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2. Solar Radiation


Solar radiation (sunlight) is the energy source that drives Earth's weather and climate. sites are outfitted with special sensors that measure the amount of radiation received by the sun. Online Resources:

The website has numerous ways that you can view solar radiation data. You can view the current value of solar radition expressed in watts per meter squared from your site here:

Another interesting way to look at solar radiation data is to view a graph of how it changes throughout the day. You can view these graphs here:


  1. Describe the solar radiation trend (black line) for this day. Some might describe the shape of the curve as a large bell, like the liberty bell in Philadelphia. What causes this bell shaped curve? Can you explain the small variations in the curve during the afternoon? What events are shown on this graph at roughly 6 AM and 9 PM?

  2. In the first example, the solar radiation curve was relatively smooth. In this example, the curve is very rough and contains alternating high and low values. Can you explain why? Also notice the temperature values during the afternoon. Can you explain why the temperature dropped a few degrees around 2 PM and then bounced up a few degrees around 3 PM?

  3. Notice the different radiation curves for these two plots and notice the difference in the temperature. Can you guess what the difference between the two days is? Notice the two slight increases in temperature on the second plot around 10 AM and again at 4 PM. Can you tell what might have caused those increases in temperature?

  4. The two plots show solar radiation curves for a summer day and a winter day. Can you explain the differences in radiation values between the two? Why are the values for summer so much higher?

Possible Answers:

  1. The bell shaped curve is caused by the rotation of the earth and our local angle to the sun. The 'bumps' in the curve during the afternoon are caused by fair weather clouds that occasionally scatter some of the sun's energy preventing it from reaching your site and the ground. The events at 6 AM and 9 PM are the sunrise and sunset, respectively.
  2. The roughness of the curve is explained by clouds. The amount of roughness can give an indication of the amount of clouds present at the time. For instance, the morning (till 11 AM) was probably characterized by thin low stratus clouds. Mid-day (11 AM till 1:30 PM) saw the development of larger clouds resulting in mostly overcast skies around 2 PM. The rest of the day probably experienced periods of clouds and sun. The temperature response during this time is very interesting. Notice the bump in temperatures at 3 PM. This was caused by the partial clearing that developed around 2:30 PM, that is, by the increase in solar radiation.
  3. The two plots shown are a classic example of what happens when you have bright sunny days and when you have clouds. For the most part, temperatures will rise when more solar radiation is recieved at the surface. The second plot shows a day that experienced a lot of clouds resulting in significantly lower temperatures.
  4. The difference in the maximum value of the solar radiation curve is can be explained by the difference in solar angle between summer and winter. With the sun higher in the sky, more radiation is directed toward the ground. With the sun lower in the sky, the solar radiation does not strike the ground directly and less radiation is absorbed by the surface.

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