Why do rain clouds appear black?

How does Rain (and Rain Clouds) affect the impact of UV Rays?

• I'm doing a science experiment on how UV Rays affect the strength of plastic bags. One bag will be out for 1 week, another will be 2 weeks, and the last will be 3 weeks. I realized that it's raining lately here in San Diego I'm worried that the UV rays will not be strong enough to go though the thick clouds & my experiment will be pointless and incomplete. So, how will my experiment be affected? Are UV Rays still going to go through the clouds just as if it was a sunny day? * For the record, I am in 8th grade. No judging please (:

• Answer:

  The clouds, as well as the angle of the sun will have an effect on the UV rays that reach your bags. The extent of the effect, though, will depend upon the amount of cloud cover. The clouds, as well as the atmosphere that the rays pass through, have a filtering effect. Not knowing the extent of the cloud cover, we can't determine the ultimate effect. Here is a link that will help to explain the UV index, and what the UV index currently is for your area. http://www.epa.gov/sunwise/uvindex.html