Sun File > Sea or Montain
The sun our skin receives depends on six main factors

Season
In the northern hemisphere in July, the risk of sunburn due to UVB rays is 100 times greater than in winter.

Latitude
The sun's intensity is greatest at the equator where the rays are vertical, thus following a shorter path through the ozone layer.

Altitude
The amount of UVB increases 4% with every 300 meter increase in altitude. This explains in part the major risk of sunburn in the mountains. But why is the sun more aggressive in the mountains? Is it because we're closer to the sun at a higher altitude? Not at all, since compared to the earth's distance from the sun (150 million kilometers), a distance of a few hundred meters is insignificant. On the other hand, the further away you get from sea level, the more rarefied the atmosphere which UV rays have to penetrate. Thus its filtering action is less effective.

Time of day
Mornings and evenings the sun's rays are oblique and have farther to travel through the ozone layer, while between 11 a.m. and 3 p.m., the UVB rays are at their strongest.

Cloud
Depending on their thickness and altitude, clouds can filter some UV rays, but it is still possible to burn under an overcast sky.

Surroundings
The reflective power of your surroundings varies, depending on whether it's snow (85%), sand (17%), water (5%), or grass (3%). Thus, the mountains combine all the dangers with more UVB and more reflection off the snow.

Are you hoisting sail for your holiday? Keep in mind that the foam from a boat's wake doubles the quantity of UV received, while the sails or a varnished deck raise it even higher.