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 laws.  In 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:
http://weather.about.com/od/cloudsandprecipitation/ig/Clouds-Types-on-a-Weather-Map/

 

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: http://www.vivoscuola.it/us/rsigpp3202/umidita/lezioni/form.htm

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!

 

Autumn Tints

The photo is a month late, but I came across this paragraph in Thoreau's Autumn Tints, and couldn't resist.  Anyay, there are some red maples still lingering in color.

A small red  maple has grown, perchance, far away at the head of some retired valley, a mile from any road, unobserved.  It has faithfully discharged the duties of a maple there, all winter and summer, neglected none of its economies, but added to its stature in the virtue which belongs to a maple, by a steady growth for so many months, never having gone gadding abroad, and is nearer heaven than it was in the spring.  It has faithfully husbanded its sap, and afforded a shelter to the wandering bird, has long since ripened its seeds and committed them to the winds, and has the satisfaction of knowing, perhaps, that a thousand well-behaved maples are already settled in life somewhere.  It deserves well of Mapledom.  Its leaves have been asking it from time to time, in a whisper, "When shall we redden"?  And now in this month of September, this month of traveling, when men are hastening to the seaside, or the mountains, or the lakes, this modest maple, still without budging an inch, travels in its reputation,--runs up its scarlet flag on that hillside, which shows that it has finished its summer's work before all other trees, and withdraws from the contest.  At the eleventh hour of the year, the tree which no scrutiny could have detected here when it was most industrious is thus, by the tint of its maturity, by its very blushes, revealed at last to the careless and distant traveler, and leads his thoughts away from the dusty road and into those brave solitudes which it inhabits.  It flashes out conspicuous with all the virtue and beauty of a maple,--Acer rubrum.  We may now read its title, or rubric clear.  Its virtues, not its sins, are as scarlet.
                                                                                                                      -- Henry David Thoreau

Nemasket River, Saturday, September 21, 2013

Saturday, October 26, 2013

Walked with boys to top of Great Blue Hill this pm to see if I could see scarlet oaks in fall color.  Got to top of hill at 5:35, just before trees to east were in complete shadow.  Didn't see much beyond the green of white pine and tan-brown of most oaks.  Very little scarlet--I saw more on the drive up than from the hill.  There were still a few red maples with leaves of yellow and some splashes of scarlet on a few individual leaves.  Beech has barely begun to turn.  sugar maples are about done.  Scrub black oak (5-lobed) at the top is partly turned.

We walked over to the Observatory, looked at the very red setting sun, and then started down by the directest trail while the rocks underfoot became increasingly hard to see.  It was nearly full dark when we got down, around 6pm.

 

Recreating Thoreau's sky

"It was the night but one before the full of the moon, so bright that we could see to read distinctly by moonlight, and in the evening strolled over the summit without danger..."

"...It was at no time darker than twilight within the tent, and we could easily see the moon through its transparent roof as we lay; for there was the moon still above us, with Jupiter and Saturn on either hand, looking down on Wachusett, and it was a satisfaction to know that they were our fellow-travelers still, as high and out of reach as our own destiny."

In A Walk to Wachusett, Henry David Thoreau gets astronomically specific in writing about the night spent on the mountain.  Since Thoreau very often puzzles events together from different trips, adds bits from his journals, and even brings in events from completely different circumstances, I thought to check this particular detail. 

He made the trip in July, 1842, as reported in Richardson's Thoreau: A life of the mind.  Stellarium* shows that Jupiter and Saturn were indeed both well up in the sky in July of 1842, and close together.  The Mecklenberg Jeffersonian (Charlotte, NC) for July 19, 1842 reports in its almanac that the full moon that month would be at 4:02am on the 22nd.  (I know Google is too powerful, but its search engine is WONDERFUL.)  Putting the 21st into Stellarium, the moon lies to one side, but on the night of the 20th they made a neat triangle with the moon almost equidistant from the two planets.

So the detail checks out, if you allow Thoreau to be one day off on the phases: the scene he describes occurred two days before full moon, rather than one.  I'm quite pleased by that little bit of verisimilitude; from a distance of one-hundred-seventy-one years, I can tell you Thoreau camped on Wachusett on the 20th of July, 1842.

*I highly recommend Stellarium--an open-source planetarium for the pc--to anyone who wants to know what is visible in the sky at any time from any location.  It is a simple, lean program that is pretty easy to learn to use.  It's light on the bells and whistles, but that's what we have the REAL SKY for!  Go to: http://www.stellarium.org/

 

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.

A Cloud Observation

Although yesterday's mid-morning sky was cloudless, there were fair-weather cumulus clouds moving in by late morning, and at the time of my son's 1:30 soccer game a varied cloudscape covered the sky.  As I stood on the sidelines, I determined to describe it, as a picture, carefully and without jargon, yet so classification would later be possible.
 

The cloud cover was nearly complete, yet seemed barely to dim the sun, which seemed to show a little less distinctly than before.  The high or mid-level clouds themselves were almost formless, but had some variation in lightness like gentle ocean waves, vaguely patterned north-south.  Shreds of blue, nearly-clear sky lay southward, and a few smaller irregular shreds elsewhere, while a stuttering of small, regular puffs ran across most of the lower altitude sky to southward running east-west.  Winds were light and variable mostly around the southern half of the compass when I began my observations. 

Returning my attention to the sky after ten  or so minutes of watching the game, I found the larger pattern still present, but the shreds of blue to have moved and multiplied and the regular puffs to have vanished completely.  After another half hour it was clear the clouds were breaking up.  The wind had steadied and strengthened from the southwest.

