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— The shadow rule is a simple way to know the peak time of day for UV exposure, Storm Center 7 Meteorologist Kirstie Zontini said.
The UV index is calculated with a computer model that looks at the strength of UV radiation with ozone concentration, forcasted cloud cover and elevation.
The shadow rule is a simple way of remembering the peak time of day for UV exposure. If there are clouds in the sky and you can see your shadow, you can still get a sunburn, according to the EPA.
If your shadow is taller than you, the UV exposure is lower, and if your shadow is shorter than you, the UV exposure if greater.
This means that midday and a few hours before and after will be when the UV index will be the highest.