Summer is sunburn season

Forrest M. Mims III

Summer is sunburn season. As shown in the full-sky photograph nearby, which was made on 3 July 2005 from the field I call Geronimo Creek Observatory, the sun is much higher in the sky during summer. This which means its ultraviolet (UV) rays travel through less of the ozone that absorbs them.

The good news is that some UV is good for us. That' s because UV serves a vital role in human health by stimulating the production of the hormone known as vitamin D. This vitamin allows the body to metabolize the calcium that forms our bones and conducts nervous signals throughout our brain and body.

If you have ever had a sunburn or a skin cancer, you already know the bad news about UV. Fortunately, most people can get all the UV they need for their vitamin D by going for a short walk in the sun several days a week.

If you have dark skin, you will need a bit more sun than folks with light skin. That' s because the melanin granules in your skin absorb UV before it can start the vitamin D cycle.

Light skinned people can burn very rapidly. They should be very cautions about how long they stay in the sun.

You can get your vitamin D quotient from sunlight fairly quickly, especially in the summer. How do you protect yourself thereafter?

The safest choice is to stay indoors during the middle part of the day. On 6 July 2005 at Geronimo Creek in South-central Texas, the sun reached its highest point in the sky at 1:36 PM Central Daylight Time. That' s called solar noon. Depending on the cloudiness, there was less than half the amount of the UV at noon than before about 9:15 AM and after about 5:00 PM.

The sun's highest angle in the sky increases before the solstice and decrease thereafter. You don' t need a table, watch or clock to find this angle. Just look at your shadow. When your shadow on the ground is longer than you are tall, the UV is less than half its intensity at noon.

If you are outdoors during the middle part of the day, it' s wise to wear a hat and sunglasses. Wraparound sunglasses are best. Unless you have dark skin, daub some sun screen on your nose, ears and arms.

Don't forget to protect children and infants from excessive UV! How sad it is to see babies lying in strollers squinting at the bright sun.

And don' t think you're protected by the shade of a tree or a cloud! Around half the UV arrives as indirect skylight that is just as potent as that arriving directly from the sun.

And watch out when fluffy cumulus clouds are near the sun when the sky is blue and clear. UV reflected from the sides of these clouds can increase your dose by 20 percent!

Forrest M. Mims III and his science are featured online at www.forrestmims.org.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.