Monday, Mar. 21, 2016

Elle King: "Can't be Loved" (3:47), "Good For Nothin' Woman" (3:02), "Playing for Keeps" (4:15), "In the Water" (4:01), "See You Again" (3:26), "Make You Smile" (2:31)

The Experiment #1 revised reports,  all but about 25 of the 1S1P reports on Ultraviolet Light, Quiz #2 have all been graded.  Also "mid term" grade summaries were prepared over the break and were available for pickup in class today.  You'll find more information about the grade summaries at the end of today's notes.

I forgot to mention a couple of things before class:
1.  A few students have reached the 45 pt. maximum number of points allowed on 1S1P reports.  You can't earn more than 45 pts so there is no reason to write any
     additional 1S1P reports. 

2.  There is a new 1S1P Topic available for your consideration.  The tentative due date is Wed., Mar. 30 (I may extend that until Fri., Apr. 1 given that I forgot to
     mention this in class today)

Between now and Quiz #3 we'll learn about humidity variables.  These are ways of measuring and tracking the amount of moisture in the air.  We'll learn a little bit about how clouds form and will learn how to identify and name clouds.  Only 2 of the 10 basic cloud types are able to produce significant amounts of precipitation.  It's not as easy to produce precipitation as you might think.  This is something else we'll be looking at. 

There was a short in-class Optional Assignment handed out in class today.  If you weren't in class and would like to do the assignment and turn it in at the start of class on Wed., you can download a copy here.

Today: humidity variables
Humidity = moisture (water vapor) in the air.
This topic and the terms that we will be learning are probably new and might be confusing.  So here's an introduction.  We will be mainly be interested in 4 variables:



Your task will be to learn the "jobs" of these variables, their units, and what can cause them to change value. 
 
Mixing ratio ( r )
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.   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 (about one cubic yard of air).  Mixing ratio is basically the same idea as teaspoons of sugar mixed in a cup of tea.  We'll use a lower case r to represent mixing ratio.




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 ( rS )



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.  And not only that, the amount depends on temperature: 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 45 F air only has 6 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.   One of the figures is almost filled to capacity, with water vapor the other is not.  That's basically what the 3rd humidity variable, relative humidity, tells us

Relative humidity (RH)




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.  RH has units of %.

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.  How full the room is 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.

What is the RH good for if it doesn't tell you how much moisture is in the air?  When the RH reaches 100% dew, fog, and clouds form.  RH tells you whether clouds or fog are about to form or not.

Dew point temperature




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.

The dew point is a temperature and has units of   oF or oC

Second the dew point tells you how much you must cool the air in order to raise the RH 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.


Example grade summary

Grade summaries were handed out in class today.  Here's an example (most of the values are class averages)


quiz1 -54.0 (215.0 pts possible) 76.7%
quiz2 -49.0 (170 pts possible) 71.2%
1.5 EC points (2.45 pts possible)
writing scores: 0.0 (expt/book report) + 27.0 (1S1P pts)
writing percentage grade estimate: 98.8%
average (no quiz scores dropped): 78.9% + 1.5 = 80.4
average (lowest quiz score dropped): 82.2% + 1.5 = 83.7%

* because you haven't completed the experiment or book report
yet (or your report hasn't been graded yet) an average score of 34 was
used to compute your writing grade


The first two items are your scores on the quizzes.  If you did the Upper Level Charts Optional Assignment and chose to have points added to your Quiz #2 score they have been included.  There are two more quizzes left this semester.

The next line shows the Extra Credit points earned so far this semester. 

Your writing score and writing percentage grade are next.  This is made up of your experiment report grade (up to 40 pts) and the number of 1S1P pts you've earned so far (this should be 45 pts by the end of the semester).  If you turned in an Expt. #1 or Expt. #2 report the grade summary will show the grade you received (the revised Expt. #1 reports have been graded and were included).  A score of 0 is shown for everyone else.  If you haven't received credit for an experiment report yet, the computer has assumed an average score of 34 out of 40 points just to show you the effect that your writing grade will have on your overall grade.  The computer has also taken into account the fact that most students haven't earned 45 1S1P pts at this point in the semester (the average student has earned 27 points so far).  The writing percentage grade has the same weight as a quiz.  You should end up with a writing percentage grade near 100% by the end of the semester. 

The 1S1P reports on Ultraviolet Light have been graded and are included in the points total.  The reports on the Equinoxes or the Seasons haven't been graded yet.

Finally two overall averages are computed:
(i) the first doesn't drop any quiz scores.  This is the score that must be 90.0 or above at the end of the semester in order to be exempt from the Final Exam.
(ii) the lowest quiz score is dropped when computing the 2nd average.  If you do have to take the Final Exam, this is the grade you would have going into the Final Exam.  Your overall grade for the class would be based on this average and the score you receive on the Final Exam.

These grade estimates attempt to predict the grade you will end up with at the end of the semester if you keep on doing as you have done so far.  If you're happy with your overall average, you need to keep up the quality of work you have done so far.  If your score is lower than you'd like there is still plenty of time for improvement. 

Be sure to check that all of the information on your grade summary is correct.