#MEwx What does the "UV Index" mean and what is the "W/M2" solar radiation reading mean?
There are two (2) readings on my new personal Weather Station (which can be reached by clicking here) that I get a lot of questions about. One is the W/M2 reading and the other is the UV index.
What follows is a basic explanation of both readings that I’ve found online. They are by no means an exhaustive explanation, so if you’re really interested in them spend a bit of time with google and sit down with some Iced Tea and read about it to your heart’s content.
By the way, my personal weather station is located on the northern most edge of the city of Augusta, Maine. I mention this so that you understand that the usual city-related ozone depletion is minimal (for UV reference) here.
What does the solar radiation reading of W/m2 mean?
Solar radiation is measured atop the meteorological mast at the shore laboratory using an Eppley Model PSP (Precision Spectral Pyranometer). Approximately 99% of solar, or short-wave, radiation at the earth's surface is contained in the region from 0.3 to 3.0 µm, which corresponds to wavelength between the ultraviolet and near infrared. Above the earth's atmosphere, solar radiation has an intensity of approximately 1380 watts per square meter (W/m2). This value is known as the Solar Constant. At our latitude, the value at the surface is approximately 1000 W/m2 on a clear day at solar noon in the summer months. The difference between this value and the Solar Constant is due to transmission loss to the atmosphere. The clear sky value is considerable less in the winter. Clouds can dramatically reduce this value by reflecting the solar radiation back out to space.
The PSP senses this radiation with a thermopile that produces a millivolt signal that is directly proportional to the downwelling solar irradiance. The PSP uses a glass dome that uniformly transits radiation between 0.285 and 2.8 µm. A calibration constant is applied to provide the total solar irradiance in watts per square meter.
Blogger’s note: I understand ‘some’ of the foregoing, but if you’re a really smart radiation geek you’ll get a lot more out of it than I did. Enjoy.
What is the UV index?
If you have read How Sunburns and Sun Tans Work, you know that your skin is sensitive to ultraviolet (UV) radiation. The UV index is a value calculated each day that helps you to know how much UV radiation will reach you if you go outside. By knowing the index value for the day, you can take appropriate precautions. These precautions help you avoid sunburn in the short term and skin cancer in the long term.
The UV index is calculated based on four factors:
· The thickness of the ozone layer over your city (detected using satellites)
· The cloud cover over your city (clouds block UV radiation to varying degrees)
· The time of year (in winter, UV radiation is lower than in the summer because of the sun's angle)
· The elevation of your city (higher elevations get more UV radiation)
The UV index value ranges between 0 and 10, with zero being minimal UV exposure risk and 10 being maximal UV exposure risk. This page lists the proper precautions to take for the different index values.
Here are some interesting links:
· How Sunburns and Sun Tans Work
· EPA: How is the UV Index Calculated?
Bloggers note: If you have kids who want to be on the beach to swim, suntan or build those all-important sand castles I strongly urge you to go to some of the foregoing links and read up on things.
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