Topic Guide: Seasonal Variation

and the EarthÕs Heat Budget


In this activity you will form groups, conduct research into Seasonal Variation and the EarthÕs Heat Budget, and report back to your lab section in the form of a presentation. At the end of this activity you will find suggestions for the format of the presentation. You will use the data you investigate as evidence for your statements. Please use these suggestions and the ŌHow to make a class presentationĶ as guidelines for your presentation.



The Earth receives the energy that drives the climate from the sun. Because the EarthÕs axis is tilted in relation to the plane of the EarthÕs orbit around the Sun, the amount of solar radiation received by the Northern and Southern Hemispheres varies with the seasons. This variation produces interesting patterns in climatic parameters, some of which we will investigate in this activity.


Incident solar energy is transformed by the EarthÕs atmosphere, clouds, and surface. It can be reflected, absorbed, and re-radiated. The datasets in Worldwatcher allow you to investigate satellite data that can be used to help understand these processes.


Key processes and concepts to review before beginning:




After completing this investigation you should be able to:

  1. Understand how solar energy is distributed over the EarthÕs surface.
  2. Understand the effect of the latitudinal imbalance in the EarthÕs radiation.
  3. Understand how heat is transferred from low latitudes to higher latitudes
  4. Understand the effect of the concentration of landmasses in the Northern Hemisphere on seasonal variations


You can go straight into exploring the data, but if you need more background information about paleoclimate, please review the websites that provide background information (found after the data section).


Incident Solar Energy:

The incident solar radiation is called the "Insolation." The solar radiation at the top of the atmosphere follows a very simple behavior.  Figure 1 shows how the angle of incidence of the solar radiation affects the solar energy per unit of area. At 45 degrees incidence, the same amount of solar energy must be spread over more area than when the incident angle is 90 degrees. In terms of latitude, the angle is (solar flux)*cosine(angle from vertical). You would expect the insolation to be highest in the equatorial regions, but to move north and south with the seasons. The angle from vertical will be the latitude plus or minus the angle of the earth's axis relative to the solar rays. Since the earth's axis tilts at 23.5 degrees from its orbital plane, the incident angle will be = (latitude Đ 23.5)degrees in summer (northern hemisphere) and = (latitude + 23.5)degrees in winter.


Figure 1. Variation of insolation with angle of incidence of the solar "rays." After:



Its variation with latitude is shown in figure 2, below.



Figure 2. Incident "available" solar radiation during the year at the top of the atmosphere for several latitudes. These values are strongly affected by the season, which affects the length of the day and angle at which the radiation strikes the earth. It is interesting that the insolation at the north pole is higher than any other location, in June. This is because it is always daylight, during which the sun is at a constant angle of 23.5 degrees above the horizon. From:


Figure 3 shows a plot of insolation averaged over the time interval July 1983 to June 1993 (



Figure 3. Incoming solar radiation (insolation) at the surface of the earth, in kilowatt hours per square meter per day.


1.     What factors affect the amount of sunlight reaching the surface of the earth? Refer to your lab 6 reading to answer this. Or, a very nice explanation can be found at:


2.     Describe the distribution of incoming solar energy look like for January. What is the latitude of maximum incident energy? Why do we see this pattern?


  1. When we look at the April insolation, what changes are happening? Can you explain what is happening to cause this change?


4.     Make a general statement about what you observe, and variations you would expect throughout the remainder of the year.


5.     Draw a diagram of the Earth and the Sun to illustrate what is happening. Include a diagram for the months of January, April, July, and September.


Puzzle: Notice that at the extreme winter months (e.g. January), the incident solar energy seems to decrease as you move to lower latitudes, but then increase near the South pole. Why should this be?

Reflected Solar Energy Data

The ratio of the reflected solar energy flux to the incident solar energy flux is called the Albedo. Click on this link to get an animated plot of the average earth albedo during the year. The color bar is in percentage. So, a 30% value would correspond to an albedo of 0.3. White values are very high reflectivity and purple values are low reflectivity. Clouds and ice have high reflectivity.


6.     Is there a pattern there? Does this pattern have anything to do with what you saw in the incoming solar energy?


7.     Make a general statement about what is happening to this data as we progress from January to July.


8.     What is the maximum and minimum albedo? Why does it occur where it does?

Surface Temperature Data

The best way to observe surface temperature is by clicking on the link: 

This site also has some very nice climate animations:


  1. What can you say about the surface temperatures at the poles?


  1. At the equatorial region?


First, concentrate on the Northern Hemisphere, stepping through the months from January to July.


11.  What do you notice about the latitudinal extent of the cold region as compared to the Southern Hemisphere?


  1. Why might this occur? A hint: what are the surface differences in the two regions?


Now watch the Northern Hemisphere as we slowly progress from January to February to March to April.


  1. Do the temperature changes progress or regress evenly?


  1. If not, why?


  1. Look at March and April. What are these remaining pockets of cold temperatures?


  1. Why are they there?


Now look at the Southern Hemisphere, stepping through the animation data from January to February to March to April.


  1. What is the general pattern that you see?


  1. Is it an even progression/regression?


  1. What is a possible reason for this pattern?


Step through the animation data from May to June to July looking at the South American continent. What do you see happening?


  1. Why is this happening?

Net Radiation Balance Data

The absorbed solar energy is the difference between the solar energy the hits the earth and that which is radiated back to space. Click to the link: Net.html to get an animation of the "Net Radiation" budget. Positive values (red) indicate that more is being absorbed that is being radiated back to space. Negative values (blues) indicate that more energy is being radiated to space than is being absorbed. There is a consequence to this fact. The entire earth must be in balance, where total incoming solar energy is equal to total radiated energy. This means that somehow the excess energy at the equator gets transported to the poles.


22.  Can you think of mechanisms that allow the excess heat at the equator to get to the higher latitudes?


  1. What pattern do you see as you step through the months January to July?


  1. Is there a region where the net energy balance remains fairly high?


  1. What are the general relationships you have derived from the data?


Background information: Please take some time to learn more about the background information available for the topic of Seasonal Variation and the EarthÕs Heat Budget. If you learn something new and interesting, please share it with the lab in your presentation.


Presentation Framework

Your presentation should include a brief overview explaining the significance of Seasonal Variation and the EarthÕs Heat Budget. You should then choose as many of the following topics as is necessary to explain the concept. Choose topics that you think might be relevant to understanding seasonal variation, heat transfer, and uneven heating of the hemispheres. Your presentation should include interesting findings from your investigations, backed up with data. You must use the physical data in your presentation.


You may choose from the following list of topics, or investigate a topic of your own. The topics in the list are examples of investigations that could be made using the data available at the URLÕs listed above.


Data driven topics:


Overview type topics:



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