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The Forel-Ule Scale is a method to approximately determine the color of bodies of water, used in limnology and oceanography. By means of mixing different chemical (The standard solutions are made with distilled water, ammonia, copper sulphate, potassium-chromate and cobalt-sulphate) a standard color scale is produced in a series of numerically designated vials (1-21) which is compared with the color of the water body. The result is a color index for the water body which gives an indication of the transparency of the water and thus helps to classify gross biological activity. The color graduations correspond to open sea and lake water colors, as they appear to an observer ashore or on board a vessel. The method is used in conjunction with the Secchi disk submersed to half the Secchi depth.
The method was developed by François-Alphonse Forel and was three years later extended with greenish brown to cola brown colors by the German limnologist Willi Ule. The Forel Ule scale, a color comparator scale, has been described by Wernand and van der Woerd (M. R. Wernand, H. J. van der Woerd, "Spectral analysis of the Forel-Ule Ocean colour comparator scale.", J. Europ. Opt. Soc. Rap. Public. 10014s Vol 5 (2010), doi:10.2971/jeos.2010.10014s) and can be seen as a simple but adequate scale to classify the color of rivers, lakes, seas and oceans. The Forel-Ule observations belong, besides temperature, salinity and Secchi depth, to the oldest oceanographic parameters dating back to 1890.