Friday Oct. 16, 2009
click here to download today's notes in a more printer friendly mode.

A couple of songs from Dire Straits ("Sultans of Swing" and "Walk of Life") were played before class today.

Quiz #2 has been graded and was returned in class today.  Please check your quiz carefully for grading errors.

The Experiment #3 materials were distributed today.  I'll bring them to class again on Monday.

A new Optional Assignment (Controls of Climate) is now available.  It's due at the start of class on Wed., Oct. 21.

A couple of new Bonus 1S1P Assignments are now available as well.

There are a couple of loose ends to wrap up from the section on the greenhouse effect before moving on to some new material.
You can use the simplified picture of radiative equilibrium to understand the effects of clouds on nighttime low and daytime high temperatures.  You'll find this discussed on pps 72a and 72b in the Classnotes. 

Here's the simplified picture of radiative equilibrium (something you're probably getting pretty tired of seeing).  By now you should be able to identify each of the colored arrows in the figure above and explain what they represent.

The two pictures below show what happens at night when you remove the two green rays of incoming sunlight.

The picture on the left shows a clear night.  The ground is losing 3 arrows of energy and getting one back from the atmosphere.  That's a net loss of 2 arrows.  The ground cools rapidly and gets cold during the night.

A cloudy night is shown at right.  Notice the effect of the clouds.  Clouds are good absorbers of infrared radiation.  If we could see IR light, clouds would appear black, very different from what we are used to (because clouds also emit IR light, if we could see IR light the clouds might also glow).  Now none of the IR radiation emitted by the ground passes through the atmosphere into space.  It is all absorbed either by greenhouse gases or by the clouds.  Because the clouds and atmosphere are now absorbing 3 units of radiation they must emit 3 units: 1 goes upward into space, the other 2 downward to the ground.  There is now a net loss at the ground of only 1 arrow. 

The ground won't cool as quickly and won't get as cold on a cloudy night as it does on a clear night.  That makes for nice early morning bicycle rides this time of the year.

The next two figures compare clear and cloudy days.

Clouds are good reflectors of visible light.  The effect of this is to reduce the amount of sunlight energy reaching the ground in the right picture.  With less sunlight being absorbed at the ground, the ground doesn't need to get as warm to be in energy balance.

It is generally cooler during the day on a cloudy day than on a clear day.

Clouds raise the nighttime minimum temperature and lower the daytime maximum temperature. 

Typical daytime highs and nighttime lows in Tucson for this time of year.  Note how the clouds reduce the daily range of temperature.

We'll use our simplified representation of radiative equilibrium to understand enhancement of the greenhouse effect and global warming.
The figure (p. 72c in the photocopied Class Notes) on the left shows energy balance on the earth without an atmosphere (or with an atmosphere that doesn't contain greenhouse gases).  The ground achieves energy balance by emitting only 2 units of energy to balance out what it is getting from the sun.  The ground wouldn't need to be very warm to do this.

If you add an atmosphere and greenhouse gases, the atmosphere will begin to absorb some of the outgoing IR radiation.  The atmosphere will also begin to emit IR radiation, upward into space and downard toward the ground.  After a period of adjustment you end up with a new energy balance.  The ground is warmer and is now emitting 3 units of energy even though it is only getting 2 units from the sun.  It can do this because it gets a unit of energy from the atmosphere.

In the right figure the concentration of greenhouse gases has increased even more (due to human activities).  The earth would find a new energy balance.  In this case the ground would be warmer and would be emitting 4 units of energy, but still only getting 2 units from the sun.  With more greenhouse gases, the atmosphere is now able to absorb 3 units of the IR emitted by the ground.  The atmosphere sends 2 back to the ground and 1 up into space.

The next figure shows a common misconception about the cause of global warming.

 Many people know that sunlight contains UV light and that the ozone absorbs much of the dangerous type of high energy radiation.  People also know that release of chemicals such as CFCs are destroying stratospheric ozone and letting some of this UV light reach the ground.  That is all correct. 

They then conclude that it is this additional UV energy reaching the ground that is causing the globe to warm.  This is not correct.  There isn't much UV light in sunlight in the first place and the small amount of additional UV light reaching the ground won't be enough to cause global warming.  It will cause cataracts and skin cancer and those kinds of problems but not global warming.

I spend a good part of the remainder of today's class telling you about an awesome field experiment that I took part in several years ago.  What is the tie in with this class?  A good part of the experiment was conducted at a relatively small island near the equator in the middle of the Pacific Ocean.  Once you read the online notes on the factors that control/determine a region's climate you will learn that there is very little change from summer to winter in regions like this.


