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Inaccuracies In Uv Index Predictions?


J07

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Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    I had been following the UV indices, as I usually do from September to April, and noticed a few instances where the measured UVI was notably above what was predicted for clear sky conditions. This struck me, because usually they are extremely accurate, they only deviate wildly if it's a cloudy day, and of course then the measured UVI is less than the predicted....and when skies clear it usually follows the predicted curve pretty impressively.

    1) The first, and possibly most notable, is that of Invercargill on the 12th November. Predicted clear sky UVI maxes out at 9, but the measured UVI gets up to about 12! This is the sort of value which will only be *predicted* on a few occasions in ~January.

    2) This one is also pretty impressive. Rarotonga at the end of November. Predicted clear sky UVI of 13, but it bursts right up to almost 16 at one point!

    3) One closer to home, just up the road from Wellington. Forecast of about 10, instead pushes near to 12.

    4) And finally, from yesterday. Forecast of 11, hits 13. No wonder the sun felt like it was really stinging!

    5) (This one is just showing normal, well behaved predictions and measurements, which is what we see on most sunny days).

    At first I thought this may be a measurement error, but I've put this to one side because it's occurring in at least 3 different locations, and it's not just an isolated outlying point, there is a very clear trend around the extreme value, showing it rise and then fall.

    So, is this a result of the ozone hole breaking up and parts of it passing over the South Pacific? And, if so, is it completely unpredictable?

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    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    More examples from 5th Dec:

    1) Wellington exceeding prediction

    2) Raro exceeding prediction

    3) Meanwhile, Central Otago pretty much perfectly predicted.

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    • 1 month later...
    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    I know I'm talking to myself, but it's somewhat enjoyable :D

    I thought UV indices would be on the slide now. But, yesterday, we recorded UV index of 13. Again, it was a bit above predicted. I wonder if it was due to a southerly change the previous day? Southerly change = very clear pollution free skies + (possible ozone hold fragments?) = extreme UV indices...

    Certainly it's kind of dangerous to have a UV index of 13, when the temperature is only 18C.

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    • 1 month later...
    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    Eh, once again it happened. Conditions at the time, there was plentiful mid cloud (altocumulus lenticularis), but it wasn't blocking out the sun here....though it may have done so in Paraparaumu.....would that be a case of UV increasing due to high cloud? I didn't think it worked on mid cloud though.

    anyway I just can't piece it all together. It goes on a downwards trend and then "overshoots" when recovering, but that's not really an explanation for the physical mechanism. <_<

    Extreme UV couldn't be further away from this country at the moment! Lot's of snow, cloud and cold!

    Extreme UV is always a long way from the UK! Still this seems like a long time ago!

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    • 1 month later...
    Posted
  • Location: Upton, Northampton.
  • Weather Preferences: Snow, storms
  • Location: Upton, Northampton.

    Flippin heck, UV index of 13!! It is strong enough here when it reaches 7-8 in the peak of Summer, I burn from being out in 7-8 for a few hours, I dread to think how bad it would be if I was out a few hours in 11-13 :|

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    • 1 month later...
    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL
    Flippin heck, UV index of 13!! It is strong enough here when it reaches 7-8 in the peak of Summer, I burn from being out in 7-8 for a few hours, I dread to think how bad it would be if I was out a few hours in 11-13 :|

    Here's a like-for-like comparison:

    1)Today's UV forecast for the UK, with clear skies.

    2) 6 months ago the UV forecast for Wellington (11) and what was actually recorded (13).

    Clear sky UV indices in Europe at comparable latitude to Wellington range from 2 to 9 depending on cloud cover.

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    Posted
  • Location: Madrid, Spain (Formerly Telford)
  • Location: Madrid, Spain (Formerly Telford)

    I never thought a UV index of 12/13 could be possible in New Zealand as ive always thought of it as being like the Southern Hemisphere version of the UK in a way but i guess its Latitude is not about 50S like the UK is 50N?

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    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL
    I never thought a UV index of 12/13 could be possible in New Zealand as ive always thought of it as being like the Southern Hemisphere version of the UK in a way but i guess its Latitude is not about 50S like the UK is 50N?

    NZ stretches from about 35N to 47S.

    If you went to 50S though, the summer sun would still be stronger than in the UK (not that there is much sun at 50S though!). Southern Hemisphere UV is 40% stronger than at similar latitudes in the Northern Hemisphere. The sun's rays down here are also stronger- when ignoring UV- due to lack of pollution and the seasons being in phase with the earth-sun distance (as opposed to out-of phase as is the case in the NH).

    Essentially, to get NZ-esque sun-strength in the northern hemisphere, you have to go to the tropics- not even the Med is enough.

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