Winter Solstice--and the Reason for the Seasons

Winter solstice this year was on December 21.  (Depending on when leap years fall, it is sometimes the 22nd.)  The solstice (means "sun stops") occurs when, from our point of view, the noonday sun reaches its lowest point in the sky in the northern hemisphere--it stops getting lower, and will begin rising again after this date.  How low it goes depends on the observer's latitude: the higher your latitude, the lower the sun, until you reach the Arctic Circle inside which the sun will not rise at all on this date. 

The winter solstice is the boundary between fall and winter--all the seasons begin and end as astronomical events, rather than changes in the weather. 

Recall that all globes come with a built-in 23 1/2 degree tilt.  That tilt represents the degree to which earth's axis of rotation is out of perpendicular to its orbit around the sun--as it would be if you put a (small) model of the sun on the same table with the globe.  Model our seasons in the following way.  Put a powerful lamp in the middle of a large table.  Put the globe on the edge of that table with the north pole tilted as far as it can go toward the sun: it is June 21st, the summer solstice.  As you spin the globe, notice how high the sun would get in the imagined sky of North America; notice also that much more than half the northern hemisphere is illuminated.  (It's hard to see this with the usual shiny-surfaced globe; I dust it with chalk dust to make clearer how much is lit.)  Because of this, the northern hemisphere day will be longer than its night, and the sun will shine more directly (so more intensely) on the surface at noon.  Both of these factors mean more heating of the northern hemisphere, bringing on warmer weather as the heat builds up. 

Now slide the globe counter-clockwise around the edge of the table to make the weeks and months pass.  While you do, be careful to keep the axis of the globe point in the same direction all the time (keep it aimed always at the same side of the room).  When you reach the opposite side of the table, you will find the north pole now tilted away from the sun.   It is now Decenber 21 and the winter solstice.  Notice that the situation is reversed from six months ago: the sun will not rise nearly as high for North America, and since most of the hemisphere is in darkness, the daytime will be short.  The earth's surface in the northern hemisphere is receiving much less heat than six months ago, so it is getting colder.

Several lines on the globe are defined by the solstices.  The Tropic of Capricorn is the line at 23 1/2 degrees south latitude where the sun will be directly overhead on the winter solstice, while on that date nothing inside the antarctic circle will get any sun at all, while inside the arctic circle on that date the sun won't even set!  Similarly for the tropic of cancer and the summer solstice.  The "tropics" is therefore that band from 23 1/2 degrees north to 23 1/2 degrees south where the sun will pass directly overhead at least once each year.

This might put a few misconceptions to rest.  First, notice that our distance from the sun is not a big factor.   In fact, the earth is actually a bit closer to the sun right now than it will be in June!  Notice also that the seasons will be opposite in the two hemispheres: summer has just begun for my friends in New Zealand, who are enjoying their longest days right now.  Notice that the tilt of the earth'a axis is not changing--just where the sun is in relation to it.  [You might remember that the north pole always points (approximately) at Polaris, the "pole star."]  Finally, don't confuse orientation with distance: plenty of students do this very activity, then get mixed up explaining that when the north pole is tilted toward the sun that hemisphere is actually closer to the sun--true, sort of, but only by a miniscule fraction of a percent! 

Remember: the changing seasons are about the length of days and the directness of rays!

One question might still occur to you: why isn't the winter sostice the coldest day, and the summer solstice the warmest?  It's strange to think that, as the days lengthen into January, the weather is still cooling off!  The secret is to think in terms of the balance between heat gain and loss: the summer solstice is when the northern hemisphere gains heat fastest, but it takes time for that temperature to rise; similarly, the decreased heating that reaches its lowest ebb on Dec 21 will take time to have its full effect.  So it really does make sense to begin winter with the winter solstice: though the fastest cooling is past, the chill will still deepen further.

 

Material Guy

I guess I am ultimately a materialist.

For a long time after we bought this house fifteen years ago I took a certain delight in fiddling with things--particularly outdoor things.  Since we had only rented before, owning property was a novel experience.  Almost before we moved in, I had begun drawing up a design for a native meadow garden to replace the "wildflower mix" garden in the strip between house and driveway, dreaming my way through catalogs from native plant nurseries.  The first spring I could not bear to cut the back lawn until I had traversed the whole on hands and knees, cataloging the striking diversity of grasses, sedges and other herbs that made up our "lawn."  (Today an artistic arrangement of dried plants from the yard adorns our wall.)  That year or the next I built the "grownup" tree platform in the tiny woods out back.  And a year or two after that I began a protracted war against the English ivy that had invaded and taken over a large part of the little woods.  Homeowner delight, naturalist-style.

