In meteorology, cloudiness or cover is the name given to an accumulation of condensed water in the earth's atmosphere that is visible to the naked eye and does not touch the earth as a whole. Clouds should therefore be differentiated from both fog and haze , which is formed by floating, solid particles in the air.
Individually recognizable elements of the cloud cover are referred to as clouds . The extent to which the sky is covered with clouds is usually estimated by the weather observer, and nowadays also determined by special sensors or weather cameras . The data from weather satellites , however, can only be used regionally. Most weather stations use the eighth-note division specified in the synoptic . One then speaks of the unit of measurement "octa".
While the degree of coverage was often estimated in tenths in the past, nowadays it is usually given in eighths (see table on the right).
Classification of the aviation weather service
The aviation weather service sums up the eighth notes: The best weather condition is “clouds and visibility okay” ( CAVOK ), the almost cloudless sky was previously with “sky clear” ( SKC ), today with “no significant clouds” ( NSC ) for no clouds below 5000 ft or no significant clouds or “no clouds detected” ( NCD ) for automatic stations. This is followed by "few" (FEW) for slightly cloudy , then with 3–4 eighths of "scattered" (SCT) or loosely cloudy , 5–7 eighths of "broken" (BKN) for broken cloud cover and 8 eighths of "overcast" (OVC) for a dense, closed cloud cover . Also 9 eighths “obscured” (OBSC) if the sky is not visible.
Classification of the CIE
The International Commission on Illumination (CIE) defines some states of the sky. Sunny Sky (Engl. Sunny sky ) is the sky where the sun appears, the degree of cloudiness is initially irrelevant. As a clear sky (Engl. Clear sky ) a heaven where less than 30 percent of the area of sky through clouds covered. As teilbewölkten sky (engl. Partly cloudy sky ) one in which is covered between 30 and 70 percent of the area of sky with clouds. As a cloudy sky (Engl. Cloudy sky ) one in which more than 70 percent of the area of sky is covered with clouds. As overcast sky (engl. Sky overcast ) a sky with a completely closed cloud cover, that is 100 percent of the area of sky is covered with clouds. These sky conditions are important for daylight planning and in photography for determining the color temperature , which is between 5500 K and 7500 K depending on the degree of cloudiness.
These sky conditions are partially used for the calculation of daylight quotients; They are also important for daylight planning and in photography for determining the color temperature , which changes depending on the degree of cloudiness.
The standard sky is a CIE standard that mathematically models the spatial luminance distribution of the sky under different weather conditions . Its aim is to be a universal basis for the classification of measured luminance distributions of the sky and to provide a method for calculating the sky luminance for daylight planning.
The standard defines relative luminance distributions, i.e. the luminance of the sky at any point in the hemisphere is described as a function of the luminance at the zenith . In order to obtain absolute luminance distributions for daylight calculations, the model can be used with given values for the zenith luminance or the horizontal illuminance .
Standard lighting situations
The following standard lighting situations can be created through the defined value assignment of the five luminance gradation parameters (a ... e):
- CIE Standard Overcast Sky - Steep luminance gradation towards zenith, azimuthal uniformity
- Overcast, with steep luminance gradation and slight brightening towards the sun
- Overcast, moderately graded with azimuthal uniformity
- Overcast, moderately graded and slight brightening towards the sun
- Sky of uniform luminance
- Partly cloudy sky, no gradation towards zenith, slight brightening towards the sun
- Partly cloudy sky, no gradation towards zenith, brighter circumsolar region
- Partly cloudy sky, no gradation towards zenith, distinct solar corona
- Partly cloudy sky, with the obscured sun
- Partly cloudy sky, with brighter circumsolar region
- White-blue sky with distinct solar corona
- CIE Standard Clear Sky, low luminance turbidity
- CIE Standard Clear Sky, polluted atmosphere
- Cloudless turbid sky with broad solar corona
- White-blue turbid sky with broad solar corona
Common ISO / CIE standard ISO 15469: 2004 (E) / CIE S 011 / E: 2003: Spatial distribution of skylight - CIE standard general sky (German: spatial distribution of daylight - general sky standardized according to CIE )
The CIE Standard General Sky contains the CIE Standard Clear Sky and the CIE Standard Overcast Sky as special cases and thus replaces the CIE S003 - 1996: Spatial distribution of daylight - CIE standard overcast sky and clear sky .