The Lake Sawyer Weather Station measures UV radiation directly and displays intensity as the UV Index. Both the current index and the one hour average index are displayed. This table shows how the UV Index relates to exposure intensity.
The EPA publishes daily UV Forecast information and a set of guidelines on sunburn risk and how much protection is recommended based on the index.
Sunburn risk is primarily a function of total exposure (UV Index times time) and skin type. The following table categorizes skin types by an individual’s tendency to tan and/or burn and estimates the minimum UV Index that will cause perceptible reddening 24 hours following one hour of exposure. (The table was extracted and modified from a comprehensive eMedicine article on sunburn.)
If persons of Skin Type III or Type IV are exposed to an average intensity of 5 for one hour, then they can expect to have some reddening of the skin 24 hours later. A person of Skin Type V would probably be OK. Note that total exposure is cumulative so exposure to a UV Index of 5 for two hours is equivalent to exposure to an index of 10 for one hour.
I need to give some warnings and a disclaimer. First, UV overexposure carries other risks including skin cancer, toughening of the skin, suppression of the immune system and cataracts. Second, the weather station takes direct measurements from the sky overhead and does not take into account the large amount of radiation reflected from water. Third, tolerance to UV varies considerably from person to person by skin-type (as discussed above) as well as other factors such as prior tanning, skin thickness, hydration, and anatomic site (e.g., face versus legs). And finally, to those people who believe they are always victims, here is the disclaimer: weather station information is intended to augment your own common sense and experience. I accept responsibility only for my own sunburn – not yours.