What is the UV Index?
UV Index & Sun Protection
Origins of the UV Index
What Determines UV Strength?
Latitude & Time of Year
Time of Day
Reflection off Water
Reflection off Snow
Clear Skies & Clouds
UV Exposure & Your Health
Immune System Suppression
For the most part, latitude, time of year, and time of day will determine UV strength. Further from the equator UV levels will fluctuate more throughout the year, being higher in Summer and lower in Winter. As we get closer to the equator UV levels tend to remain at a high to extreme level year round. It is only necessary to worry about UV levels during daylight hours. UV strength usually peaks at solar noon, so as a very basic rule of thumb, the shorter your shadow is, the stronger the UV will be. Here's some detailed info on what determines UV strength.
Latitude and Time of Year
In general, the closer you are to the Equator
the stronger the UV will be. The main reason for this is that at the Equator the Sun is more directly
overhead, and therefore has less Ozone and atmosphere in general to filter the UV rays whilst traveling to the Earth's surface.
Depending on your location UV strength may vary a lot or a little throughout the year. As a rule of thumb, the closer you are to the Equator the higher the UV strength will be and a higher level of Sun protection will be needed year round.
Some locations, such as the Northern third of Australia (Brisbane Northwards), the Southern parts of the United States (Southern California, Arizona, New Mexico, Texas, Louisiana, Mississippi, Alabama, Georgia and Florida) are more likely to have year round moderate to extreme UV Index values.
The strongest year-round UV will occur generally between the Tropic of Cancer and the Tropic of Capricorn. Places like the Caribbean, Central America, South East Asia, Indonesia, Southern India, most of Africa, are likely to have year-round moderate to extreme levels of UV.
For places North of the Tropic of Cancer and South of the Tropic of Capricorn UV levels will vary a lot more on a yearly basis. UV levels will be higher in the Northern and Southern Hemisphere Spring, Summer, and Autumn, with Low to Moderate levels during the Winter.
Time of Day
UV radiation is generally strongest in the hours around solar noon. For safety sake, you can take this to mean between the hours of 10am and 4pm.
Solar noon is not the same as noon on a clock (12pm). Solar noon can vary by more than an hour before or after 12pm depending on location and
on time of year. Even across states or provinces within the same time zone solar noon can vary a lot.
This doesn't mean UV will not be strong outside of these hours, it is, but for any given day UV radiation will usually be strongest between 10am and 4pm and so your UV protection should also be greater during these hours.
UV Index levels will change from hour to hour, day to day, and place to place. One day it may be Low all day long and the next day it may be Extreme for just 2 hours, or most of the day. Don't get caught out. Always be aware of the UV level especially if you have babies or young children playing outdoors, as they are particularly at risk. Use our UV Forecast as a guide to UV levels for each hour of the day to determine appropriate protection.
The thickness of the Ozone layer also plays its part in determining the UV strength at any given location.
Countries such as Australia have large areas of thinned Ozone, which means more of the Sun's UV will penetrate
through to the Earth's surface. Reduced Ozone also lets a larger amount of UV-B through, which is very harmful
to humans. So if you live in an area of thinned Ozone, it is particularly important to be aware of the UV levels
on a day to day basis.
Australia is not the only place with thinned Ozone. There are many parts of the World that have temporary or semi-permanent Ozone holes. Such mini-holes can mean an increased risk of UV exposure even in places where you would normally expect a low to moderate UV exposure level. This can be particularly dangerous as you will be caught unaware and unprotected.
UV radiation is reflected or scattered to varying extents by different surfaces. It is important to be aware of this especially when near or on water or snow, or other highly reflective surfaces.
Activities such as swimming, canoeing, sailing, surfing, snorkeling, fishing or any other activity where
you are close to water (even sun bathing beside a pool) can increase UV exposure. Sea foam can reflect UV by about 25% and dry beach sand about 15%.
Water (and sweat) can wear off sun lotions fast, depending on brand and type used. Remember to re-apply sun lotion if necessary.
UV reflection from snow is a big one to watch out for. Fresh snow is a particularly good reflector and almost doubles a person's UV exposure.
Fresh snow can reflect as much as 80% of UV radiation! Recurring incidences of snow blindness, or photokeratitis, in skiers
emphasize that UV protection measures must take ground reflection into account.
On top of this, as snow is usually encountered at higher altitude the atmosphere is thinner, and so the UV can be even stronger in these places.
What makes this situation particularly dangerous is that many people may be unaware of this increased risk of exposure to UV as often times it is cooler at higher altitudes, and so Sun protection is less considered. Always remember to cover up, use a hat and sunscreen, and use UV protective goggles when skiing or snowboarding, or having other kinds of fun in the snow.
At higher altitudes the atmosphere is thinner and so absorbs less UV radiation. This increases the strength
of UV and your risk of exposure.
Be aware that with every 1000 meters increase in altitude UV radiation levels increase by 10% to 12%!
So, for example, if you managed to hike to the top of Mount Rushmore, the UV levels there could be 15-18% stronger than at sea level, just from the change in altitude alone. And, if you are thinking about climbing to the top of Mount Everest you could be looking at a 100% and greater increase in UV strength!!!
Increased UV strength at higher altitudes compacted with the highly reflective nature of snow is what makes skiing and snowboarding particularly risky. A lot of the time most of your body will be well covered in thick layers of clothes, but your face, neck, hands, and particularly your eyes can be at risk.
Always wear quality goggles that have the best UV protection you can find.
Clear Skies & Clouds
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 as the reflection by different surfaces and thus increase total UV radiation levels.
Haze in the atmosphere can even increase UV radiation exposure.
It is a common misconception that you can't get burnt on a cloudy day. You can! In fact, up to 80% of solar UV radiation can penetrate light cloud cover.