Solar UV Index

UV forecast

Excessive sun tanning is harmful and may cause skin cancer, cataract and contribute to reducing the organism’s natural immunity. UV tolerance depends on the skin type.

Basic information about the UV radiation

The amount of the UV radiation reaching the surface of the Earth depends on:

- sun’s height above the horizon, i.e. latitude, time of day and year. The highest UV radiation occurs in the tropical zone and the lightest in the polar areas. In Poland, the highest radiation occurs in summer at midday.

- height above sea level: the highest UV index occurs in Tatra Mountains

- aerosol and water vapour diffusion in the atmosphere

- the amount of ozone in the Earth’s atmosphere, e.g. 1 percent change of ozone causes a 1.1-1.3 percent change of UV radiation

- absorption and reflection of the UV radiation by the Earth’s surface: the ratio of diffusely reflection is called albedo and equals 0.5 for ice/snow and 0.03 for flora respectively. The albedo effect is most prominent in mountains (reflection of the snow and rocks) and at the sea area (reflection of sand and water).

- cloud cover – the UV index is in a large scale absorbed by the clouds

Cloud cover is measured and expressed in oktas. Therefore, the cloud codification 0/8 refers to clear skies while 8/8 oktas indicate overcast.

UV radiation effects

For a great number of people Sun is a sign of joy and life. Yet, sun rays having its pros and cons may cause serious damage. Being not aware of it we tend to sun tan for much longer and more often than it is allowed. Because of that our skin may suffer. Too much exposure to sun may lead to skin cancer, cataract, accelerate aging of the skin, cause sunburns, skin irritations etc.

The sun emits ultraviolet radiation in the UVA (320-400nm), UVB (280-320nm) and UVC (100-280nm) bands.

The lethal Ultraviolet subtype C radiation (used to sterilise surgical instruments) is filtered out by the atmosphere.

UVB rays are blocked by the Earth’s ozone layer. Exposure to UVB radiation cause prompt skin sunburns. Luckily, only a few percent of these rays reach the Earth’s surface.

UVA radiation is unfortunately the least blocked by the atmosphere. It may result in skin and immune system problems.

Therefore, we have to be cautious and wise upon exposure to UV radiation. It is especially prudent to protect children. In particular, children under the age of three must not be exposed to sun rays. The level of UV damage depends on the radiation dose but also on the skin type. Usually, there are four most common skin types which are explained in the following section.

Characteristics of the European population skin type:

Skin type	Description	Reaction to Sun tanning	Suntan	Notes
I	Skin: Pale, fair, freckles Hair: Blond Eyes: Blue	Always burns	Never tans	Avoid sunbathing at all cost
II	Skin: Fair Hair: Blond or brown Eyes: Blue, green or grey	Usually burns	Sometimes tans	Be wise while sunbathing
III	Skin: Light brown Hair: Light coloured/dark	May burn	Usually tans	Allowed to sunbathe
IV	Skin: Olive blond Hair: Dark	Rarely burns	Always tans	Allowed to sunbathe
Regardless of the skin type you must not allow sun burns. One should avoid too much sun exposure. It is mandatory to avoid sun tanning during midday. Children under the age of 3 must not tan.

In order to forecast UV index for the following day, the total ozone level in the atmosphere is measured. The data is gathered from the NOAA/TOVS satellite and 48-hour meteorological forecast from the ALADIN mesoscale model.

The UV index forecast is based on the statistical model of the total ozone level provided by the Centre of Aerology and LIBRATRAN radiation transfer model.

Due to significant cloud cover variations, two UV index forecasts are made.

Sun tanning period for different skin types and UV index:

How to determine when will the skin burns or irritates?

Example:

Valid for index UV=8, frequent in summer time in Poland. The sun tanning period for the particular skin types is as follows:

Skin type:

I – under 20 minutes, II – over 20 minutes, III – 30 minutes, IV – approximately 40 minutes

Sun tanning limit is defined as a maximum sun exposure time without any protection e.g. sunscreen (sun cream) during which sun tan will not appear. It is also regarded as skin tolerance threshold when the redness of the skin appears.

In order to use the UV index information it is crucial to recognise your skin type.

UV index is issued to make people aware of its harmful effects and to inform the general public about the necessity of taking safety precautions. In case of including cloud cover and other meteorological factors in UV measurements the modification factors are ought to be implemented.

UV index is an international standard measurement of the ultraviolet radiation related to Erythema (redness of the skin) caused by sunburn.

Recommended means to effectively protect oneself from UV light:
UV index	High tolerance - I skin type and infants:	Itching and redness of the skin:
10+ Very high		under 15 minutes
7-9 High		20 minutes
4-6 Medium		30 minutes
1-3 Low	No need for protection
UV index	Medium tolerance - III skin type:	Redness of the skin:
10+ Very high		Under 30 minutes
7-9 High		40 minutes
4-6 Medium		60 minutes
1-3 Low	No need for safety precautions
The best way to protect skin is clothing (including hats) e.g. shirts, hats, trousers or sunglasses with special lenses that block UV light. The parts of body not protected against UV radiation ought to be sunscreen applied. Preferably, these designed to block both UVB and UVA radiation.

IMGW-PIB

Solar UV Index