What is the UV index

Many of you have heard of the term UV index.  If you are a frequent beach goer you might even tend to look up the UV index before leaving the house.  But what does it actually mean?

Every day the sun gives off radiation or what we know as UV or ultraviolet.  These rays are what cause that summer time tan (or sunburn for others).  Some exposure to UV radiation may be enjoyable but too much may be dangerous!  Over exposure to radiation may leave you with instant affects such as sunburn.

However, some long-term affects could mean skin cancer and even cataracts.  The National Weather Service along with the Environmental Protection Agency created the UV Index which is a scale of how powerful the suns rays are on a given day.

The UV scale is a number system, along the same lines as the pollen count.  The numbers fall into a separate category: Low, Moderate, High, Very High and Extreme.  The scale goes from 0-11+, there is technically no cutoff.  Anything over 11 falls into the extreme category with burn times 15 minutes or less.

sun What is the UV index

There are four main factors in the calculation of UV Index. Thickness to the ozone layer, cloud cover, time of the year and elevation.

Ozone layer: Help to absorb UV radiation

Cloud cover: Clouds help to block UV radiation

Time of the year: Suns angle is important, summer time more direct rays, lower angle in winter time

Elevation: Higher elevation areas receive more UV radiation

The easiest way to explain how UV is calculated is the measurement of current total ozone amounts in the atmosphere which are figured out from two satellites.  For example, on an average summer day the strength of the radiation may be between 252 nm and 340 nm. We then input the values of radiation into a calculation called the McKinlay-Diffey Erythema action spectrum. This calculation will adjust the different values of radiation coming into the earth’s atmosphere.

Wavelengths that are higher in value are less sensitive to the skin and wavelengths with lower values are more sensitive to the skin. This calculation will weigh the strength of wavelengths and adjust accordingly giving us a final value. There are of course some factors that help reduce the UV index.

Clear skies allow 100% of UV to pass

Scattered Clouds 89% of UV to pass

Broken Clouds 73% of UV to pass

Overcast 31% of UV to pass

Elevation: 6% increase every kilometer

So with an ozone value of 280 nm, broken clouds and with an elevation of 1 KM the calculation would look like this.  280*.73*1.06=216.7.  The final step is to divide the final number by 25.  216.7/25= 8.7.  So the UV index would be 9.

Here’s the Chart!

0-2: Low

  • Low danger from UV rays
  • Wear sunglasses on bright days
  • More than an hour burn time

3-5: Moderate

  • Moderate risk for unprotected exposure
  • Wear SPF 30+ even on cloudy days
  • Try and be in shade during midday when sun is strongest
  • 45-60 minutes burn time

6-7: High

  • High risk for unprotected exposure
  • Reduce time in sun between 10AM-4PM
  • UV blocking sunglasses and wide brimmed hat
  • Apply SPF 30+ every 2 hours
  • About 30 minutes burn time

8-10: Very High

  • Very high risk, take extra precautions
  • Avoid sun exposure between 10AM-4PM
  • Seek shade as much as possible
  • Apply SPF 30+ every 2 hours
  • Burn time between 15-25 minutes

11 or More:  Extreme

  • Very high risk, take extra precautions
  • Avoid sun exposure between 10AM-4PM
  • Seek shade as much as possible
  • Apply SPF 30+ Every 2 hours
  • Burn time less than 15 minutes

Looking for your shadow can tell you how much UV exposure you are getting.  If your shadow is taller than you are then your UV exposure is likely to be on the lower side.  If your shadow is taller than you are, you are being exposed to higher levels of UV radiation. To check the UV index for your hometown, click here!

Thanks for reading

Meteorologist Kevin Arnone

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