Friday Jan. 11, 2013
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Time for only one song before that start of class today.  You heard "Somebody That I Used to Know" from Gotye.

Exactly 49 sets of Experiment #1 materials were checked out before class today.  Those of you that have materials will eventually find your name on the Expt. #1 signup list.  Even though your report isn't due until Feb. 4 the experiment can take several days, maybe even a week, to run to completion so don't wait too long to get it started; this weekend would be a perfect time.  You may need to check the experiment fairly frequently at the beginning (every hour or two).  It slows down somewhat as it progresses.  You'll find more information about the experiment here.

Once you've completed the experiment and return the materials you'll receive a supplementary information handout that will help with the analysis portion of the experiment.  You should try to return the materials well before the report due date. 

Because I'm only teaching one section of Atmo 170A1 this semester I have plenty of additional materials.  I'll bring some more Expt. #1 materials to class on Monday. 

Signup sheets for all four experiments were circulated in class, though I only got 3 of them back at the end of class.  So some of you may not be signed up.  Check on the appropriate list and please let me know (in class or by email) if your name isn't there. 

A strong cold front passed through Tucson Thursday night.  We won't be covering fronts for another 2 or 3 weeks but I wanted to give you a little information about them so that you can better understand the cold weather that we will be experiencing the next few days.


A cold front is a boundary between warm and cold air.  A cold front is found at the leading edge of a bunch of cold air that is moving into a region.  A cold front is shown on on the figure above and on surface weather maps using a blue line with "points" on it.  The points identify it as a cold front and show what direction the front is moving.

About this time yesterday a cold front was located in western Arizona and was headed toward Tucson.  At that time we were in the warm air found ahead of the front (figure above).   The front passed through during the night and was accompanied by some gusty winds and light rain showers.


By this morning the frontal boundary had moved east of Tucson and we found ourselves in the cold air behind the front (figure above).  But there is still even colder air (and drier air) to our west that will move in the next day to two.

Some very cold (for Tucson anyways) nighttime temperatures (for Tucson anyways) are predicted for the next several nights.  You can find the latest predictions here (from the National Weather Service office here in Tucson). 

A couple of time lapse videos of cold fronts moving through Tucson were shown in class The first was recorded on Apr. 4, 1999 (Easter Sunday).  It was cold enough for snow to fall in the valley here in Tucson though it melted once it hit the ground.  The 2nd video was from Feb. 12. 2012.

Something like a cold front, though on a much smaller scale, is produced by the cold air falling from a thunderstorm (the thunderstorm downdraft).

The cold air hits the ground and spreads out radially underneath the storm.  These winds can sometimes reach 100 MPH and, in Arizona at least, stir up a lot of dust.  You would see a cloud of dust moving outward from underneath the thunderstorm.  The front edge of this dust storm is called a gust front or haboob (a more descriptive name might be dust front).  They're a pretty common occurrence in Arizona during the summer thunderstorm season and we'll talk more about them later in the semester.

Here is a gallery of images of a dust storm in Australia that just occurred a day or two ago.  Have a look at Image #11 first because it shows the dust cloud about to move out over the ocean.  Once the dust cloud moved out over the ocean it began displacing warm moist air.  The air was moist enough that it formed a cloud at the top edge of the dust cloud.  This type of cloud is called a shelf cloud.  You can see that clearly in Image #1.

We had a quick look back at Wednesday's notes.  I wanted to be sure you knew that the notes contain some information that wasn't covered in class.

Then back just briefly to the dew point.  It has two "jobs".  The first is to give you an idea of how much moisture is in the air.  Air with a 60 F dew point temperature contains more water vapor than air with a dew point of 30 F.


The dew point has another job.

If you cool air next to the ground to its dew point, water vapor will condense and coat the ground with water.  The ground will be covered with dew.  If a little thicker layer of air is cooled fog will form.


At this point we were about 40% of the way through the class period and we still had the main topic of the day,  the origin and evolution of the atmosphere, to cover.

The atmosphere we have today (mostly nitrogen, oxygen, water vapor, and argon) is very different from the earth's original atmosphere which was mostly hydrogen and helium. 



This original atmosphere either escaped (the earth was hot and the gases were moving around with enough speed that they could overcome the pull of the earth's gravity) or was swept into space by the solar wind (click on the link if you are interested in learning more about the solar wind, otherwise don't worry about it). 


With the important exception of oxygen (and argon), most of our present atmosphere is though to have come from volcanic eruptions.



Don't worry about remembering all of the gases listed above (you might find slightly different lists depending on the source you check).  Volcanoes emit a lot of water vapor and carbon dioxide.  As the earth began to cool the water vapor condensed and began to create and fill oceans.  Carbon dioxide dissolved in the oceans and was slowly turned into rock.  Smaller amounts of nitrogen (N2) are also emitted by volcanoes.  Because nitrogen is relatively nonreactive it remained in the air and its concentration began to built up over time. 

There are also lots of poisonous gases such as sulfur dioxide emitted by volcanoes.  We'll learn a little more about sulfur dioxide, in particular, next week when we cover air pollutants.

Two or three years ago, air travel to Europe was being severely disrupted by the eruption of the the Eyjafjallajökull volcano in Iceland.  Here are some really amazing pictures published in the Boston Globe.  Here's another set of photos also from the Boston Globe.

Volcanoes didn't add any of the oxygen that is the atmosphere.  Where did that come from?



