However, they addressed the worst case scenario i.e. zenithal sun, rarely found in common outdoor activities.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference

Therefore zenithal sun exposure conditions, corresponding to summer sunlight at noon and maximizing UVB proportion are rarely found.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference

However they represent extreme solar UV exposure conditions with a quasi zenithal sun irradiance i.e. a UVA to UVB irradiance ratio of less than 18, representative of a high UVB level.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference

To assess more realistic solar UV exposure, a non-zenithal UV spectrum has been defined as standard daily ultraviolet radiation (DUVR) spectrum, with a UVA to UVB irradiance ratio of around 27.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference

He improved the methods of astronomical measurement, invented the zenithal collimator, suggested and applied photography and electricity to astronomy, and dealt with problems of physical astronomy, the shape of comets, the spots of the sun, meteors, etc.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 6: Fathers of the Church-Gregory XI] Reference