Monday Mar. 22, 2010
click here to download today's notes in a more printer friendly format

I wasn't able to go to Paris for Spring Break, but a friend at work told me about Tryo a French group.  That's who you heard before class today ("Serre-Moi", "Pour un Flirt Avec La Crise", and "La Main Verte").  Trying to hear and understand the lyrics made me realize how much further I need to go before I'm really able to understand and speak French.

The quizzes have been graded together with 1S1P Topic #2 (El Nino).  If I forget again to distribute the papers like I did in class today, please remind me.

Here's what we will be trying to cover in the next couple of weeks.
humidity, measuring humidity, heat index
dew and frost
condensation nuclei and cloud formation
identifying and naming clouds
satellite photographs
precipitation formation and types of precipitation

We started today by learning about 4 common humidity variables.  There was an in-class optional assignment distributed today.  If you download it, answer the questions, and turn in the assignment at the beginning of class on Wednesday you can receive at least partial credit.

We needed a bit of a break after the brief introduction to humidity variables, so we had a quick look at a completely different topic.  The spring equinox was last Saturday.  That's a big event as far as I'm concerned.


The figure above shows the earth orbiting the sun.  On or around Dec. 21st, the winter solstice, the north pole is tilted away from the sun.  Days are less than 12 hours long in the northern hemisphere and the sun is low in the sky.  Both factors reduce the amount of sunlight energy reaching the ground.  On June 21st, the summer solstice, the north pole is tilted toward the sun.  Days are more than 12 hours long in the northern hemisphere and the sun is high in the sky at noon.  A lot more sunlight energy reaches the ground; that's why it is summer.

The equinoxes are a time of transition.  On the equinoxes, the day and night are each 12 hours long everywhere on earth (except perhaps at the poles).  On the equinoxes, the sun rises exactly in the east and sets exactly in the west.  The picture below shows the position of the sun at sunrise (around 6:30 am on the spring and fall equinox in Tucson).


At noon you need to look about 60 degrees above the southern horizon to see the sun (the sun only gets 34.5 degrees above the southern horizon on the winter solstice in Tucson and is 81.5 degrees above the horizon, nearly overhead, at noon on the summer solstice).


The sun sets exactly in the west at around 6:30 pm on the equinoxes in Tucson


This is the 2 pm class.  Most of you are more likely (perhaps) to see the sun set than see the sun rise.  The figure below shows you about what you would see if you looked west on Speedway (from Treat Ave.) at sunset.  In the winter the sun will set south of west, in the summer north of west (probably further south and north than shown here).  On the equinoxes the sun sets exactly in the west.


If you aren't careful, you can get yourself seriously injured, even killed, on or around the equinoxes. 




December 21, the winter solstice, is the shortest day of the year (about 10 hours of daylight in Tucson).  The days have slowly been getting longer all semester. This will continue up until June 21, the summer solstice, when there will be about 14 hours of daylight.  After that the days will start to shorten as we make our way back to the winter solstice.

The length of the day changes most rapidly on the equinoxes. 

We spent a little time trying to understand first why there is an upper limit to the amount of water vapor that can be found in air and second why this depends on the air's temperature. 

We first must understand the rate at which water evaporates depends on temperature (see p. 84 in the photocopied ClassNotes).  Hot water evaporates more rapidly than cold water.  Wet laundry hung outside on a hot day will dry much more quickly than it would on a cold day.

Before talking about water, have a look at the grade distribution below.  The average appears to be about 77%.  Students with grades equal to or greater than 90.0% are exempt from the final. 


If I added 5 pts to everyones grade,
Would the curve shift to the RIGHT  or the  LEFT?
Would the average grade  INCREASE,  DECREASE  or remain the  SAME?
Would the number of people that don't have to take the final  INCREASE,  DECREASE  or remain the  SAME?

 



It seemed like most everyone understood that the curve would shift to the RIGHT, the average grade would INCREASE, and the number of people getting out of the final exam would INCREASE.

The next question is very similar.  Instead of grades, the figure below shows the distribution of the kinetic energies of water molecules in a glass of water.  There's an average and some of the water molecules (the ones at the far right end of the curve) have enough kinetic energy to be able to evaporate (similar to students that are exempt from the final exam).  You'll find this figure on p. 84 in the photocopied ClassNotes.



If the water were heated, would the curve shift to the  RIGHT  or the  LEFT.  Would the average kinetic energy of the water molecules  INCREASE, DECREASE  or  remain the  SAME?.  Would the number of water molecules, with enough kinetic energy to be able to evaporate  INCREASE,  DECREASE,  or remain the  SAME?  The new curve is shown below  




The value of the average kinetic energy would increase and more molecules would lie to the right of the threshold and be able to evaporate.  Thus we conclude that hot water evaporates more rapidly than cold water.  This is shown pictorially below (the number of arrows is a measure of the rate of evaporation).

 


And now a completely different type of question.  The situation is shown below.



When the front door is first opened people will start streaming into the Walmart.  The number of people in the store will start to increase.  Some fraction of the people inside will start to leave.  Eventually the number inside will grow to the point that the number of people leaving balances the number entering.  The question is how many people would have to be inside the Walmart in order for the two rates to be equal?

In the rate of people entering the store were higher, the number inside would increase.  If the rate were to decrease then the number of people inside would get smaller.  This is as far as we got in class on Monday.

The Walmart problem is very similar to saturation of air with water vapor which is shown on p. 85 in the photocopied ClassNotes.


The evaporating water in Picture 1 is analogous to people entering a Walmart store just as the store opens in the morning.  There is initially no water vapor in the air in the covered glass but it will begin to buildup (Fig. 2).  Some fraction of the water vapor molecules will condense (even though they might have just evaporated), this is shown in Fig. 3.  The rates of evaporation and condensation aren't yet equal in Fig. 3 so the water vapor concentration will increase a little bit more until eventually the rate of condensation balances evaporation (Fig. 4).  The air is saturated at that point.  The water vapor concentration won't increase further.  Saturated air has a relative humidity (RH) of 100%. 

Cups filled with cold and warm water are shown at the bottom of the figure.  Because of different rates of evaporation (slow in cold, rapid in warm water) the water vapor concentrations at saturation are different.  Cold saturated air won't contain as much water vapor as warm saturated air.  Note that the two glasses have different amounts of water vapor but that the relative humidities are the same.