davis uv sensor

[dalecoy]

That could be interesting... AWOS stations do measure UV, but it is not a part of a normal METAR.

I asked WU for the source of UV number they are showing for my local airport. Turns out that it's not a measured value:

"We compute UV Index data based on the time of day, elevation, latitude and cloud cover.  UV will update on our site as frequently as the local airport updates conditions which should always be at lease once an hour."

Thanks for getting and posting that information. 

So, WU isn't even using something like solar radiation - it's just "we think UV should be this much". 

[graculus]

Thanks for getting and posting that information.  

So, WU isn't even using something like solar radiation - it's just "we think UV should be this much".  

I guess they are working with only the data in the metar, i.e. no solar. Quite clever really, but they should footnote the source. I am near the airport but unfortunately I don't have a UV sensor to validate the result.

[dalecoy]

I guess they are working with only the data in the metar, i.e. no solar. Quite clever really,...

I'm not sure I would call it clever.

If they clearly labeled it "estimated UV", then that would be OK.  On the other hand, now we get to wonder which of the other data items that WU reports, are actually "calculated" or "estimated". 

[jerryg]

I just got an answer to my email and it was the same as above. How accurate is the estimated uv using those items? I would have to assume there is some margin for error. They say elevation is one factor, how much change in elevation makes a difference? The metar nw of me is at 115 feet and the on to the se of me is at 29 feet and i am at 59 feet thats not alot of difference in height. 

[Weather Display]

maybe its also estimated on a clear sky
pollution cuts down the UV alot

[dalecoy]

I just got an answer to my email and it was the same as above. How accurate is the estimated uv using those items? I would have to assume there is some margin for error. They say elevation is one factor, how much change in elevation makes a difference? The metar nw of me is at 115 feet and the on to the se of me is at 29 feet and i am at 59 feet thats not alot of difference in height. 

Those small elevation differences will make no difference at all.  And estimating UV using only the terms that WU stated, is bound to have a very large error margin.

Cloud cover?  Where do they get that information, and what's the accuracy?  [And cloud altitude(s) and ceiling would also affect UV]
Humidity (dew point) has an appreciable effect on UV.
As mentioned, things like pollution, etc. etc.

I would call WU's estimate "better than nothing", but that's about it.