If you only start to apply sun screen when you are already under the sun, your skin will have a longer period of being unprotected and a higher chance of getting sun burn.
Wear sunscreen on a cloudy day
Clouds cannot block UV rays. On a cloudy day, up to 80 percent of the sun’s UV rays can pass through the clouds. UV rays can also be reflected off sand and snow, so you can even get a sunburn in the shade.