Outdoor UVB at this time of year

[Alan1]

This time of year being the closest to midsummer's day it's obviously going to be the best for longest available useful UVB. Now that we've passed midsummer's day it will be interesting to see if it deteriorates at the same levels as before we got to that point - like May = July and April = August and March = September or whether other factors like heat in the ground will affect the thickness of cloud etc, it probably will.

Anyway for those interested (and it will depend on where you are in the UK, I am in Southern Scotland at 55.46 degrees north) the UVB from the sun that is equal to a mercury vapour bulb that is still producing acceptable levels of UVB starts at around 9am and you are still getting it at 5pm.

It will be different wherever you live but I'm not sure how it will compare. I noticed on the news the other day on midsummer's day it said the sun rose at Stonehenge at 5.19am, well the sun is rising here at 4am and setting after 10pm.

In the south, by the time the sun has risen it has already been burning away the haze for an hour here but at the same time the sun will be hotter in the south so by the time you get to about 9am the UVB levels may be about the same as here, I don't know really. Here yesterday, after the sun had been out for 2 hours the UBV level was only 9 uW/cm2.

The afternoons are a bit better because the sun burns through the haze as the day goes on. Even when it looks as though the sky is clear and blue there is always a haze or thin cloud which can't be seen.

yesterday's readings:
June 22nd 2010: 6am cear sky, 9 -- 7am clear sky, 36 -- 8am clear sky, 96 -- 9am clear sky, 152 -- 10am slight haze, 209 -- 11am slight haze, 258 -- 12 noon slight haze, 277 -- 1pm slight haze, 297 -- 2pm clear sky, 285