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Extreme UV intensity

2008-07-31 13:51:35

UV Index Satellite Maps

http://www.temis.nl/uvradiation/world_uvi.html

With the world s highest rates of skin cancer, New

Zealanders should be aware of the health risks posed by

overexposure to UV radiation. A long-established UV

advisory service regularly reports the risk, which is

heightened by (mostly) fair skins, outdoor lifestyle, and a

climate that encourages sun-seeking. Relatively high UV

intensities also play a part. The erythemal (sun-burning)

strength of UV radiation is usually given in terms of the

UV Index (UVI), a scale first used in Canada and defined

to range from 1 to 10 there. In the NZ summer, UVI

values regularly exceed 13, even in the south of the

country where UV is less intense. A recent study showed

that peak UVI values in NZ are about 40% more than at

similar latitudes in North America (McKenzie et al.,

2006).

Compared with equatorial regions, our UV is not

particularly strong contrary to anecdotal evidence from

travellers. The main factors controlling UV, in desdending

order of importance, are:

Solar elevation causes the diurnal, seasonal and

latitudinal range.

Clouds reflect and scatter radiation. Heavy overcast

conditions can reduce UV intensity by more than 90%,

while clouds near the Sun can cause short-term UVI

increases of ~20%. On average, clouds reduce clear sky

UV by about 30%.

Ozone absorbs UV. UVI decreases by about 1% for

every 1% increase in ozone, which varies from a

minimum of about 100 Dobson Units1 inside the

Antarctic ozone hole to a (seasonal) maximum of about

500 DU at mid latitudes. The global average ozone

column is about 300 DU, and away from the ozone hole

it tends to be lowest (~ 200 DU) in the tropics.

Aerosols extinction can reduce UVI by 20-30%.

Altitude reduces the air column for scattering and

absorption; even in the most pristine air, UV increases

by about 5% per kilometre.

Snow reflects UV for backscatter from air. It can add

20-40% to UVI, but less at high altitude.

The elliptical orbit of the Earth about the Sun is also

significant; at closest approach in early January, the UV

intensity at the top of the atmosphere is about 7% more

than in July.

So where on Earth s surface would we expect the

highest UV? Peak UV should occur within the tropics

(high sun, low ozone), at a high altitude site, in the

Southern Hemisphere. Near the Tropic of Capricorn,

overhead Sun occurs during the period when the Earth-

Sun separation is a minimum.