Understanding The UV Index Scale



Use the Navigation Bar Above to navigate each Section

When dealing with all this summer sunshine, it’s important to understand how dangerous it can be and when it’s a good idea to just stay in the shade. The UV Index was created 15 years or so ago by the National Weather Service (NWS) and Environmental Protection Agency (EPA). For the US, the UV Index is a scale from 1 (low) to 11+ (extreme), which is meant to give the public an idea of how strong the solar radiation is.

Hot-sun/p>

The UV Index starts getting into the “high” and “extreme” levels during the late spring and summer months and is usually “low” in the winter. The levels tend to start “high” and “moderate” in the fall and gradually decrease in severity as the season wears on. So, why is that? Well, first lets understand the earth… it moves in an ellipse around the sun. It is also tilted roughly 23 degrees. Because the earth is tilted, different parts of it are tilted toward the sun at different times along it’s orbit. That little tilt is what gives us seasons. Take this graphic:

Seasons

In June (at the Summer solstice), the Earth is actually far away from the sun on the ellipse, but the Northern hemisphere is tilted toward it. The sun’s rays (imagine they are in a straight line) are pointing directly between the North Pole and the Equator. This is why the UV Index is so high during the summer months. The opposite is true in the winter: the Earth is pretty close to the sun in the ellipse, but the southern hemisphere is tilted toward the sun-which is their summer. At this time, the UV Index here is low, but in say, Australia, the UV Index is high.

The Index also varies with the time of day as well. When a UV Index number is forecasted, that is the expected value at solar noon, when the sun’s rays are directly overhead. As the sun moves farther down toward the horizon, the UV Index number will decrease. This happens because the solar radiation wavelengths are diffused in the atmosphere-and the closer to the horizon, the more atmosphere it has to travel through (which is also why we get lots of color in the sunset this time of year too). And here is a little extra tidbit: most people think the hottest time of the day is at noon too, when the sun’s rays are directly overhead, but that actually is not the case. It takes about 3 or 4 hours for the earth to heat up-like turning on a stove top (but a little longer obviously). The warmest time of the day is usually between 3 and 5pm.

So, why do we care about solar radiation? Well, I think most of us are not fans of sunburn. The UV Index can let you know how long it will take unprotected skin to burn. Here is the Index in detail (from the EPA):


2 or less Low: A UV Index reading of 2 or less means low danger from the sun’s UV rays for the average person. But remember: snow and water reflect the sun and can double the UV rays in some cases. Skiers wear suntan lotion to protect their faces!


3-5 Moderate: A UV Index reading of 3 to 5 means moderate risk of harm from unprotected sun exposure.

Take precautions, such as covering up, if you will be outside.

Stay in shade near midday when the sun is strongest.


6-7 High: A UV Index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Apply a sunscreen with a SPF of at least 15. Wear a wide-brim hat and sunglasses to protect your eyes.

Protection against sunburn is needed.

Reduce time in the sun between 10 a.m. and 4 p.m.

Cover up, wear a hat and sunglasses, and use sunscreen.


8-10 Very High: A UV Index reading of 8 to 10 means very high risk of harm from unprotected sun exposure. Minimize sun exposure during midday hours, from 10 a.m. to 4 p.m. Protect yourself by liberally applying a sunscreen with an SPF of at least 15. Wear protective clothing and sunglasses to protect the eyes.

Take extra precautions. Unprotected skin will be damaged and can burn quickly.

Minimize sun exposure between 10 a.m. and 4 p.m. Otherwise, seek shade, cover up, wear a hat and sunglasses, and use sunscreen.


11+ Extreme: A UV Index reading of 11 or higher means extreme risk of harm from unprotected sun exposure. Try to avoid sun exposure during midday hours, from 10 a.m. to 4 p.m. Apply sunscreen with an SPF of at least 15 liberally every 2 hours.

Take all precautions. Unprotected skin can burn in minutes. Beachgoers should know that white sand and other bright surfaces reflect UV and will increase UV exposure.

Try to avoid sun exposure between 10 a.m. and 4 p.m.

Seek shade, cover up, wear a hat and sunglasses, and use sunscreen.


UV protection

If you see a UV Index of 11+, it’s best to not even go in the sun. You can burn in less than 15 minutes! When the Index is “Very High” you will burn in 20 minutes or less. So remember, even on a dry, cooler day this summer, use that sunscreen!