Daylight climatology in Athens, Greece: types of diurnal variation of illuminance levels

In the present work, the diurnal variation of illuminance levels in Athens, Greece, is studied during winter and summer. The database consists of 5‐minute values of global horizontal illuminance for the period 1992–1996 recorded at the National Observatory of Athens (NOA). For each season, two sets of data are determined (sunrise to noon and noon to sunset) consisting of the semi‐diurnal courses of illuminance anomalies from the mean semi‐diurnal course of each day of the year. By using the multivariate statistical methods Factor Analysis and Cluster Analysis, the identification of the most characteristic modes of diurnal variation of global illuminance is attempted. It is found that six types of semi‐diurnal courses (sunrise to noon and noon to sunset) of global illuminance are the most characteristic for winter, while for summer, six types of courses are found for sunrise to noon and five for noon to sunset. Some of these courses present the almost smooth ascending/descending intra‐daily variation of illuminance, while others show significant disturbances during various daytime periods due to development of clouds. Copyright © 2009 Royal Meteorological Society