The US Environmetal Protection Agency has launched their daily Ultra Violet Index
website, where you can search for your zip code's daily UV Index rating.
Tomorrow's Los Angeles UV Index rating is 9: "Try to avoid the sun between 11 a.m. and 4 p.m. Otherwise, seek shade, cover up, wear a hat and sunglasses, and use sunscreen. "
The UV Index is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence (incoming radiation level) on the ground, forecasted cloud amounts, and the elevation of the forecast cities.