Forel-Ule scale

The scale provides a visual estimate of the color of a body of water, and it is used in limnology and oceanography with the aim of measuring the water's transparency and classifying its biological activity, dissolved substances, and suspended sediments.

[2] The classic Forel-Ule Scale uses a set of liquid vials of multiple colors.

Together, the liquid vials represent a standardized color palette created by using a set of small transparent glass tubes containing colored water by adding different concentrations of stable inorganic salts.

[1] The Forel-Ule scale observations, along with temperature, salinity, bathymetry, and Secchi depth, are some of the oldest oceanographic parameters dating back to the 1800s.

Citizen scientists estimate the color of the water with the Forel-Ule scale using a smartphone app called “Eye on water.” [4] [5] [6]

A researcher uses a Forel-Ule scale to measure the transparency and color of water in Kyiv and Kaniv reservoirs, Ukraine.
Illustration of the Forel-Ule Scale on a pamphlet titled “Colors of the Sea” in Dutch.