Causes of the Seasons

This section gives a brief explanation of the causes of the seasons.  We'll also list and describe some of the surprising changes that occur at different locations on the earth during the year.

First a review of some very basic information concerning the orbiting earth and its moon.  When we cover this material in class I ask students to fill in the three blanks below (you'll find this figure on p. 73 in the photocopied ClassNotes).



Many people would have missed the 3rd question and might have said the moon orbits the earth in about a day.  This is because we see it in about the same position in the sky on successive nights.  The following figure explains what is really occurring.

On the first night in Fig. A the person looks up and sees the moon.  One day later on night B,  the earth has completed one rotation on its axis and the person is looking up at the same point in the night sky.  The person doesn't see the moon in exactly the same position as the night before; the moon has moved a little bit in its orbit.  We see the moon again in the night sky mainly because the earth has spun around is again looking in the same direction as it did the previous night.  The moon is in a slightly different position because it has moved a little bit in its orbit around the earth.  In Fig. C, a little more than 24 hours after Fig. A, the person again sees the moon overhead.  If you were to make a note of the time the moon rises you would notice it rises a little later each successive night.

The earth's orbit around the sun is elliptical, it's not circular.

Many people know this and think this is the main cause of the seasons. 

The earth is closer to the sun in January than in July (this is called the perihelion, the point at which the earth is furthest from the sun is the aphelion).  If this were the main cause of the seasons, summer in Tucson would be in January and winter would be in July.  If you spend the full year in Tucson you know this is not true.  Summer and winter would also both occur at the same times in both hemispheres.  This doesn't happen either.  The changing distance between the earth and the sun does have an effect but it is not the main cause of seasonal changes.

The main cause of the seasons is the fact that the earth is tilted with respect to its orbit around the sun.  This is shown in the next figure.

This figure shows the tilted earth at four locations in its orbit around the sun.  Note how the N. Pole tilts away from the sun on Dec. 21st and how there are 24 hours of night north of the Arctic Circle on that date.  This is the winter solstice in the Northern Hemisphere.  The N. Pole is tilted toward the sun on June 21.  The sun is in the sky all 24 hours of the day north of 66.5 N. latitude making this the summer solstice in the Northern Hemisphere.  Try to imagine what this picture would look like if, instead of standing at Point A, you moved to the other side of the scene and looked back toward the sun from Point B.  

While seasons on the earth are caused by the changing orientation of the earth relative to the sun, the figure above doesn't really explain why this is true.



n the summer when the sun reaches a high elevation angle above the horizon (at noon), an incoming beam of sunlight will shine on a relatively small area of ground.  The ground will get hot.  The two people sharing the shaft of summer sunlight will get a sunburn.  Shadows at midday are small and it is hard to find shade.

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.  Notice the longer shadows in the winter picture.

These area differences can be illustrated using three pieces of PVC pipe.  The end of one piece is cut perpendicularly.  The ends of the 2nd and 3rd pieces are cut at 30o and 60o angles.  You simply trace around the cut end of each piece of pipe on a piece of paper.  Some actual tracings are shown below at right for the pipe orientations at left.


The end of the 30o pipe covers twice the area of the 90o pipe.  The end of the 60o pipe is a little larger but not a lot larger than area of the end of the 90o pipe.



As sunlight passes through the atmosphere it can be absorbed or reflected.  On average (over the globe) 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 (2 of the original 6 rays reach the ground in the right picture above) is less intense than beam that passes through the atmosphere more directly (4 of the 6 make it to the ground in the left picture).


In most locations, the sun is in the sky longer in the summer than in the winter.  In Tucson the days (daylight hours) are around 14 hours long near the time of the summer solstice.  In the winter the sun only shines for 10 hours on the winter solstice.  Days are 12 hours long on the equinoxes.
 

The day to day changes in the length of the daylight hours is most noticeable (most rapid rate of change) at the times of the equinoxes.

Sun path diagrams are a way of showing the path that you would see the sun follow during the day.  We'll look at how the sun's path changes during the course of the year at Tucson, and we'll also see how the sun's path is different at different locations on the globe.

The situation is probably the simplest on the equinoxes, we'll start there (most of the figures that follow are on page 77a in the ClassNotes)

Notice first of all that the line separating day from night passes through the poles.  That is how you know this is one of the equinoxes and not the summer or winter solstice.

1.     The line separating day from night passes through the north and south poles.  As the earth spins on its axis, a person standing anywhere on the globe will spend exactly half the day on the nighttime side of the picture and half the day on the daytime side of the picture.  Thus the day and night are both 12 hours long.  This is true everywhere except at the poles.  We'll see what happens at the poles later.

2.     Imagine standing at the equator.  At point A you are positioned in the middle of the nighttime side of the globe; it is midnight at Point A.  6 hours later you will be standing at Point B where you will move from night to day; this is sunrise.  To see the sun you must look exactly back along one of the rays of light coming from the sun.  You must turn and look straight east to do this.  One the equinoxes, the sun will rise in the east (not just somewhere in the east but exactly due east).  This only happens on the spring and fall equinox.  The rest of the year the sun will rise south or north of east.

3.     Six hours later you arrive at Point C; it is noon.  Now to see the sun you must tilt your head and look straight overhead.  The sun passes directly overhead at noon at the equator on the equinoxes.

The picture above shows the earth viewed from outer space.  We will next look at the sun's path in the sky viewed from the ground where most of us will spend our entire lives.


3.     This shows the path of the sun at the equator.  The sun rises in the east at 6 am, passes directly overhead at noon, and sets in the west at 6 pm. 

