Air pollution
Air pollution or smog can reduce the amount of UV by reflecting it back towards space or absorbing it.
Altitude
The air is thinner and usually cleaner at higher altitudes over mountains. With less absorbing atmosphere available, more UV reaches the Earth's surface.
Bushfire smoke
Bushfire smoke can absorb and reflect incoming radiation and reduce UV levels. The thicker the smoke, the greater the reduction in UV. UV levels can still be harmful, particularly during the height of summer when many bushfires happen.
Cloud cover
Cloud cover, especially thick unbroken clouds and rainfall can reduce UV, as thick clouds reflect and absorb more UV than thin cloud cover. However, the sides of clouds can also reflect UV and increase the amount of UV at the Earth's surface.
Dry sand
Dry sand can reflect UV and increase your exposure by 20%. Even in the shade you may be exposed to UV from the sand.
Latitude
The most intense radiation is found when the sun is directly overhead. This is at the equator in the spring and autumn, at the Tropic of Cancer in the northern hemisphere summer and at the Tropic of Capricorn in the southern hemisphere summer.
Ozone
Less ozone in the atmosphere leads to higher UV levels.
Snow
Snow is highly reflective. Fresh snow cover can increase UV by around 90%. Old snow can increase UV by around 50%.
Time of day
On any day, the greatest amount of UV reaches the Earth around midday when the sun is at its highest point. When the sun is low in the sky, solar energy must travel further through the atmosphere. This means more UV is scattered and absorbed.
Time of year
UV changes throughout the year. The highest levels are in summer months, and lowest levels in winter months.
Water
Water reflects and can increase UV above its surface by 10%. If you're in shade on a boat, for example, you may be protected from direct UV but exposed to 10% UV reflected from water. You could also be exposed to another 10% reflected off the boat.