The ozone fields are generated with the operational data assimilation system TM3-DAM, which is driven by the meteorological fields (wind, surface pressure, temperature) from the European Centre for Medium-Range Weather Forecasts (ECMWF).

On the basis of these fields, it is possible to give a forecast for the ozone fields for the next four days. Knowledge of these ozone fields enables a clear-sky UV index forecast at local noon for today and the following four days. Clicking on the pictures below will show the forecasts of the ozone field and the clear-sky UV index forecast at local noon for the following four days. You may notice that there are no data available for some parts of the Earth surface and wonder why it is possible to forecast the ozone levels for the whole Earth. Indeed, the GOME instrument measures the backscattered sunlight and, so, depending on the season there are parts of the Earth not illuminated by the sun at all times. However, these gaps can be filled by using assimilation techniques which are modelling the movement of the ozone using ECMWF windfields.

*Updated every hour to produce a global map for local noon (GMT) but is continuously updating as more measureements come, giving an increasingly accurate picture.  
 

Ozone field	Clear-sky UV index
Forecast for tomorrow
Forecast for 2 days ahead
Forecast for 3 days ahead
Forecast for 4 days ahead

 
 
How to read the Ozone forecast
 
Background on Dobson Unit:

The 'Dobson unit' is named after professor G.M.B. Dobson (1889 - 1976), who has from the 1920s onwards done research on the ozone layer. Around 1930 he built the first 'Dobson spectrophotometer', with which reliable measurements of the ozone layer became possible.

The 'Dobson unit' indicates how much ozone there is in the air above a certain point on Earth. A proper unit would thus be 'Kilogram per square meter'.

The unit introduced by Dobson arrises as follows. Suppose that all the ozone in the air would be in a (gas) layer just above the ground, at standard pressure (1013.25 hPa) and at standard temperature (0.0 Celsius). The amount of ozone is then indicated by the thickness of this layer, expressed in 0.01 millimeter. (This is why the ozone layer is sometimes referred to as being 'thick' or 'thin'.)

1 Dobson unit (DU) is:
2.6867E+20 molecules per square meter
4.4615E-04 MOL per square meter
2.1415E-05 kilogram ozon per square meter

Averaged over the entire world the ozone column has a value of about 300 DU. During spring on the southern hemisphere, September-November, the so-called 'ozone hole' develops, with ozone values (well) below 200 DU.
 
 
How to use the UV forecast
 
UV index:

The UV index is the effective UV irradiance (1 unit equals 25 mW/m2) reaching the Earth's surface. This UV index is valid for clear-sky conditions and will usually be lower for cloudy skies or polluted areas. Under partially cloudy conditions the UV index can be higher for short time periods. The UV index can be up to a factor 2 higher when the Earth's surface is covered with snow.

The UV index is a measure for the harmful Ultraviolet sunlight. It is no measure for the "visible" sunlight or for the warmth of the Sun.

• the values given above are the highest UV index. Elsewhere you can find examples of the daily course of the UV index

• to convert the UV index into the number of minutes sunshine it takes before the unaccustomed skin starts to colour red ('sunburn'), the following list can be used:
  • Skin type 1
    Maximum time in the Sun = 67 minutes / UV index
  • Skin type 2
     Maximum time in the Sun = 100 minutes / UV index
  • Skin type 3
    Maximum time in the Sun = 200 minutes / UV index
  • Skin type 4
    Maximum time in the Sun = 300 minutes / UV index

    Example: At UV index 4 and skin type 2 your skin will start to sunburn after 25 minutes of sunshine (100 minutes / 4 = 25 minutes)

• The above mentioned times are for skin not yet accustomed to sunlight and skin which is not protected with a sunburn lotion. If you use a sunburn lotion, you can multiply the number of minutes with the 'factor' of the lotion mentioned on the bottle/tube.

  Example: At UV index 4, skin type 2 and the use of a protection factor 6 lotion, your skin will start to sunburn after 2.5 hours of Sunshine ([100 minutes / 4] * 6 = 150 minutes)

  Note that this statement is true only when people apply the correct amount of sunscreen and re-apply it frequently. In addition, bathing, rubbing and towelling will remove the sunscreen from the skin and the actual Sun Protection Factor (SPF) will be lower than predicted by the label. In sunscreen testing in a laboratory, 2 mg/cm² of sunscreen product is applied, in order to get reproducible results. Usually people tend to apply less than 2 mg/cm², resulting in an actual SPF lower than the labelled one. (This may be an argument for considering high SPF products).

• more information on the UV index can be found on the web site of the US Environmental Protection Agency

• the UV index values are valid for clear sky and broken cloud conditions. For overcast conditions the UV index is about half

• the UV index values given above are for sea-level. If there is no snow, the UV index increases by about 5% per kilometer height. The distribution of the UV index on the maps has been corrected for the average height above sea-level. Keep in mind that the real UV index on a mountain is of course higher than in a valley.

• if there is snow in the mountains, the UV index can be twice as large as indicated above. Remember that standing on snow covered ground you also get light from 'below', as he sunlight is reflected by the snow. Your chin and nose thus get more sunlight than you may be used to. And if it is cold, your skin is more sensitive to sunlight.