Wednesday, October 30, 2013

For Spacious Skies -part 3

You now know that a cloud forms in moist air when that air cools below its dew point--the temperature at which the evaporation of water from cloud drops slows enough that the relatively greater condensation rate causes those drops to grow.  You also know why the temperature influences evaporation.

Now we attack the question of what cools the air in the first place.  The most common is that something lifts the air upward, resulting in adiabatic cooling (definition later).  This involves the structure of the atmosphere, plus a nifty bit of physics called the ideal gas laws.

First the atmosphere.  You probably know that the air is thinner as you go upwards, so that the air where commercial jets fly is too thin even to breathe successfully.  You can feel the difference even driving in hilly country as the changing pressure affects your eardrums.  The reason for this is simple: air is compressible.  The vertical structure of the atmosphere is a bit like a giant  stack of pillows: the topmost pillows are light and fluffy, but as you go downward they are compressed more and more under the weight of pillows above.  The bottom pillows will be squashed flat, dense, under a lot of pressure.   As you move upwards in in the atmosphere, air pressure AND air density decreases for the same reason.

 Now the perfect gas lawsIn a nutshell, three things are interrelated in any body of gas: its volume, its pressure, and its temperature.  Changing any one of these three things affects one or both of the others.  If a gas is compressed into a smaller volume, its pressure and/or temperature rise.  If the gas is allowed to expand, its temperature and/or pressure falls.  So if a mass of air rises upward, the lowered pressure causes its temperature to fall.  (This is called adiabatic cooling.)  If that temperature falls below its dewpoint, cloud droplets grow as condensation of water vapor onto particles in the air wins out over evaporation.  A cloud forms.  Conversely, sinking air warms, evaporating any cloud that is present.   Because the altitude at which air reaches its dew point is fairly constant in a given situation, clouds often have pretty flat bottoms, all of which line up; the illusion created is that the clouds are sitting on some sort of invisible surface.  (In reality, rising air is rising through a sort of boundary line where cloud drops begin growing rapidly.)
The next time you see light, fluffy cumulus clouds apparently floating in the sky, imagine air rising where the cloud is, and sinking in between, continuously creating the appearance you see.

Watch the low, passing clouds for signs of evaporation.  (Follow the small isolated bits.)

Well then: why does the gas rise?  Usually one of two reasons, and each results in a different basic cloud type.  First, air can be heated, and that warm air rises, buoyed up by the cooler, denser air around it.  This often happens to air in contact with the warm ground on a sunny day.  Each rising mass of air begins to form a cloud when it reaches the altitude at which its temperature drops below the dew point.  This results in the puffy, separate clouds called cumulus.  Second, an enormous area of air can be lifted all at once (for example, by an approaching front.  This results in a layer cloud called stratus.



Cumulus mediocris and congestus over Swifts Creek, Australia (Wikipedia Commons)

Stratocumulus stratiformis perlucidus over Galapagos, Tortuga Bay  (Wikipedia Commons)

Make your own cloud!  Take the label off a soda bottle (the bigger the better) so you can see inside more conveniently.  Get a match ready to light.  Put a little water (a tablespoon or two is enough) into the bottle and shake it.  Now light the match, let it burn a moment, then blow it out and drop it, still smoking, into the bottle.  Put your mouth to the open end of the bottle and blow, increasing the air pressure in the bottle.  After a few seconds--and while watching what is happening inside the bottle--release the air.  There: do you see it?  The air went cloudy the moment you released the pressure.  Clouds you make this way will sometimes last several minutes.

Can you explain what happened from what you have learned?  (Try on your own before reading on.)

Shaking the water in the bottle allowed as much as possible to evaporate quickly, increasing the humidity in the bottle to near-saturation.  The smoke particles from the smoldering match provided condensation nuclei.  When you forced additional air into the bottle, the pressure increased, causing the air to warm slightly, so more water could evaporate, raising the dew point.  When you released the pressure, the temperature in the bottle dropped below the dewpoint, the condensation rate overcame the evaporation rate, and water vapor condensed on the smoke particles forming tiny cloud droplets.  There!  A cloud of your own!

For nice (though small) photos of all the more common cloud types, see:

It's worth mentioning that water vapor is a powerful carrier of energy in the atmosphere.  As water evaporates due to solar heating, potential energy becomes stored in the water vapor that results.  We think of this as potential energy because the rapid motion of the gas particles prevents (on the whole) their condensing back into clusters (drops) bound by their electrical attraction.  (This is analogous to a rock at the top of a cliff or hill: it has potential energy that it can give up as it falls or rolls downward due to the force of gravity.  In the case of water vapor molecules, the electrical attractions are the "gravity," while their rapid motion is the "height.")  Just as the water absorbs energy in evaporation, it gives off energy as it condenses. 

 Let's see how that energy can be a powerhouse.  The sun warms moist ground or a lake, causing water to evaporate and form a warm and humid body of air.  The warm moist air begins to float upward in the cooler air around it because warm moist air is lower in density.  As it rises, the drop in pressure causes that body of air to expand, lowering its temperature below the dew point.  If this warm air were dry, it would cool enough to be the same temperature as the cooler air around it, its density would match that of the surrounding air, and it would stop rising--end of story.  BUT because that air is humid, water vapor begins to condense into cloud drops.  The process of condensation produces heat (that potential energy is no longer just potential!) that prevents the air from cooling any further, so it continues to rise.  Condensation continues in the rising air, causing the cloud to tower higher and higher, until the supply of water vapor is small enough that condensation ceases, the air stops rising, and the cloud stops building.  If there is enough water vapor to build the cloud into a thunderhead, a thunderstorm may result.  And if there is enough warm, humid air (say, over the subtropical Atlantic Ocean in summer) then the energy of the condensing water vapor may power a hurricane--a kind of runaway freight train of condensation--and the most destructive of all storms. 

So when you see a cumulus cloud boil upward, increasing in height over time, you are seeing the power in water vapor.

More coming about particular clouds...

For more information:

source for "how do clouds form?" "that distinguishes convective vs stratiform process

In somewhat the same vein as above--five different situations that can cause cloud formation (though this page does refer to air "full of" moisture)

Here is one that has a little animation that gets it wrong

A nice diagram of adiabatic cooling, though it still mentions air's ability to hold moisture:

A nice, comprehensive look atclouds and their phenomona
The only (repeat: ONLY) site I've found that gives explanations for some particular cloud shapes

ExTREMELY cool time-lapse video of storm overspreading a city

The slower but larger high-def version takes much longer to load, but IS WORTH IT!


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