Although small amounts of UV radiation are beneficial to health, overexposure can lead to skin cancer. The main determinant of UVI is the sun's elevation angle. UVI values are thus greatest in the summer, especially near solar noon. At mid-latitudes, peak UVI can reach summer peak values greater than 10, but in winter, the UVI values are generally less than 1. Peak UVI values can exceed 20 in the tropics, where seasonal variations are much smaller. The highest values in the world occur in the high altitude altiplano region of Peru, where they can exceed UVI=25.

This app provides information about the current UVI and how it will vary throughout the day at any location. Sites can be selected by three methods: (1) GPS, (2) Drop-down menu of over 300 preloaded sites, (3) selection by touching the global map, using sliders to fine tune. Your own favourite locations can be specified and stored.