The position of the sun in late spring and early summer can cause a more-severe sunburn. Altitude. At a higher altitude it is easier to become burnt, because there is less of the earths atmosphere to block the sunlight. UV exposure increases about 4% for every 1000 ft (305 m) gain in elevation. Proximity to the equator (latitude). Between the polar and tropical regions, the closer to the equator, the more direct sunlight passes through the atmosphere over the course of a year. For example, the southern United States gets fifty percent more sunlight than the northern United States. Because of variations in the intensity of UV radiation passing through the atmosphere, the risk of sunburn increases with proximity to the tropic latitudes, located between 23.5° north and south latitude. All else being equal (e.g., cloud cover, ozone layer, terrain, etc. ), over the course of a full year, each location within the tropic or polar regions receives approximately the same amount of UV radiation. In the temperate zones between 23.5° and 66.5°, UV radiation varies substantially by latitude and season. The higher the latitude, the lower the intensity of the UV rays. Intensity in the northern hemisphere is greatest during the months of May, June and July—and in the southern hemisphere, November, December and January. On a minute-by-minute basis, the amount of UV radiation is dependent on the angle of the sun.
However, all skin types can develop sunburn.Fitzpatricks skin phototypes:

Type 0: Albino
Type I: Pale white skin, burns easily, does not tan
Type II: White skin, burns easily, tans with difficulty
Type III: White skin, may burn but tans easily
Type IV: Light brown/olive skin, hardly burns, tans easily
Type V: Brown skin, usually does not burn, tans easily
Type VI: Black skin, very unlikely to burn, becomes darker with UV radiation exposureAge also affects how skin reacts to sun. Children younger than six and adults older than sixty are more sensitive to sunlight.There are certain genetic conditions, for example xeroderma pigmentosum, that increase a persons susceptibility to sunburn and subsequent skin cancers. These conditions involve defects in DNA repair mechanisms which in turn decreases the ability to repair DNA that has been damaged by UV radiation. Medications
The risk of a sunburn can be increased by pharmaceutical products that sensitize users to UV radiation. Certain antibiotics, oral contraceptives, antidepressants, acne medications, and tranquillizers have this effect. UV intensity
The UV Index indicates the risk of getting a sunburn at a given time and location. Contributing factors include:
The time of day. In most locations, the suns rays are strongest between approximately 10 am and 4 pm daylight saving time. Cloud cover. UV is partially blocked by clouds; but even on an overcast day, a significant percentage of the suns damaging UV radiation can pass through clouds. Proximity to reflective surfaces, such as water, sand, concrete, snow, and ice. All of these reflect the suns rays and can cause sunburns. The season of the year.