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#### Updated on May 06, 2013

Many people are surprised that they can become sunburned on a cloudy day. While it is true that most clouds block at least some of the UV radiation from the sun, the amount that is blocked depends on the type of cloud and the amount of cloud cover. Small breaks in clouds can also let brief bursts of UV radiation through. But even more surprising is that some clouds can actually increase the amount of UV radiation on the ground by reflecting and refracting the sun’s rays.

This project measures the strength of solar UV radiation reaching the earth’s surface after passing through different types of clouds.

The goals are to test the student’s initial hypothesis about the effectiveness of clouds in blocking UV radiation through the collection of data, and then to use that data to revise the initial hypothesis (if necessary).

### Problem:

Does the UV Index vary with amount of cloud cover?

### Materials:

• UV detector, calibrated to the National Weather Service’s UV Index
• Materials can be found on the internet (Amazon.com).

### Procedure

1. Familiarize yourself with the various types and characteristics of clouds.
2. Formulate a hypothesis to explain why ultraviolet radiation might pass through some clouds more easily than others, and why it might be amplified by some.
3. Using a handheld UV detector, measure the UV Index – the strength of the solar UV radiation on a scale of 1 (low) to 11 (extremely high), at noon.
4. Record the date, percent cloud cover, and type of clouds when you make the measurement.
5. Collect the same data each day over the following two months.
6. Analyze your data to determine the relative effectiveness of the various types of clouds in blocking UV radiation.
7. Compare the results of the experiment with your initial hypothesis. If necessary revise it and make additional measurements to test the revised version.
Dr. Frost has been preparing curriculum materials for middle and high school students since 1995. After completing graduate work in materials science at the University of Virginia, he held a postdoctoral fellowship in chemistry at Stanford. He is the author of The Globalization of Trade, an introduction to the economics of globalization for young readers.