 

My strip of meadow.  It will be more impressive in a month or so. 

Twice as old now, the grasses form a solid mass as tall as me in late summer.  (6/1/2006)

 

 

 

In some years I couldn't bear to mow down all these beautiful plants until July. (7/2/2005)

Then I got too busy to use my little tree platform, the meadow thrived until it no longer needed me much, other interests distracted me until the ivy had grown back, and plumbing and other issues made me sometimes yearn for the days when I could just call my landlord for a fix.

But through it all I've never lost my interest in our slightly mysterious property lines.  The border with the neighbors on each side hasn't much wiggle-room, but the woods of a number of neighbors runs together with ours, and property lines don't all run straight, so I've never been exactly sure where our land ended, so have never exactly known how much fiddling I could get away with out there.  There are (were) some majestic red oaks back there that I hoped were ours, but probably weren't.  And what about the little stand of spindly, old white oaks way back there?

New impetus for the settling of our property lines came when the accumulation of standing dead trees came to include a small elm near the property line that clearly threatened two houses.  Was it our problem? or would the bill go to the neighbors?  (It was too tiddly a situation to be safely taken down by me and my little electric chain saw--the diy savings would have been overwhelmed by the cost of repairing at least one home and also our relations with the neighbors.)

This at last sent me in search of plot plans for our neighborhood on the net.  Lo and behold, they were there--something I guess we wouldn't have found thirteen years ago.  The scanned image of the old drawing announced that we owned a bit more land than I'd thought, and it extended a bit farther back.  They also gently informed me I'd be paying a hefty bill to professional arborists.

 

My "new" scarlet oak. (12/11/2013) 

The upside is that I suddenly have more trees, including at least one fine, strapping scarlet oak and a multiple-trunked pignut hickory, as well as some of the spindly white oaks.  Pride of ownership in my majestic 4/10 of an acre--complete with ramshackle house and leaky roof--swells once more.

For a bit of contrast, here's Thoreau in Walden:

I have frequently seen a poet withdraw, having enjoyed the most valuable part of a farm, while the crusty farmer supposed that he had got a few wild apples only.  Why, the owner does not know it for many years when a poet has put his farm in rhyme, the most admirable kind of invisible fence, has fairly impounded it, milked it, skimmed it, and got all the cream, and left the farmer only the skimmed milk.

If the shortest day is Dec 21st, why is the earliest sunset weeks before?

A recent post at EarthSky explains a confusing fact: the earliest sunset of the year is NOT on the shortest day of the year--December 21st--but weeks earlier.

The days shorten until December 21st or so, due to the earth's revolving around the sun to the point at which the earth's tilt has the northern hemisphere tilted as far as possible away from the sun.  It would be reasonable to expect that the days would shorten equally at both "ends"--so later sunrises and earlier sunsets--but that turns out to be wrong.

On the right is the situation we're approaching: notice that

much of the northern hemisphere in darkness, so that our daylight hours are few.

I first discovered this years ago as a junior high school science teacher.  I liked to get out of the book sometimes, and do big outdoor things.  One favorite was to make the school yard into a giant sundial using the flagpole as a gnomon.  (My hope was that such a tall pointer would make a shadow you could watch move just standing there for a few minutes, but the shadow turned out to be too indistinct to work well.)  I wanted to use the shadow to establish the exact direction of south by looking at the shadow at "local noon"--the time when the sun is directly over your meridian, that is, your longitude line.  I figured to find that time as the half-way point between sunrise and sunset.  (That didn't work as planned, owing to the definitions of sunrise and sunset!)  It was in studying the newspaper almanac day after day that I became confused by the seeming lack of a pattern in the times. 

Note: I've discovered that the explanation below (as well as the EarthSky post referenced above) has errors. I will fix these in a post later in January. (Edited 1/3/14)

EarthSky describes the reason for the mismatch this way: "The time difference is due to the fact that the December solstice occurs when Earth is near its perihelion – or closest point to the sun* – around which time we’re moving fastest in orbit. Meanwhile, the June solstice occurs when Earth is near aphelion – our farthest point from the sun – around which time we’re moving at our slowest in orbit."

That explanation is good, but has gaps I think need filling.

