The most important factor controlling UV radiation is the sun's angle above the horizon. At low angles, the sun's rays pass through much more of the atmosphere, reducing the radiation. As a rule of thumb, if the length of your shadow is more than twice your height (when the angle of the sun is less than 30°), then the UV Index will generally be less than 3 (UVI 'low'), and there is little risk of UV damage. If your shadow is less than twice your height, some protection may be needed. If your shadow is less than your height (the sun's elevation is greater than 45°), the UV Index can exceed 7 (UVI 'very high'), and protection is required.
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