4.     The cloud shown next to Point 4 above refers to a band of clouds that circles the globe at the latitude where the sun passes overhead at noon.  This marks the position of the "intertropical convergence zone (ITCZ)" (we'll learn more about the ITCZ later in the semester).  You can usually make out this band of clouds on a full disk satellite photograph of the earth. 

5.     A list of a few cities that are located on or very close to the equator (Kapingamarangi Atoll will probably also come up later in the semester)

6a.     This sun path diagram shows the path that the sun follows in the sky on the equinoxes in Tucson (or another city located at 32o N latitude).  The sun rises in the east (just like it does elsewhere on the globe) at around 6:30 local time (the precise time depends on your location within a time zone), reaches its highest point in the sky (58o above the southern horizon) just after noon and sets in the west at about 6:30 pm.

Because the sun rises in the east and sets in the west, crossing an east-west oriented street near sunrise or sunrise can be dangerous on or near the equinoxes.  The article below is from the The Arizona Daily Star (the city newspaper) and appeared near the time of the fall equinox.

In this case the car driver would have had the sun shining directly in his/her eyes and might really not have seen the pedestrian crossing the street.

6b.     Sydney Australia is located at  32o S latitude.  In the southern hemisphere the sun rises in the east, travels into the northern sky and then sets in the west.

7.     At Minneapolis the sun rises in the east, doesn't get quite as high in the sky at noon (only 45o above the southern horizon) and sets in the west.  Even though the sun shines for the same amount of time in Minneapolis as it does in Tucson (12 hours), Minneapolis will receive less energy during the day because of the lower elevation angle.  Remember that when the sun is low in the sky the sun's rays must pass through a longer path of atmosphere.  A larger percentage of the sunlight is absorbed and reflected.  Once this attenuated sunlight reaches the ground it illuminates a larger area on the ground.



8a.     At the north pole the sun really doesn't rise or set.  At 6 am you would find the sun right on the horizon in the east.  At noon it would be positioned in the south.  The sun would be visible at midnight in the north.

8b.     The sun also circles the sky at the horizon at the south pole.  It just travels in the opposite direction than at the north pole. 

Next we'll look at the situation on the Northern Hemisphere winter solstice.  We'll review just the main points.  These winter solstice pictures can be found on page 78a in the ClassNotes.


The North Pole is tilted away from the sun.  There are 0 hours of daylight, 24 hours of night everywhere north of 66.5 N latitude (the Arctic Circle).  There are 24 hours of daylight south 66.5 S latitude (the Antarctic Circle).  The equator is halfway between the poles, the days are 12 hours long at the equator.  Days are less than 12 hours long in the Northern Hemisphere.  Days get shorter and shorter as you move from the equator to higher northern latitude.



On the winter solstice you need to turn to the southeast to see the sun rise.  The sun sets in the southwest.


The sun will pass overhead at noon at 23.5 S latitude, at the Tropic of Capricorn.

Next we will compare the sun's path in the sky in Tucson on the Winter Solstice (left figure below) and the Equinox (right figure).


On the equnoxes the days are 12 hours long and the sun rises to about 60 degrees above the southern horizon at noon.  On the winter solstice the days are shorter, 10 hours long, and the sun only manages to get about 35 degrees above the horizon at noon.  The two main factors (angle of the sun and number of daylight hours) that control the amount of sunlight energy arriving at the ground are working together to reduce the energy arriving at the ground.  This reduction in incoming sunlight energy is what causes winter in Tucson. 

The situation on Dec. 21 in Minneapolis is even "worse".  The days are only 8 hours long and the sun only gets just over 20 degrees above the horizon at noon.

This brings up another important point:


The sun is always pretty high in the sky (at noon) at the equator; not always overhead but always pretty high (66 degrees or more).  That coupled with the fact that days are 12 hours long throughout the year means that there is very little seasonal change in the amount of sunlight energy arriving at the equator.  Seasonal variability increases as you move away from the equator toward higher latitude. 

We'll finish with the summer solstice.  These figures can be found on page 79 in the ClassNotes.


The North Pole is tilted toward the sun.  There are 24 hours of daylight north of the Arctic Circle.  There are 0 hours of daylight (24 hours of night) south of the Antarctic Circle.  At the equator the days are always 12 hours long.


If you look east from position X in the picture you won't see the sun because you won't be looking back along the ray of light coming from the sun.  You must turn to the north.  The sun rises in the NE on the summer solstice and sets in the NW.


Rays of sunlight strike the globe perpendicularly at 23.5 N latitude (the Tropic of Cancer) at noon on the summer solstice.

Now the situation in Tucson on the summer solstice compared to the equinoxes.


On June 21 the sun rises in the NE and sets in the NW.  The sun is 81.5 degrees above the southern horizon at noon, for all intents and purposes its directly overhead.  The days are 14 hours long.  There is a lot more sunlight energy arriving at the ground in summer than on the spring and fall equinoxes.

On the winter solstice we found that the two factors controlling the amount of sunlight energy arriving at the ground worked together.  As you moved toward higher latitude there was less and less energy reaching the ground because the days became shorter and the sun was lower in the sky.

On the summer solstice the two factors don't work together.  If you move from Tucson to Minneapolis, the days become longer but the sun is lower in the sky.  The overall result is less energy reaching the ground in Minneapolis in a day than in Tucson.  This is examined further in the next figure.


At high latitude the days are long but the sun is low in the sky.  At low latitude (23.5 degrees) the sun is overhead but the days are shorter (just a little over 12 hours long).  At some point in between these two extremes there must be an optimal combination of sun angle and number of daylight hours, a combination that will result in the maximum amount of sunlight energy arriving at the ground.  This "optimum" location is near 30 degrees latitude.  The hottest locations on earth are found near 30 degrees latitude.  Tucson is located at 32 N latitude, that is a big part of why it gets so hot here in the summer.