The Sun Sets Exactly in the West at Around 6:30 Pm on the Equinoxes in Tucson

Tuesday Mar. 23, 2010
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 Bonus Assignment (Surface map analysis) and Topic #2 (El Nino). I should have the Expt. #1 revised reports done by Thursday.

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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 Thursday you can receive at least partial credit.

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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 (enough of an event for me to wear a tie, something I've never done before in NATS 101 - probably never it do it again either).

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

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.

This article appeared in the Arizona Daily Star on or around the fall equinox.

The driver was looking straight at the setting sun. If his windshield was as dirty as the one on my car often is, he wouldn't have been able to see the pedestrian. I have a yellow 1980 Toyota Celica. If you see that car driving west on an east-west oriented street around sunset,
LOOK OUT!

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.

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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.

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.

We had time to work the first of four example problems. This way you will learn about the 4 humidity variables (mixing ratio, saturation mixing ratio, relative humidity, and dew point temperature). You'll see what they do and what can cause their values to change.

Here is the first sample problem that we worked in class. You might have a hard time unscrambling this if you're seeing it for the first time. The series of steps that we followed are retraced below:

We're given an air temperature of 90 F and a mixing ratio (r) of 6 g/kg. We're supposed to find the relative humidity (RH) and the dew point temperature.
We start by entering the data we were given in the table. Once you know the air's temperature you can look up the saturation mixing ratio value; it is 30 g/kg for 90 F air. 90 F air could potentially hold 30 grams of water vapor per kilogram of dry air (it actually contains 6 grams per kilogram in this example). A table of saturation mixing ratio values can be found on p. 86 in the ClassNotes.
Once you know mixing ratio and saturation mixing ratio you can calculate the relative humidity (you divide the mixing ratio by the saturation mixing ratio, 6/30, and multiply the result by 100%). You ought to be able to work out the ratio 6/30 in your head (6/30 = 1/5 = 0.2). The RH is 20%.

The numbers we just figured out are shown on the top line above.
(A) We imagined cooling the air from 90F to 70F, then to 55F, and finally to 45F.
(B) At each step we looked up the saturation mixing ratio and entered it on the chart. Note that the saturation mixing ratio values decrease as the air is cooling.
(C) The mixing ratio doesn't change as we cool the air. The only thing that changes r is adding or removing water vapor and we aren't doing either.
(D) Note how the relative humidity is increasing as we cool the air (Item 6). The air still contains the same amount of water vapor it is just that the air's capacity is decreasing.
Finally at 45 F the RH becomes 100%. This is kind of a special point. You have cooled the air until it has become saturated. The dew point temperature in this problem is 45 F.
What would happen if we cooled the air further still, below the dew point temperature?

35 F air can't hold the 6 grams of water vapor that 45 F air can. You can only "fit" 4 grams of water vapor into the 35 F air. The remaining 2 grams would condense. If this happened at ground level the ground would get wet with dew. If it happens above the ground, the water vapor condenses onto small particles in the air and forms fog or a cloud. Now because water vapor is being taken out of the air (and being turned into water), the mixing ratio will decrease from 6 to 4.
In many ways cooling moist air is liking squeezing a moist sponge (this figure wasn't shown in class)

Squeezing the sponge and reducing its volume is like cooling moist air and reducing the saturation mixing ratio. At first when you sqeeze the sponge nothing happens, no water drips out. Eventually you get to a point where the sponge is saturated. This is like reaching the dew point. If you squeeze the sponge any further (or cool air below the dew point) water will begin to drip out of the sponge (water vapor will condense from the air).