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Ship-borne measurements of spectral as well as biologically effective UV irradiance have been performed on the German research vessel Polarstern during the Atlantic transect from Bremerhaven, Germany (53.5° N, 8.5° E), to Cape Town, South Africa (33.6° S, 18.3° E), from 13 October to 17 November 2005. Such measurements are required to study UV effects on marine organisms. They are also necessary to validate satellite-derived surface UV irradiance. Cloud free radiative transfer calculations support the investigation of this latitudinal dependence. Input parameters, such as total ozone column and aerosol optical depth have been measured on board as well. Using these measured parameters, the modelled cloudless noontime UVA irradiance (320–400 nm) shows the expected dependence on varying minimum solar zenith angles (SZA) at different latitudes. The modelled cloudless noontime UVB irradiance (290–320 nm) does not show this clear dependence on SZA due to the strong influence of ozone absorption in this spectral range. The maximum daily dose of erythemal irradiance of 5420 J m−1 was observed on 14 November 2005, when the ship was in the tropical Atlantic south of the equator. The expected UV maximum should have been observed with the sun in the zenith during local noon (11 November). Stratiform clouds reduced the dose to 3835 J m−1. In comparison, the daily erythemal doses in the mid-latitudinal Bay of Biscay only reached values between 410 and 980 J m−1 depending on cloud conditions. The deviation in daily erythemal dose derived from different instruments is around 5%. The feasibility to perform ship-borne measurements of spectral UV irradiance is demonstrated. [ABSTRACT FROM AUTHOR]