The photograph above appeared on the cover of the April 1994 issue of the Bulletin of the American Meteorological Society.  If you look closely you'll notice your NATS 101 instructor (he had been given the nickname "Wilbur" by one of the members of the group, the other bald man's name was Orville).  This photo was taken on Kapingamarangi Atoll (shown on the map below), shortly before all the men were about to board ship and leave Kapingamarangi.  The two women (Erica at left, Maureen in the middle) were going to remain behind and operate all of the research equipment.  The scene looks happy enough, but "Wilbur" revealed that he had taken a liking to one of the two women and was anything but happy.

What we were doing on Kapingamarangi?  We were a small part of a much larger field experiment.  Wilbur and Orville's job was to install the tall white lightning detector at the left edge of the photograph.  They would later travel to Rabaul (on New Britain island) and Kavieng (New Ireland island) in Papua New Guinea and install two more detectors.  Papua New Guinea would turn out to be a very different place.  Until recently some of the highland tribes there practiced cannibalism.   You can also get malaria in Papua New Guinea.





To get to Kapingamarangi you first need to fly to Pohnpei (an island in the Federated States of Micronesia).  The route is shown above.  Then you take a cargo ship for about a 4 day sail to Kapingamarangi.  We had intended to fly to Pohnpei, set sail for Kapinga the next day, and then spend about a month on Kapingamarangi.  The ship however was delayed 3 weeks.  That gave us plenty of time to visit the island of Pohnpei but ultimately meant we could only spend a few days on Kapingamarangi..



Pohnpei is a fairly large island and, together with some of the other Micronesian islands, is a popular, world-class, snorkeling and scuba diving destination.   Pohnpei also has a weather station that is operated by the US National Atmospheric and Oceanic Administration (NOAA). 

Here's a reminder of how temperatures change during the year in Tucson.

Pohnpei is located at low latitude in the middle of the Pacific Ocean.  Both of those factors will reduce the annual range of temperature.  How large do you think the annual range is?


The following precipitation data show that Pohnpei is also one of the rainiest locations on earth

Close to 400 inches of rain may fall in the interior of Pohnpei.  The rainiest location on earth is in Hawaii with about 460 inches of rain per year.

Pigs are also an important part of daily life on Pohnpei, Kapingamarangi, and the other islands in Micronesia.


The Micro Glory (shown below) sails back and forth between Pohnpei and Kapingamarangi about once a month.  The ship carries supplies to the people on Kapingamarangi and some other small islands.  They pay for the supplies with pigs (the pigs are sold on Pohnpei).  We shared deck space on the Micro Glory on the trip back to Pohnpei with 20 to 30 pigs (they were hoisted aboard in nets)


Most of the lower deck in the photo above (under the hoists) was occupied by pigs on the return trip.  One of the pigs died on the return trip - that was a very serious matter.

We also had a chance to sample some of the local beverages.


Drinking sakau (as it is called on Pohnpei) turns your mouth and throat numb.  It is supposed to relax you, make you sleep more fully, and doesn't seem to have any after effects.  Until fairly recently you could buy kava in pill form at local supermarkets.  However, because of reports that it can cause serious liver problems, that is no longer the case.  There are no reports of liver problems when drinking kava that has been prepared in the traditional way.  Here is a link to a Wikipedia article on kava.

We never tried betelnut.  Areca nuts are wrapped in betel leaves and chewed together with lime (lime is pretty caustic, that is one of the reasons I didn't try betelnut).  The resulting mixture is a mild stimulant (some people add tobacco to the mix).  The most interesting aspect, however, is that chewing betelnut colors your mouth and teeth bright red.  You don't swallow betelnut, you spit it out.  You see the bright red stains on sidewalks and the ground wherever you go.  Most hotels will also have a large sign near the entrance reminding guests not to chew betelnut inside the hotel.  You can read more about betelnut here.

Try to read through the material below before class on Monday.

The following is an introduction to an important new topic: humidity (moisture in the air).  This topic and the terms that we will be learning and using can be confusing.  That's the reason for this introduction.  We will be mainly be interested in 4 variables: mixing ratio, saturation mixing ratio, relative humidity, and dew point temperature.  Our first job will be to figure out what they are and what they're good for.  Then we see what can cause the value of each variable to change.  You will find much of what follows on page 83 in the photocopied ClassNotes. 


Mixing ratio tells you how much water vapor is actually in the air.  Mixing ratio has units of grams of water vapor per kilogram of dry air (the amount of water vapor in grams mixed with a kilogram of dry air).  It is basically the same idea as teaspoons of sugar mixed in a cup of tea. 