The oxygen is thought to have first come from photo-dissociation of water vapor and carbon dioxide by ultraviolet (UV) light (the high energy UV light is able to split the H20 and CO2 molecules into pieces).  The O and OH then react to form O2 and H.

By the way I don't expect you to remember the chemical formulas in the example above.  It's often easier and clearer to show what is happening in a chemical formula than to write it out in words.  If I were to write the equations down, however, you should be able to interpret them.  Ultraviolet is a high energy form of light and it's probably also good to remember that ultraviolet light is capable of breaking molecules apart.




Once molecular oxygen (O2) begins to accumulate in the air UV light can split it apart to make atomic oxygen (O).  The atoms of oxygen can react with molecular oxygen to form ozone (O3). 

Ozone in the atmosphere began to absorb ultraviolet light and life forms could then begin to safely move from the oceans onto land.  Prior to the buildup of ozone, ocean water offered protection from UV light. 

Photosynthesis is the 2nd and now the main source of atmospheric oxygen. 



Photosynthesis in its most basic form is shown in the chemical equation above.  Combustion is really just the opposite of photosynthesis and is shown below.


We burn fossil fuels (dead, undecayed plant material) to generate energy.  Water vapor and carbon dioxide are by products.  Combustion is a source of CO2.  We'll see these two equations again when we study the greenhouse effect (CO2  is a greenhouse gas ) and global warming.

And something I didn't mention in class (and something you don't need to remember).  The argon we have in the atmosphere apparently comes from the radioactive decay of potassium in the ground.  Three isotopes of potassium occur naturally: potassium-39 and potassium-41 are stable, potassium-40 is radioactive and the source of the argon in the atmosphere.

The following figure is the first page in the packet of photocopied ClassNotes.


This somewhat confusing figure shows some of the important events in the history of the earth and evolution of the atmosphere.  There were 5 main points I wanted you to take from this figure.

First, Point 1: the earth is thought to be between 4.5 and 4.6 billion years old.  If you want to remember the earth is a few billion years old that is probably close enough.  Something I didn't mention in class, it's in small type above.  The formation of a molten iron core was important because it gave the earth a magnetic field.  The magnetic field deflects the solar wind and keeps it from blowing away our present day atmosphere.

Stromatolites (Point 2) are column-shaped structures made up of layers of sedimentary rock, that are created by microorganisms living at the top of the stromatolite (I've never actually seen a stromatolite, so this is all based on photographs and written descriptions).  Fossils of the very small microbes (cyanobacteria = blue green algae) have been found in stromatolites as old as 2.7 B years and are some of the earliest records of life on earth.  Much older (3.5 to 3.8 B years old) stromatolites presumably also produced by microbes, but without microbe fossils, have also been found. 




Blue green algae grows at the top of the column, under water but near the ocean surface where it can absorb sunlight.  Sediments begin to accumulate on top of the algae and start to block the sunlight.  The cyanobacteria would then move to the top of this sediment layer and the process would repeat itself.  In this way the stromatolite column would grow a layer at a time.  This isn't a geology class, we're learning about stromatolites because the cyanobacteria on them were a very early form of life on the earth and were able to produce oxygen using photosynthesis.

Living stromatolites are found in a few locations today.  The two pictures above are from Coral Bay (left) and Shark's Bay (right) in Western Australia.  The picture was probably taken at low tide, the stromatolites would normally be covered with ocean water.  It doesn't look like a good place to go swimming, I would expect the top surfaces of these stromatolites to be slimy.

Point 3 refers to the banded iron formation, a type of rock formation.  These rocks are 3 billion years old (maybe older) and are evidence of oxygen being produced in the earth's oceans.  Here are a couple of pictures of samples of banded iron formation rock that I will pass around in class at some point.


The main thing to notice are the alternating bands of red and black.  The next paragraph and figure explain how these formed.

Rain would first of all wash iron ions from the earth's land surface into the ocean (at a time before there was any oxygen in the atmosphere).  Oxygen from the cyanobacteria living in the ocean water reacted with the dissolved iron (the iron ions) to form hematite or magnetite.  These two minerals precipitated out of the water to form a layer on the sea bed.  This is what produced the black layers.




Periodically the oxygen production would decrease or stop (rising oxygen levels might have killed the cyanobacteria or seasonal changes in incoming sunlight might have slowed the photosynthesis).  During these times of low oxygen concentration, red layers of jasper would form on the ocean bottom.  Eventually the cyanobacteria would recover, begin producing oxygen again, and a new layer of hematite or magnetite would form.  The rocks that resulted, containing alternating layers of black hematite or magnetite and red layers of jasper are known as the banded iron formation.  In addition to the red and black layers, you see yellow layers made of fibers of quartz in the samples passed around class.   The rocks are fairly heavy because they contain a lot of iron, but the most impressive thing about them in my opinion is their age - they are a few billion years old! 

We were out of time at this point.  Here are the 2 remaining points (not mentioned in class) just to finish up this topic.
 Eventually the oxygen in the ocean reacted with all of the iron ions and was free to move from the ocean into the atmosphere.  Once in the air, the oxygen could react with iron in sediments on the earth's surface.  This produced red colored (rust colored) sedimentary rock.  These are called "Red Beds" (Point 4).  None of these so-called red beds are older than about 2 B years old.  Thus it appears that a real buildup up of oxygen in the atmosphere began around 2 B years ago. Oxygen concentrations reached levels that are about the same as today around 500 to 600 million years ago (Point 5 in the figure).