Monday Mar. 18, 2013
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I heard Joe Bonamassa for the first time a couple of nights ago on a PBS special that aired during one of their pledge drives.  He's an amazing guitarist.  You heard "Driving Towards the Daylight" before class.  Listen to this Acoustic Guitar Solo if you'd like to get an idea of what he can do.

You'll find mention of an Optional Assignment hidden in today's notes.

I did manage to get the Experiment #1 revised reports graded over Spring Break.  There are still a few graded Expt.#2 that haven't been picked up.  Both the revised Expt. #2 reports (if you'd like to do one, it's not required) and the Expt. #3 reports are due next Monday, Mar. 25.  You should try to bring in your Expt. #3 materials this week so that you can pick up the Supplementary Information handout.

The due dates for the first two topics in 1S1P Assignment #2 are coming up: Friday Mar. 22 and Monday Mar. 25.  See the 1S1P page for more details.

Midterm grade summaries were also distributed in class today.  More about them toward the end of today's notes.

With Quiz #2 behind us it's time to start a new block of material.  Here's an introduction to the first topic we will be covering: humidity (moisture in the air).  This topic and the terms that we will be learning are probably new and might be confusing.  That's the reason for this introduction.  We will be mainly be interested in 4 variables:


Your task will be to figure out their "job", what they're good for, and what can cause them to change value.  The bottom half of the figure below can be found on p. 83 in the ClassNotes.


Mixing ratio tells you how much water vapor is actually in the air.  You can think of it as just a number: when the value is large there's more water vapor in the air than when the value is small.  It's not a difficult concept to grasp.  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).  A kilogram of air is about one cubic meter of air.   Mixing ratio 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 (with one exception when 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 and depends on the air's temperature; warm air can potentially hold more water vapor than cold air.  It doesn't say anything about how much water vapor is actually in the air (that's the mixing ratio's job).    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 ClassNotes.


The sugar dissolved in tea analogy is still helpful.  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.  The numbers 15 and 5 came from the table on p. 86.


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

After looking at the figure above you might start to guess at what relative humidity might mean.


The relative humidity is the variable most people are familiar with.  It tells you how "full" the air is with water vapor, how close it is to being filled to capacity with water vapor, how close the air is to being "saturated" 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 actual 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.

The figure below goes back to the volumes (1 kg each) of 70 F and 40 F air that could potentially hold 15 grams or 5 grams of water vapor.

Both the 70 F and the 40 F air each contain 3 grams of water vapor.  The 70 F air is only filled to 20% of capacity (3 of the 15 open circles is colored in) because this warm air's capacity, the saturation mixing ratio, is large.  The RH in the 40 F is 60% 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 very different values of relative humidity.  You could just as easily have two volumes of air with the same relative humidity 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.  This is something we learned early in the semester.

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).  This idea of cooling the air until the RH increases to 100% is important and is something we will use a lot.


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 the student/classroom analogy



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.

Doe_J
quiz1 -55 (195 pts possible) 71.8%
quiz2 -52 (170 pts possible) 71.5%

1.0 EC points (2.1 pts possible,class average is 0.95)

writing scores: 34.0 (expt/book report) + 15.5 (1S1P pts (average is 15.5))
writing percentage grade estimate: 89.2%

average (no quiz scores dropped): 75.2% + 1.0 = 76.1%
average (lowest quiz score dropped): 76.1% + 1.0 = 77.1%


Your grades on the two quizzes are shown first in dark green (I didn't record your score on the Practice Quiz and it isn't shown).  There are two more quizzes this semester. 

Next in dark brown are the number of extra credit points you have earned from turning in Optional Assignments.  It is possible to have earned 2.1 pts at this point, a handful of students have.  The class average was 0.95 point of extra credit.  By the end of the semester you will have had an opportunity to have earned at least 3 pts of extra credit (probably a little more than that).

Your score on either an Expt. #1, Expt. #2 or a book report is shown next in purple.  Many students haven't yet turned in a report (speaking of which you should check to see if your name is on this list).  They'll find a 0 listed here and a short message at the bottom of their grade summary saying that an average score was used by the computer to provide a reasonable estimate of their writing grade.  The report score is followed by the total number of 1S1P points you have earned (the class average is 15.5 which is right on target to earn 45 pts by the end of the semester).  The report points and the 1S1P points are added and a writing percentage grade is computed.  The computer has taken into account the fact that you can't have earned 45 1S1P points at this point in the semester.  By the end of the semester if you have a decent experiment report score and 45 1S1P pts the writing percentage grade should be close to, maybe a little over, 100%.

Finally the quiz scores and the writing percentage grade are themselves averaged, the extra credit is added on and your overall grade is shown in this reddish color.  No quiz scores have been dropped in the first average.  This is the average that has to be 90.0% or above on the last day of classes in order to get out of the Final Exam.  If you do have to take the Final Exam, the second average (with your lowest quiz score dropped) will be used together with your Final Exam score to determine your overall grade.

The grade estimate attempts to determine what you will end up with at the end of the semester if you keep doing like you have done up to this point.  With two quizzes left and lots of writing still to do there is time for significant improvement.  It is also possible for your grade to drop between now and the end of classes if you stop performing as you have been.

I'll try to handout another grade summary following Quiz #3 and a last grade summary for sure after Quiz #4 so students will know whether they need to take the Final Exam or not.

NOTE:  Please check to be sure the grades listed on your summary are correct.  And, as far as graded work is concerned, we're at about the halfway point.  There is still time to earn a lot of 1S1P points and there are two quizzes left to take.  So your grade can change significantly between now and the last day of classes.


Finally we spend the last 10 minutes or so trying to understand why it is possible to saturate air with water vapor.  I.e. why there is an upper limit to the amount of water vapor that can be found in air and why that limit depends on temperature.  I've placed that material in a special supplementary reading section.  While your reading through that material keep an eye out for a Hidden Optional Assignment.