 

Someday soon I will have to lay on my back for half an hour in the right conditions with a notebook and see if I can follow the changes in the entire visible sky as quickly as they happen; I expect it will be a challenge. 

For Spacious Skies

Take some time each day to look at the sky.  I am trying to begin each day with a few minutes outside, coffee in hand, looking up.

Block Island Sound, RI, September 7, 2013, by the author.

 

Thanks to the efforts of the late Jack Gordon's For Spacious Skies, I find myself looking at clouds more and more.  But I make two mistakes: either (1) I enjoy their beauty without thinking about how they form and relate to weather, or (2) I think about how they form and relate to weather without enjoying their beauty.  I probably get nearest the happy medium when, rather than simply gazing, or fixating on whether they're stratocumulus or altostratus, I instead see a cloud as dynamic atmosphere made visible: air roiling, temperature changing, and water molecules ricocheting around before my very eyes.  To "see" clouds this way, you need to know a little physics and chemistry.  And after another post or two, you'll be able to see clouds this way, too.

 

To start with, clouds do NOT form because the air is like a sponge that can hold only so much water vapor, and cooling the air is like squeezing the sponge more and more, yada yada yada.  That was what I was taught, believed for many years, and taught many 7th graders (may they forgive me).  As an explanation, it works pretty well, but is simply wrong.  So what is the truth?

The first thing to understand is that water is evaporating and condensing all the time.  That means that a cloud is less a "thing" than it is a dynamic PROCESS.  To understand it, we will start with the fundamentals. 

1. All molecules and atoms move, and they move all the time.   Gas molecules bounce off each other, molecules packed together in a liquid jostle each other, and molecules locked into a solid simply vibrate--but all are in motion.  In a gas, molecules are separated, so they zing around and ricochet off each other like pool balls--with the difference that, unlike on a pool table, the collisions of gas molecules are perfectly elastic--so the balls never "run out of energy" the way pool balls eventually do.*

image from: http://saburchill.com/chemistry/chapters/chap033.html

2. Moving molecules have kinetic energy which is a consequence of their mass and speed.  (The equation for kinetic energy is E_K=mv², which means that heavier, faster-moving molecules have more energy than lighter, slower ones.  And the speed (v, for velocity) has a big impact on energy, because this number is squared.)  We experience this energy of movement as temperature, because temperature is proportional to the average kinetic energy of a sample of molecules.  In short, the warmer something is, the faster its molecules are moving.  Try to picture this when you hold a hot cup of coffee!

3. Every water molecule has a little positive charge at one end, and a little negative charge at the other.  Because of this, water molecules cling to anything with an electrical charge--including and especially each other.  (Remember learning in school that "opposites attract."

What does all this have to do with clouds?  Next time we'll look at evaporation and condensation.

Sunset over Block Island Sound, RI, September 7, 2013, by the author.

*By the way, those gas molecules are moving fast.  As a rule, the speed of gas molecules is about equal to the speed of sound in that same gas.  In air under normal conditions, that's rather more than seven hundred miles per hour.  But they don't usually get too far, since on average air molecules are only spaced 10^-5cm apart, so that a single molecule collides on average five times per nanosecond.  (1ns=one billionth of a second.)  That's fast-moving but mighty small game of pool!
Info from:
http://en.wikipedia.org/wiki/Speed_of_sound
http://www.ems.psu.edu/~bannon/moledyn.html

In Wildness is the Preservation of the World -Henry David Thoreau

I have been reading the 19th century trancendentalist and nature writer, Henry David Thoreau.  I come to Thoreau late in life, after an influential high school English teacher biased me against him (my teacher was a devotee of Thoreau's mentor Ralph Waldo Emerson).  A year ago I picked up Thoreau's most famous book, Walden, at the library.  Though at first I found some of it tough sledding, and ignored the more mystical bits as not speaking to me, I was still hooked at once. 
I reread it almost immediately, an effort to get more out of the book than I was first capable of.  I have since gone on to read A Week on the Concord and Merrimack Rivers, the only other book published in his lifetime, his Cape Cod, and some of The Maine Woods, both published posthumously, and--most riveting for me--some of his enormous journal.  I just picked up a biography--the second--and I expect to be working my way through his writings on and off for years to come.  To encapsulate my interest in him, I need only say I recently paddled in the Concord River in a home-made kayak I named after his (Musketaquid, the indian name for that river), I have gazed at his few belongings in the Concord Museum, and I have contemplated his grave.  I have not yet become a member of the Thoreau Society--but only because I'm short of cash.

The Concord River

My Musketaquid.  Henry's was also homebult, but more of a dory.

Ralph Waldo Emerson's grandfather's house, by the north bridge 

famous for the "shot heard round the world."

Henry's grave, beside those of his siblings and parents, is a pilgrimage site.

Thoreau has been hailed as an early environmentalist and ecologist, inspiration for Gandi and M.L.K, Jr., saw himself a philosopher in the large sense, something of a mystic, was a keen observer of his age and place.  He was devoted to "wildness" in the settled agrarian and industrial town of Concord, Massachusetts, where he lived his entire life.  (The title above is a quote that now graces the bumper of my car.)

I am most drawn to Thoreau's relationship to nature, but he was very much more than a naturalist and nature writer--in even the most "scientific" of his published work he seldom writes about nature alone.  His philosophy of life lived deliberately is a continuing goad to me.  Lately he has been my historical guide into the New England of a century-and-a-half ago, as well as the man who knows every flowering plant in Concord, and when it blooms.  In his journal, he is an odd but interesting and charismatic friend.  He is often over-simplified and sometimes caricatured, but Thoreau was a complex and paradoxical man, and I will return to some of his many facets in future posts. 

I leave you with another quote that continually comes to mind.  "Only that day dawns to which we are awake."