| Sun
Elevation |
|
| The
higher the sun in the sky, the higher the UV radiation level. Thus
UV radiation levels vary with time of day and time of year. Outside
the tropics, the highest levels occur when the sun is at its maximum
elevation, at around midday (solar noon) during the summer months. |
Source (pdf
file):
Global
Solar UV Index - a Practical Guide
|
| Latitude |
| The
closer to equatorial regions, the higher the UV radiation levels. |
| Cloud
Cover |
| UV
radiation levels are highest under cloudless skies but even with
cloud cover, UV radiation levels can be high. Scattering can have
the same effect the reflectance by different surfaces and thus
increase total UV radiation levels. |
| Altitude |
| At
higher altitudes, a thinner atmosphere absorbs less UV radiation.
With every 1000 metres increase in altitude, UV radiation levels
increase by 10% to 12%. |
| Ozone |
| Ozone
absorbs some of the UV radiation that would otherwise reach the
Earth’s surface. Ozone levels vary over the year and even across
the day. |
| Ground
Refection |
| UV
radiation is reflected or scattered to varying extents by different
surfaces, e.g. fresh snow can reflect as much as 80% of UV
radiation, dry beach sand about 15% and sea foam about 25%. |