UV Index tutorial
Find out why you should make our UV Index forecasts part of your daily routine.
Protect yourself from the sun by using sunscreen, sunglasses and learning about how UV radiation is measured and how to reduce exposure
Bigsalty Weather now offers full UV Index forecasts to help you look after yourself on the water and on the beach. So, how do you use these forecasts and what are they measuring? If you’ve ever finished your day with that uncomfortable feeling that you’ve been caught out by too much sun then this article is for you.
Our UV graphs
The UV index charts we publish on our web app for each of our weather locations plot UV Index against time. Bands of UV index are grouped by colour and the peaks of the graph indicate higher UV exposure levels. A quick glance at our charts should indicate that peak UV exposure occurs near the middle of the day when the sun is also at its peak (solar noon) and most powerful. This is a simple but useful patten to remember when limiting sun exposure but more importantly we should understand what the UV Index numbers actually mean and think about our exposure throughout the whole day. More about this in a minute...
About UV radiation
We all have times when we forfeit health over convenience, enjoyment or lack of preparation. But in general there should be no reason not to lube up with sunscreen and prepare for the day outdoors. If you are reading this article you are most likely an avid water sports enthusiast and spend a lot of time in the sun which makes this point even more acute. UV radiation has health benefits in moderation including the generation of vitamin D but over exposure may result in skin ageing, DNA damage, skin cancer and eye damage.
So water sports peeps - listen up!
The UV radiation emitted by the sun falls in to bands of UVC (absorbed entirely by the atmosphere) then the bands that our important to our health - UVA and UVB. About 10% of the lower wave length UVB rays reach the earth’s surface in addition to most UVA rays. As a consequence we require protection from both UVA and UVB radiation and a broad spectrum sunscreen is required for this. SolRX sunscreen is an example of a sunscreen that is designed to protect against both UVA and UVB rays.
What is UV index?
UV Index (or ultraviolet index) measures the strength of the UV radiation capable of causing sunburn. This scale increases in a linear fashion - so for example a UV index of 6 is twice as strong as a UV index of 3. Or put another way, 30 minutes of sun exposure at a UV index of 3 is roughly equivalent to 15 minutes of sun exposure at a UV index of 6. This is a really important point because it really highlights how UV index relates to our daily sun exposure time and this is the most effective way of thinking about it. The implication is that it’s not enough to just stay out of the sun at the peak time of day when our graph hits the maximum reading if you are going to be balls out for the rest of the day!
With any numeric scale it’s useful to have some benchmarks. A UV index of 0 is generally the case at night time. A UV Index of 10 corresponds roughly to midday summer sunlight with a clear sky in Toronto, Canada (this is where UV index was originally designed). UV indexes in the teens common in tropical latitudes, mountainous areas and locations with ozone depletion issues.
Our UV Index forecast graphs are a result of predictions from the GFS forecast model and are therefore not direct readings of spectral radiation but are entirely suitable for your daily use and planning. The UV Index rating itself is worked out using calculations that observe sun elevation & distance, ozone levels, cloud cover, air pollutants, altitude, surface reflectivity (albedo) and is weighted in priority of those UV wave lengths that effect human skin. All this is factored in to the number given that corresponds to burn time.
Sun protection and reducing sunburn?
We know UV index relates to exposure time but how does that relate to us as individuals? As a general rule if you have light skin and you begin to get sun burn in 30 minutes at UV Index 5, then the same level of burning will only take 15 minutes at UV Index of 10. Recommendations are broadly as follows:
| 1-2 | 3-5 | 6-7 | 8-10 | 11 |
Green - Low risk. If you burn easily cover up and use SPF 30+. Wear UV blocking sunglasses on bright days.
Yellow - Moderate risk. Stay in shade near midday, use SPF 30+. Wear U blocking sunglasses.
Orange - High risk. Protection against skin and eye damage is necessary. Reduce time in the sun. Apply SFP 30+ sunscreen and use UV-blocking sunglasses
Red - Very high risk. Minimise sun exposure between 10am and 4pm. Seak shade. Apply SPF 30+ sunscreen and use UV-blocking sunglasses.
Violet - Extreme risk. Unprotected skin and eyes can burn in minutes. Avoid exposure between 10am and 4pm. Apply SPF 30+ sunscreen and use UV-blocking sunglasses. Stay in shade and wear sun protective clothing.
The SPF factor in sunscreens also have a linear scale. UV radiation exposure for someone using an SPF 30 sunscreen will be half that of the same person using an SPF 15 rated sunscreen. Partly for this reason it is generally recommended to use a factor 30SPF or higher and proper application is very important.
In summary
Plan your day ahead and think of your overall sun exposure for the entire day and prepare accordingly. Check our UV index graphs to confirm what the expected highest UV is for the day and the spread of the UV index as this will make it obvious if there is a high risk of immediate burning for the day. Otherwise approximate the sun exposure you and/or your family will receive and judge how long you can safely enjoy exposure whilst using your hats, sunscreens, UV blocking sunglasses and any protective clothing.
A final thing to consider is the type of environment you are spending time in because typically beaches, water, concrete and snow have a higher albedo than other environments and can lead to a higher rate of exposure.