from your skin and makes you cooler.  If the air is very dry such evaporation happens so quickly that you may not even be aware you are sweating, but in humid air there will be so little net evaporation that--instead of cooling you--your sweat will simply accumulate.  Yuck!

One more point and then we'll close for today.  If surfaces cool below the dewpoint, you get dew.  But if cool ground chills the air above it so that water vapor condenses into cloud droplets near the ground, we have fog.  And of course if the air cools higher up, a cloud forms. 

One more piece will make the puzzle pretty complete: why does air cool and warm in the first place?  That will be next time.


I set out to show cloud in dynamic change.  (This despite my entry-level camera.)  Watch these passing low clouds especially for signs of evaporation: smaller whisps of cloud "disappear" as they evaporate.

*A site that debunks the misunderstanding that "warm air holds more water than cool air," so that "air is like a sponge" is Bad Clouds.

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