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Activity Number 20 - Today'S UV

Purpose

To increase understanding of daily variation in UV.

This activity could be done in conjunction with a track and field meet or other activity that requires your class to be outdoors for an extended period

Materials

  • UV meter
  • graph paper
  • pencil
  • ruler
  • notepad

Method

  1. Prepare graph axes and labels, with the UV Index listed vertically on the left. Mark the vertical axis with UV Index units from 0 to 11 in increments of 0.5 units. The horizontal axis will show time in increments of ½ hour. Label both axes. Add a title to your graph, including the date and location. Include a space for the UV Index forecast.
  2. Check the day's UV Index forecast maximum for the location nearest you from the newspaper, radio or TV, or from the public forecast on any of Environment Canada's web sites. Record this value in the space provided.
  3. Every 30 minutes, use the meter to take a UV reading in an open sunny area. Class members can take turns. Note if any cloud was blocking the sun at the time you took the reading.
  4. Record the time and UV Index reading for each observation. Then enter each reading as a point on the graph. Circle the point if the sun was behind cloud at the time.
  5. Join the points on the graph. Compare the highest point on the graph to the forecast maximum obtained earlier. Identify the period of time when the UV Index was above 3. Observe what happens to the UV Index when the sky is cloudy.

Observations

On a clear day, a graph of UV Index measurements will have a smooth bell shape, peaking at "solar noon" -- roughly noon Standard Time or 1 p.m. Daylight Time. In southern Canada, the UV Index would normally be high or very high from 11 a.m. to 4 p.m. on sunny summer days. Thick cloud will reduce UV and produce irregularities in your graph.

Conclusions

During the early afternoon, UV is generally twice as strong as in the early morning or late afternoon. That's why protection from the sun is particularly important then. There is less UV when significant cloud is present.

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