Days are significantly dimmer and nights are completely dark without artificial sources of light. Overcast weather is when the sky is completely covered by a cloud blanket. A cloudy day may result in some weak showers, snowfalls, or even thunderstorms, although a thunderstorm temporarily overcasts the sky as it happens.
The clouds may temporarily drop the temperature when they hide the Sun during the day, which is particularly helpful during a heatwave.Ĭloudy weather is when a significant amount of clouds is covering the sky (at least half the sky). Partially cloudy weather is when some clouds are hovering above the horizon level however, most of the sky remains clear and there is no precipitation of any kind. There is practically no sunny/clear day under the Inter-tropical Convergence Zone since it rains almost on a daily basis. However, sunny/clear weather is quite rare for the rest of the world. Tops of mountains also enjoy plenty of sunny days since few clouds reach them this is why the most powerful telescopes were built in mountains. Sunny/clear weather is commonly found in deserts. Thus, sunny weather causes the most important variations in temperatures within the same air mass. Sunny weather can help temperatures to rise under the Sun's influence but a clear sky during the night allows temperatures to significantly drop, especially in winter, as heat is radiating from the surroundings and there is no cloud cover to keep it from escaping to space the decrease in temperature keeps going until an hour after sunrise. This also means that the sky is always sunny/clear above the highest altitude at which clouds can generate, which is about 85,340 meters (53 miles). If one is at the top of a mountain or travel in a plane, possible clouds and fogs don't count as long as they remain below the horizon level. The results are available for Europe.A sunny day, or clear sky during nighttime, is when there are clouds above the horizon. The analysis of daily surface solar radiation has been conducted in cooperation with the University of Frankfurt, Institute for Atmospheric and Environmental Sciences, as part of a Master Thesis. Both, SARAH and CLARA, are freely available data sets based on high-resolution satellite observations and provide information on the solar radiation at the ground at the daily and monthly time scales (also hourly for SARAH data). This analysis is based on the climatological CM SAF SARAH and CLARA data sets SARAH stands for Surface Solar Radiation Data Set - Heliosat, and CLARA stands for CM SAF cLouds, Albedo and Radiation dataset from AVHRR data.
With these definitions and based on more than 3 decades of daily surface solar radiation data the likelihood for a certain day being a sunny day and/or being part of a sunny period has been determined. In this analysis, a day is defined as a sunny day if the daily surface solar radiation exceeds 80% of the clear-sky surface solar radiation a 5-day sunny period is defined as five consecutive sunny days.
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