1 Institute of Construction and Architecture, Slovak Academy of Sciences, Bratislava, Slovakia 2 National Observatory of Athens, Institute of Environmental Research and Sustainable Development, Atmospheric Research Team, Athens, Greece Daylight levels during the solar eclipse of 11 August 1999 S. Darula 1 , H. D. Kambezidis 2 , and R. Kittler 1 With 4 Figures Received May 19, 2000 Revised October 17, 2000 Summary Solar eclipses are unique phenomena not only for astronom- ical and space observations but also for terrestrial; they create unique conditions of sunbeam blockage which cause not only the reduction of direct sunlight but also the dimming of skylight from the whole sky vault. Very favorable con- ditions were met during the recent August 1999 solar eclipse in Athens, Greece and Bratislava, Slovakia. General class daylight stations operate within the International Daylight Measurements Program in the two cities. One-minute data of global/diffuse illuminance and zenith luminance from those stations have been used to provide information about their levels and the daylight reduction rate during the eclipse. An approximate formula for the estimation of sunlight and skylight illuminance levels as well as zenith luminance using relative luminance sky patterns is also presented in this work. To achieve this, recently developed sky standards together with their parameterizations are utilized. 1. Introduction Many meteorological parameters, e.g. tempera- ture, wind, cloudiness, ozone as well as minor ionospheric and atmospheric changes, have been studied during past solar eclipse events (e.g., Anderson and Keefer, 1975; Arendt, 1972; Bojkov, 1968; Chimonas, 1970; Mims and Mims, 1993; Segal et al., 1996). However, the study by Shiozaki et al. (1999) is the only one that reports on daylight effects up to now. The shortage of such results in the international literature is due to the following factors. (i) Solar eclipses are rare astronomical phenom- ena. They occur when sunlight is totally or partially blocked by the moon leaving a trail of shadow on the surface of the Earth. The level of the sunlight blockage is de®ned by the Eclipse Magnitude (EM), i.e. the extent of the shade on the ®ctitious Sun diameter normalized by the unshaded diameter seen on the surface of the Earth. (ii) During the eclipse period, which usually lasts about 2.5 hours, clear-sky conditions have to prevail to prevent cloud in¯uence on sky patterns. (iii) Stations operating within International Day- light Measurement Program (IDMP) under the supervision of the International Commit- tee of Illumination (CIE), and World Meteorological Organization (WMO) must be selected in the region of the total or partial eclipse. Such stations are capable of measur- ing irradiance/illuminance data with a rela- tively rapid sampling rate. (iv) It is favorable to record the daylight levels before, during and after the solar eclipse maximum in order to register and evaluate relevant illuminance reduction. Such unique and ideal conditions were met in Bratislava as well as in Athens on August 11, 1999. On the other hand, the recent solar eclipse is the only one that occurred in the 90's with the Meteorol. Atmos. Phys. 76, 251±256 (2001)