First, why should the speed of the earth in its orbit around the sun affect sunrise and sunset times?  For convenience, let's count a day as being from one solar noon to the next.  (The Royal Navy used to do this.)  We think of the 24-hour day as the result of the earth's rotation on its axis, but in fact the sun is also moving about one degree of its 360 degree annual trip around the sun in that same day.  That means that the earth not only has to rotate one degree MORE than 360 degrees on its axis in order for our location to return to pointing at the sun--that is, back to solar noon.  As long as the earth kept a steady speed in it's orbit, all would be well.  But instead the earth speeds up a bit as winter approaches, and begins slowing again after early January.  Because it is going more than one degree around the sun in December, the earth must rotate farther to bring it around to the same solar noon, making everything (all things being equal) a bit later.  The opposite occurs when the earth is at its slowest, in June and July.  This effect combines with the shortening days of fall to create the odd timing of sunrises and sunsets.

Here's an animation that makes the below clearer.

Imagine you are standing on earth where the left-pointing arrow begins. From "Day 1" to "Day 2" your location has rotated 360 degrees PLUS an additional amount to point back toward the sun, since the earth has moved a bit further around the sun.

At least one more question occurs: why does the earth change speed in the first place?  That has to do with the elliptical shape of earth's orbit, and a law first discovered by Johannes Kepler centuries ago (soon after Copernicus and Kepler established that the earth revolved around the sun, instead of the reverse).  Earth's orbit is an ellipse (oval) that is not quite circular, with the sun a bit nearer to one end of the oval.  Kepler discovered that planets move slowest when they are farthest from their parent body, and fastest when closest.  To be precise, any orbiting body sweeps out equal areas of its orbit in equal times.  (You can think of these areas as pie-slices of the whole orbit: it will take a wider pie slice to cover the same area when the wedge is shorter.) 

Enough for now.  More about the "reasons for the seasons" soon!

*This surprises a lot of people, who assume that summer is warm because the earth is closer to the sun, while winter is colder because it is farther--but the opposite is true.  It turns out that the difference in earth-sun distance from summer to winter is not very great, and the reason for the seasons lies elsewhere, as I'll explain at the solstice in a couple of weeks.

The Generation Passing



hairy woodpecker & northern flicker photos from Cornell's
http://www.allaboutbirds.org/guide/search.aspx

We have been blessed to hear the percussion of woodpeckers in recent years, and occasional sightings of the striking black and white plumage with red accent of (most likely) the hairy woodpecker.  (We've also several times hosted northern flickers, a ground-feeding relative.)   The downside of their frequency is the reason for it: we have lost a number of our trees.

 A pair of big red oaks died on their feet about six or seven years ago, the second coming down in a blizzard only last winter. (Enjoying the blizzard in Thoreauvian fashion at the time, I dodged it by noticing how alarmingly it swayed in the wind.) A pretty little scarlet oak in the backyard died about the same time. I am embarrassed to say I didn't realize these had died until a year or more had passed. These three were apparently in the prime of life, and may have succumbed to the winter moth caterpillars that infested us around that time.

The big old three-stemmed black cherry that supported the kids' tree fort has been dying gradually. One stem came down in a rainstorm last spring: the break was twenty feet up, in an area weakened (unbeknownst to us) by insects.

And a small elm in the narrow space between us and a neighbor--a bit of a weed, really--has been leafless for several years, and without bark for the last year.

Most of these are reason to be a little sad, especially without their children waiting in an understory to spring up to take their place, as would be typical in a more natural setting. But the little oak in the backyard threatened our garage, while the elm could not be felled at all in usual way without taking off part of our roof and maybe the neighbor's as well. It was time to call in the professionals, and to figure out how to pay the bill.

    Once a strapping young adult scarlet oak.                  Worldly remains of a young elm.

Now then, how will I get the woodpeckers back?

 

A Moment in the Life of a Tree

Among the forgettable Norway maples making up most of the trees that lounge around my property, there are a few natives, including a tall, stately white ash.  When the wind was at its height two days ago, I found myself out in the backyard watching it sway and dance in gusts that surely topped 40 miles per hour.  I wondered how much of this the tree could take. Then I reflected on the life of a tree--a life of immobility, of standing against whatever came--unlike for most animals, for a tree there is no escape.

I don't know how old the tree is, no idea how many annual rings lie under that bark, but the tree is about two feet in diameter at chest height, and stands over sixty feet tall.  It has undoubtedly stood against dozens of storms stronger than this in its decades as a full-sized adult.  I relaxed.

How do I know she's over sixty feet?  The rake leaning against the base is six feet tall,

and I estimate the easily-measured part at ten rakes, and there is at least five or ten feet more.

Whenever you look at a big tree, you are necessarily looking at a survivor. You don't get to be a grand-dad like that if you can't take a bit of wind, the odd week of sub-zero cold, the ravages of winter moths, a month of severe drought now and again, and a few false springs that first encourage you to flower and then kill all your flowers. You don't get to be a grand-dad tree if you're merely human.

