Long-term Observation of Nighttime Clouds over São João do Cariri (7.4º S, 36.5º W)

Abstract

Using images collected by an all sky imager deployed in São João do Cariri (7.4º S, 36.5º W) from September 2000 to November 2010, the nighttime incidence of clouds has been observed and studied. Most of the nighttime cloudy occurred from February to July, which is related to the rainy season in the area of the observatory. The best period to run optical observations over São João do Cariri starts in the early night up to 22:00 LT (Local Time) throughout the period between August and December. An anti-correlation between the F10.7 cm solar flux index and the number of cloudy nights was observed, which is an evidence of the cosmic rays activity in the lower earth's atmosphere. The location of the all-sky imager provides reliable optical observations all year through.