The value of the mixing ratio won't change unless you add water vapor to or remove water vapor from the air.  Warming the air won't change the mixing ratio.  Cooling the air won't change the mixing ratio (unless the air is cooled below its dew point temperature and water vapor starts to condense).  Since the mixing ratio's job is to tell you how much water vapor is in the air, you don't want it to change unless water vapor is actually added to or removed from the air.


Saturation mixing ratio is just an upper limit to how much water vapor can be found in air, the air's capacity for water vapor.  It's a property of air, it doesn't say anything about how much water vapor is actually in the air (that's the mixing ratio's job).  Warm air can potentially hold more water vapor than cold air.  This variable has the same units: grams of water vapor per kilogram of dry air.  Saturation mixing ratio values for different air temperatures are listed and graphed on p. 86 in the photocopied class notes.  You may be clicking on the words highlighted in blue.  You're probably finding that these aren't links.  One of them is a link however, it will take you to a hidden optional assignment that will be due at the start of the next class.

Just as is the case with water vapor in air,
there's a limit to how much sugar can be dissolved in a cup of hot water.  You can dissolve more sugar in hot water than in cold water.

 The dependence of saturation mixing ratio on air temperature is illustrated below:



The small specks represent all of the gases in air except for the water vapor.  Each of the open circles represents 1 gram of water vapor that the air could potentially hold.  There are 15 open circles drawn in the 1 kg of 70 F air; each 1 kg of 70 F air could hold up to 15 grams of water vapor.  The 40 F air only has 5 open circles; this cooler air can only hold up to 5 grams of water vapor per kilogram of dry air.


Now we have gone and actually put some water vapor into the volumes of 70 F and 40 F air.  The same amount, 3 grams of water vapor, has been added to each volume of air.  The mixing ratio, r, is 3 g/kg in both cases.


The relative humidity is the variable most people are familiar with, it tells you how "full" the air is with water vapor.

In the analogy (sketched on the right hand side of p. 83 in the photocopied notes) 4 students wander into Classroom A which has 16 empty seats.  Classroom A is filled to 25% of its capacity.  You can think of 4, the number of students, as being analogous to the mixing ratio.  The classroom capacity is analogous to the saturation mixing ratio.  The percentage occupancy is analogous to the relative humidity.

Instead of students and a classroom you could think of the 70 F and 40 F air that could potentially hold 15 grams or 5 grams, respectively of water vapor.  Maybe this is the Optional Assignment I mentioned I would hide in these notes.  It will be due at the beginning of class on Mon., Mar. 23.

Here are the relative humidities of the 70 F and 40 F air that each contain 3 grams of water vapor.  The 70 F air has a low RH because this warm air's saturation mixing ratio is large.  The RH in the 40 F is higher even though it has the same actual amount of water vapor because the 40 F air can't hold as much water vapor and is closer to being saturated. 

Something important to note: RH doesn't really tell you how much water vapor is actually in the air.  The two volumes of air above contain the same amount of water vapor (3 grams per kilogram) but have different relative humidities.  You could just as easily have two volumes of air with the same relative humidities but different actual amounts of water vapor.



The dew point temperature has two jobs.  First it gives you an idea of the actual amount of water vapor in the air.  In this respect it is just like the mixing ratio.  If the dew point temperature is low the air doesn't contain much water vapor.  If it is high the air contains more water vapor. 

Second the dew point tells you how much you must cool the air in order to cause the RH to increase to 100% (at which point a cloud, or dew or frost, or fog would form).


If we cool the 70 F air or the 40 F air to 30 F we would find that the saturation mixing ratio would decrease to 3 grams/kilogram.  Since the air actually contains 3 g/kg, the RH of the 30 F air would become 100%.  The 30 F air would be saturated, it would be filled to capacity with water vapor.  30 F is the dew point temperature for 70 F air that contains 3 grams of water vapor per kilogram of dry air.  It is also the dew point temperature for 40 F air that contains 3 grams of water vapor per kilogram of dry air.Because both volumes of air had the same amount of water vapor, they both also have the same dew point temperature.



Now back to our students and classrooms analogy on the righthand side of p. 83.  The 4 students move into classrooms of smaller and smaller capacity.  The decreasing capacity of the  classrooms is analogous to the decrease in saturation mixing ratio that occurs when you cool air.  Eventually the students move into a classroom that they just fill to capacity.  This is analogous to cooling the air to the dew point.

If the 4 students were to move to an even smaller classroom, they wouldn't all fit inside.  The same is true of moist air.  If you cool moist air below the dew point, some of the water vapor will condense.