Naked-eye observation of sky cloud cover has widely resisted automation. Automatic cloud cover detection systems suitable also for nighttime operation often demand large equipment investments and expensive data processing. An automatic partial cloud amount detection algorithm (APCADA) is presented, based only on accurate measurements of longwave downward radiation, temperature, and relative humidity at screen level height. APCADA provides cloud cover estimates every 10 min during daytime and nighttime and is applicable to radiation stations without knowledge of synoptic cloud observations. Naked-eye observations from seven radiation sites spanning from arctic to tropical climates have been compared to APCADA estimates. Results show that about 86% of all cases agree within ¿1-octa cloud amount difference for sites with moderate climate, 82% for sites with arctic climate, and 78% for the site with tropical climate. For a maximum ¿2-octa cloud amount difference, average site percentages range from 90% up to 95%.