Monday Mar. 7, 2011
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Got to class a little early this morning so that I could play three songs from B.B. King and Eric Clapton ("Key to the Highway", "Worried Life Blues", "Hold On! I'm Coming" )

The In-class Optional Assignment from last Friday was returned in class today.  You should have found a green card if you asked for one.  Otherwise you received 0.5 pts of extra credit.

Today's (not so high quality) picture of the day.  I saw a larger clearer image in the current issue of Scientific American magazine.



The item of interest is the narrow band of clouds in the center of the picture.  It is called the Intertropical Convergence Zone.  We'll learn a little more about it today and again later in the semester.  The ITCZ is not always as clearly defined as it is in this picture.  You can probably identify it on this current satellite photograph but it probably isn't as continuous a band of clouds as in the photo above.

The Spring Equinox is just about 2 weeks away.  That's something we celebrate in NATS 101 and today seemed like a reasonable time to review and maybe learn something new about the causes of the seasons on the earth.

We start with three questions that any college graduate should be able to answer (the picture below is on p. 73 in the photocopied ClassNotes)

Most everyone will know the answers to the first two questions, but not necessarily the third.  Once you think you know the answers to the questions above, click here
The earth's orbit around the sun is elliptical not circular.  The perihelion is the point in the earth's orbit at which it is closest to the sun.  This occurs around January 3.  The aphelion is the point at which the earth is farthest from the sun (the a in far reminds me of aphelion).  This occurs around July 4.

Once you think you can answer the question above, click here

The real cause of the seasons is the tilt of the earth relative to the plane of its orbit around the sun.

The north pole is tilted away from the sun on the northern hemisphere winter solstice (it is the summer solstice in the southern hemisphere).  The north pole is tilted toward the sun on the summer solstice in the northern hemisphere.

If you were on the far side of this picture looking back toward the sun.  Here is what you would see

Can you identify the northern hemisphere summer and winter solstices & the spring and fall equinoxes in this picture?
Click here when you think you have the answer.

Because the earth is sometimes tilted toward the sun, sometimes away from the sun, the angle of the sun in the sky varies during the year.  This will partly determine how much incoming sunlight makes it to the earth's surface and how effectively it can warm the ground.


In the summer when the sun reaches a high elevation angle above the horizon, an incoming beam of sunlight will shine on a small area of ground.  The ground will get hot.  The two people sharing the shaft of summer sunlight will get a sunburn.

In the winter the sun is lower in the sky.  The same beam of sunlight gets spread out over a larger area.  The energy is being used to try heat a larger amount of ground.  The result is the the ground won't get as hot.  4 people are able to share the winter sunlight and won't get burned as quickly.

As sunlight passes through the atmosphere it can be absorbed or reflected.  Both prevent energy from reaching and warming the ground.   On average (over the globe) only about 50% of the sunlight arriving at the top of the atmosphere actually makes it to the ground.  A beam of sunlight that travels through the atmosphere at a low angle (right picture above) is less intense than beam that passes through the atmosphere more directly (left picture).  4 out of the 6 arrows taking the short path through the atmosphere make it to the ground in the left figure (67%).  In the right figure only 2 of the 6 survive the trip (33%).

There's something else that changes also. 


Click here when you think you have the answer.


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).


The figure at left traces out the path will follow in the sky on the equinox.  The sun rises in the east, moves to the southern sky and begins to get higher in the sky


At noon you need to look about 60 degrees above the southern horizon to see the sun.  The sun is only 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.


On the equinoxes the sun sets exactly in the west at about 6:30 pm.

This is a 10 am class.  Most of you are more likely (perhaps) to see the sun set than to 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.


Several years ago I positioned myself in the median near the intersecton of Treat and Speedway and pointed my camera west.  I took a multiple exposure photograph of the sun over a 2 hour period that ended at sunset.  I'll try to bring the slide photograph to class one of these days.

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





June 21, the summer solstice, is the longest day of the year (about 14 hours of daylight in Tucson).  The days start to shorten after that.  They're 12 hours long on Sept. 21, the fall equinox and continue to get shorter until December 21, the winter solstice, when there will be about 10 hours of daylight.  After that the days will start to lengthen as we make our way back to the summer solstice.

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

I'm trying to resist my natural inclination to beat this topic to death (have a look at pps 77a - 80 in the photocopied ClassNotes if you want to see what that would entail).  There are a few more points that are worth covering however (though this remaining material won't be on this week's quiz)

The sun passes overhead at noon on the equinoxes on the equator.  That's the 3rd main thing to remember about the equinoxes.  This is shown at the right edge of the figure below (a portion of p. 77a in the ClassNotes). 


The sun rises in the east, passes directly overhead at noon and sets in the west at the equator.  The sun also rises in the east and sets in the west (and the days are 12 hours long) in Tucson and Minneapolis but it doesn't get as high in the sky at noon.

Now do you remember the thin band of clouds in the satellite photo at the beginning of class.  That band of clouds, the intertropical convergence zone (ITCZ) tends to be found wherever the sun passes directly overhead at noon.  The band of clouds will be found near the equator on the equinoxes.

Next we'll look briefly at the winter solstice.

24 hours of night (0 hour days) at the N. Pole, 12 hour days at the equator (that is true year round), and 24 hours of day at the S. Pole.  As you move northward from the equator toward the N. Pole days go from being 12 hours long to eventually ending up 0 hours long north of the Arctic Circle).  Days get shorter with increasing latitude in the N. Hemisphere.

Next some more sunpath diagrams showing the situation on the winter solstice

In Tucson the day is only 10 hours long and the sun never gets very high in the sky.  There is less sunlight energy arriving at the ground now that than there is at other times of the year.  That's why it's winter in Tucson in December (and in January and February).

As you move northward from the equator, the days get even shorter and the sun is lower in the sky at noon.  The two main factors that determine how much sunlight energy reaches the ground during the day (angle of the sun and length of the daylight hours) are working together and reducing the amount of sunlight as you move toward higher latitude.  It gets colder and colder as you move toward the N. Pole.

Now we'll look at the summer solstice.

Now days are getting longer and longer as you move north of the equator.  There are 24 hours of daylight north of the Arctic Circle on the summer solstice.

The sun passes overhead at noon at 23.5 N latitude, the Tropic of Cancer.  That's where the band of clouds moves.  In Tucson there are 14 hours of daylight and the sun is nearly overhead at noon.  That's a lot more solar energy arriving at the ground during the day and explains why it gets so hot in Tucson in the summer.

Now note as you move northward the days get longer but the angle of the sun is lower and lower in the sky.  The two main factors that determine how much energy arrives at the ground during the day are working against each other.  Where do you think the optimal combination of length of day and angle of the sun in the sky occurs?  It turns out to be around 30 latitude (the big red arrow in the figure above).  That's why the hottest locations on earth are found near 30 latitude and not at the equator.

And that's probably enough material on the causes of the seasons.