What Causes Earth's Seasons?

Associate Professor Brian Hornbuckle
Department of Agronomy
Iowa State University of Science and Technology
bkh@iastate.edu

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What causes the seasons?

Many people have misconceptions about what causes the seasons, even graduates of our very best universities. It's not because Earth gets closer to and farther away from the Sun as the year goes by, because when it is winter in the United States and Europe, it is summer in Argentina and Australia! So don't feel bad if you don't know why Earth has seasons. Instead, use the resources on this page to find out.

Why is winter cold and summmer warm? Basically three reasons!

1. Earth-Sun Geometry

Use the period of daylight simulation to plot on this figure the hours of daylight at the middle of each month (January 15, February 15, etc.) for the following cities: Quito, Ecuador, (at the equator, 0 degrees latitude); New Orleans, Louisiana, (30 degrees N latitude); and Anchorage, Alaska, (60 degrees N latitude). Then answer these questions.

  1. Which city has the most hours of daylight at any one time during the year?
  2. Which city has the least hours of daylight at any one time during the year?
  3. For which city does the hours of daylight change the least during the year?

Use the solar angle simulation to plot on this figure the zenith angle of the Sun at solar noon at the middle of each month (January 15, February 15, etc.) for the following cities: Quito, Ecuador, (at the equator, 0 degrees latitude); New Orleans, Louisiana, (30 degrees N latitude); and Anchorage, Alaska, (60 degrees N latitude). We all know the Sun rises and sets each day. At sunrise, the Sun zenith angle is 90 degrees. At sunset, the Sun zenith angle is also 90 degrees. The zenith angle at solar noon is the smallest zenith angle that will occur on any given day. You can also think of the zenith angle at solar noon as corresponding to the highest point that the Sun will reach in the sky that day. Then answer these questions.

  1. Which city has the largest Sun zenith angle at solar noon at any one time during the year?
  2. Which city has the smallest Sun zenith angle at any one time during the year?
  3. For which city does Sun zenith angle at solar noon change the least during the year?

Use the Earth-Sun geometrical relationship simulation to answer these questions.

  1. What happens around March 20? June 21? September 22? December 21?
  2. Does the tilt of Earth's rotational axis, relative to the Universe, change as it orbits the Sun?
  3. Does the tilt of Earth's rotational axis, relative to the Sun, change as it orbits the Sun?
  4. Why does the zenith angle of the Sun at solar noon in Des Moines (41 degrees N latitude) change over the year?
  5. Why do the hours of daylight in Des Moines, Iowa, (41 degrees N latitude) change over the year?
  6. Why is the yearly behavior of the zenith angle of the Sun at solar noon so different in Quito as compared to New Orleans, Des Moines, and Anchorage? The picture at right illustrates what is called "solar declination" and how it changes during the year. It may help you answer this question.

solar_declination

2. Cosine Law and Beam Depletion

Draw four straight lines ~40 cm length and ~20 cm apart on a table with a washable marker. Spread shaving cream over the table to make a rectangle that is ~30 cm by ~1 m that covers the lines. Be sure to leave ~5 cm of the bottom of each line and ~5 cm of the top of each line visible. Cut a ~10 cm by ~10 cm square of cardboard.

Then answer these questions.
  1. Which orientation allowed the piece of cardboard to scrape away, or intercept, the most shaving cream?
  2. Which orientation intercepted the least shaving cream?
  3. Why do you think this is called the "cosine law?" Use this figure to explain. (Note: the solar angle simulation calls this phenomenon "beam spreading" since at larger Sun zenith angles the same solar beam illuminates a larger land area.)
  4. Now think about the Sun and Earth. How much solar radiation is intercepted by the ground in Anchorage in the middle of June? How about in the middle of December? How is this different for New Orleans? For Quito? For Des Moines?

Spread some more shaving cream on a table, in a rectangle about ~30 cm by ~1 m. Run your finger through the shaving cream in a line that is the shortest possible route through the shaving cream. Then run your finger through the shaving cream in a line that is rotated 30 degrees from the first path. Then run your finger through the shaving cream in a line that is 60 degrees from the first path. Imagine the shaving cream is our atmosphere. Solar radiation from the Sun travels through the atmosphere. Most of the solar radiation gets to Earth's surface. Some is scattered back into space and some is absorbed by the atmosphere. The amount that is scattered and absorbed depends on how far the solar radiation must travel through the atmosphere before reaching Earth's surface. Answer these questions.

  1. What happens to the length of the path through the shaving cream as you increase the angle of the path?
  2. Which path would allow the most solar radiation to get to Earth's surface?
  3. Which path would result in the least solar radiation getting to Earth's surface?
  4. Now think about solar radiation traveling through Earth's atmosphere. For what Sun zenith angle would beam depletion (solar radiation lost due to absorption or scattering) be the least? When the Sun zenith angle increases, does beam depletion increase or decrease?

3. Heating the Atmosphere

Earth's surface absorbs energy in the form of solar radiation from the Sun. Most of the solar radiation is transmitted through the atmosphere. The Earth's surface absorbs this solar radiation, heats up, and then heats the lower part of our atmosphere (where we live). You can simulate this heating by using a stove and a pot of water. Turn on the stove and it gets hot, just like Earth's surface gets hot when it absorbs solar radiation. The stove heats the water in the pot, just like Earth's surface heats the atmosphere. Answer these questions.

  1. What happens to the temperature of the water in a pot if you "turn up" the stove to a higher cooking level?
  2. What happens to the temperature of the lower atmosphere when Earth's surface absorbs more solar radiation?

Now put it all together!

Here is a figure that shows the total amount of solar radiation available each day (also called the "daily insolation") at the top of the atmosphere for Quito, New Orleans, and Anchorage.

  1. Which line is for which city?
  2. Which city has the strongest seasons (biggest difference between winter and summer)?
  3. Which city may not have seasons at all?
  4. Quito, New Orleans, Des Moines, and Anchorage are all in the Northern Hemisphere. Buenos Aires, Argentina, lies in the Southern Hemisphere. During what months does winter occur in Buenos Aires?

Think you got it?

What other planets in the Solar System have seasons?

What do you need to know about the planet to find out if it has seasons?

Which planet has the most extreme seasons (biggest difference between summer and winter)? Least extreme?

agronomy