 

 

Citizen Science Phenology

Poking around the Web as I investigated the idea of phenology*--a preoccupation of Thoreau's and many 19th century contemporaries--I discovered that there is still data to be gathered--it's not simply a 19th century hobby--and WE CAN HELP.  The National Phenology Network (whoda thunk?) has a citizen science project called Nature's Notebook that allows anyone who is interested to enter a location they want to observe over time, choose species of plants and/or animals they want to collect data on, and then upload all this on a regular basis to their database.  The nature of the observations needed differs depending on the species.  If you were watching a white oak, for example, you would record any buds opening, leaves expanding, flowers blooming, leaves turning, and so on.  For an animal like the wooly bear caterpillar (which metamorphoses into a tiger moth), it might be the presence of caterpillars, their feeding, the presence of adults, their mating, and so on. 

Of what use is this data?  One urgent need is to track the effects of global warming on ecosystems, and because phenology records have been kept for a long time, these are particularly valuable.  (Henry Thoreau's own century-and-a-half-old records have even been pressed into service to show that spring temperatures have been arriving earlier than ever before.)  The Nature's Notebook site lists several recent discoveries made with their data.

I was so pleased to find usefulness for my interest in nature, that I immediately signed up; yesterday I uploaded my first observations.

If you'd like to get involved, here are some tips.

· Think about where you can observe (your site) that is very accessible and not too close to a building: you choose when you want to observe, and how intensively, but in times of rapid change (spring, fall) frequent observations (every few days) will be more useful in pinpointing timing. (I chose my own yard to avoid the time needed to travel.)

· When you begin to choose species to observe, you will notice that data is only being collected on certain species. (I was disappointed to discover I would not be able to enter data on my beautiful scarlet oak.) Some, called calibration species, are especially valuable to observe because they are widely distributed, and so allow comparisons over much of the US. In the case of plants, it may be valuable to observe several individuals (though not near neighbors). Of course, make sure you correctly identify what you will observe!

· Be careful not to get in over your head; choose just one or a few species to start with. (I blithely signed up for three tree species and two grasses, and was surprised to find out how long it took to observe all the details and then upload them; I'm hoping I get faster with practice!) When you have become familiar with the time commitment, you can always add more species, or more individuals of the same (plant) species.

· Finally, I had trouble getting through the site set-up process, maybe because I was using Internet Explorer; the website is optimized (I was told later) for Firefox and Google Chrome.

· Other questions you might have will probably be answered in the FAQ

To see the big picture, maybe figure out what is most needed in your area, you can use the Phrenology Visualization Tool to see where in your state and nationwide data is being collected, and on what species. (I found some surprises here.) You can even get data to analyze yourself. These pages are back at the original USA-NPN site.

NOTE that there are other citizen science initiatives that might interest you. Internet-based citizen science started years ago with Seti@home, which put idle home computers to work analyzing radio telescope data from space in search of intelligent extraterrestrial signals. A modern one I've participated in is Zooniverse, which began by putting peoples' eyes and minds to work classifying objects from the zillions of space telescope images, and which has now branched out in interesting ways. These do not get you outdoors the way Nature's Notebook will, though!

*Phenology refers to key seasonal changes in plants and animals from year to year—such as flowering, emergence of insects and migration of birds—especially their timing and relationship with weather and climate. --Nature's Notebook

Why do leaves turn color in the fall?

Poking around looking for a site on fall fall foliage, I lucked out to hit this on the first try.  It has good science, simply and concisely written.   --I know: I could take a lesson...

On the theme of blindness, today I "discovered" a scarlet oak on my very own property!  (No, at eighteen inches in diameter it isn't exactly inconspicuous, but I was never this interested in scarlet oaks until I read Thoreau, and I typically ignore my neighbor's trees; I recently realized that the property line is not where I'd thought it was, giving me a sliver more land and a few nice trees to boot!)

Rut

Friday, November 15, 2013

Our two little dogs get a mile-long walk around the "block" most days.  Our "block" is an oddly-shaped bit of land, but the shortest loop by public roads that includes our house is an honest mile.  Lately my wife usually walks them; sometimes I do.  A week ago I decided to alter the route to take in a row of trees--oaks--still in color, so I could identify them up close then compare their foliage at a distance from home.  The dogs were overjoyed by having new places to nose around and explore and mark.

Remembering their pleasure, I decided to go in another direction last Friday.  The dogs were delighted--but so was I.  Almost instantly I was treated to new sights.  Houses of unfamiliar architecture.  A towering scarlet oak that dwarfed the front yard in which it stood.  A curiously designed housing complex.  A pretty little fixed-keel yacht in someone's driveway.  --all of this within blocks of my home.

