Researchers who follow hunter-gatherer tribes in tropical and dessert areas have found a nearly universal pattern: during the very hottest hour or two of the day, the members of the tribe get out of the sun completely, to relax and eat in the shade.
Over the course of a summer day, the UV index – the amount of UV radiation reaching ground level – varies hugely. At 12:30pm today in New York, for example, the UV index was at 10, enough to cause burns in just 10 minutes, blazing through even strong sunscreen. Whereas by 1:15, the index had dropped to a 6, allowing for a half hour before burning without protection, and for several hours of happy sun time with a layer of (full-spectrum) sunscreen applied.
A team of outdoorsy engineers in New Zealand recently released a free app, UV Lens, which provides daily hyper-local UV forecasts. With the app in hand, you can easily plan your schedule to mimic the wisdom of our hunter-gatherer ancestors: enjoy the sun in the morning, take a brief, strategically timed mid-day lunch break in the shade, and then head back out once the very highest UV stretch of the day has passed. That way, you can spend far longer outside overall, while still greatly reducing the risk of sun-damage and burn over the course of the day.