 

This beauty, rising in tiers above the manicured lawn,

is about three feet across at chest height.

The experience reminded me what a rut I normally live in, taught me how unfamiliar is my own neighborhood. Tomorrow Golda, Linkin and I will explore further.


PS: I DID explore further just today.  My son Stephen came along, and obliged me by acting as a ruler, standing beside the tree so I could measure its height in a photo.  I make its height out to be about 65 feet, give or take five feet.  From a distance, it is clearly only one of several tall trees on that street.  Afterwards, we investigated new neighborhoods.

Stephen is the tiny figure in gray on the sidewalk.

 

Why is Sunset Red?

The sun gives off the full spectrum (rainbow) of wavelengths of light, from the longest (red) to the shortest (violet).  (We will ignore the infrared and ultraviolet also given off, since these are not visible to our eyes.)  Together, a pretty even mixture of these wavelengths appears as "white light."  Most of these light waves travel through air pretty much unchanged, but those near the violet end of the spectrum, because their short waves are similar in size to air molecules, are scattered in all directions.  That is why a clear daytime sky is blue: that blue glow is short-wave light scattered off air molecules.  As the earth rotates and causes the sun to approach the horizon, the direct light will appear reddish, since more blue light has been scattered out of it.

Now consider a sun setting behind a few clouds.  As the sun drops, the cloud tops, shining by indirect, scattered light, appear blue.  Meanwhile--at a critical moment--the bottoms of the clouds, where they get the direct light "left over" from scattering, reflect this reddened light to your eyes.  Together this color combination can make breath-taking sunsets. 

This limited time adds to the value of a sunset: the time when the angle between the sun and the earth at your location is allows the sunlight to sneak between clouds and ground lasts only a moment.  If, during a sunset you watch the sky to the east, you can often see where this moment has already past, and the clouds have gone blue, while in the east that moment is still to come.

I don't see sunsets often, because of the trees west of my home.  But I caught this sunset last evening as I was coming out of a store.  I actually regretted my impulse to get out my camera: the most glorious moment passed while I was fumbling with it, and the photo does not come close to capturing it.  I should have skipped the camera, and captured it in my memory.  Besides being "quicker on the draw" than a camera, the eye and mind can focus in on what's important and ignore distractions such as steet lights,  trees and power lines.

a nice simple source of info is: http://spaceplace.nasa.gov/blue-sky/en/

 

Kayaking Massasoit State Park

At work on Friday, I overheard a coworker recommended a pond in Massasoit State Park for canoing, and I determined to try it.  Saturday, November 3rd turned out to be unseasonably warm, so I grabbed the opportunity.  I and my middle son spent about an hour-and-a-

half on the water.  Lake Rico, less than a mile long and irregular in shape, is a beautiful place, and the fall colors made it more so. 

 The sky alone was the worth the price of admission.

 

A scarlet oak against white pines.

 

Yellow fall foliage of silver maple

(Acer saccharinum, or what Thoreau called white maple).

 

Identifying a few plants by fall foliage.

 

 

Venus with a Fingernail Moon

Walking into our polling place this evening, my wife pointed out one of the thinner crescent moons I've been blessed to see.  With Venus there, and a serene cloudscape, I needed a photo.  The aerial wires weren't exactly part of the plan--but I was running out of time to find a vantage point, so I grabbed the tripod and went out on our little bit of flat roof.  Not too bad for my little point-and-shoot camera, I think.

Paying Attention

Saturday, November 02, 2013

The commonplace also deserves our attention.  Taking the dogs around the block at midmorning, I came upon a red maple in full color.  I stopped, stood on the dogs' leashes, and took photos--some against the sun, others against a beautiful cloudscape, and at different distances--in an effort to capture this rarity.  Continuing on, I immediately encountered no fewer than three more, each as beautiful or more so than the first that I'd lavished attention on. 

I walk this same route regularly, yet was almost thunderstruck today by trees that must have been in their full glory on at least one earlier walk.  So what happened?  After reading in Autumn Tints, I "knew" that red maples are about the first to turn, and sugar maples only later.  I'd attended to sugar maples in full color, and then watched them drop their leaves.  I'd paid more attention to Thoreau's description of "reality" than the thing itself.  So I was surprised.  As well as paying more attention to the commonplace, I need to pay more attention, period.

The scarlet oaks I was disappointed of at Blue Hill

turn out to be fairly common in my neighborhood.

 

I liked the combination of colors here: sugar maple (I think) overhanging,

paper birch, then scarlet oak in the background.

Scarlet oak.

 

 

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.