Which type of evaporimeter is considered the standard type used globally?

The Standard Evaporimeter: The U.S. Class A Pan

When measuring evaporation from open water surfaces, one instrument stands out as the global benchmark—the United States Class A evaporation pan. Despite advances in meteorological technology, this simple yet effective tool remains the most widely used evaporimeter worldwide.

Designed as a cylindrical tank made of galvanized iron or stainless steel, the Class A pan measures 120.7 cm in diameter and 25.4 cm deep. It's typically placed on a wooden platform to allow air circulation beneath, minimizing external heat influence. The pan is filled with water to a specific level and left exposed to the elements. Daily measurements of water loss, combined with precipitation data, allow scientists to calculate the net evaporation rate.

Although modern automated sensors and satellite-based estimates are gaining traction, the Class A pan endures because of its simplicity and consistency. It serves as a reference point against which newer technologies are often validated. Organizations like the World Meteorological Organization (WMO) recognize it as a standard for comparative evaporation studies.

While it does require manual readings and corrections—such as applying empirical coefficients to account for differences between pan and natural water body evaporation—it remains a cornerstone in hydrological monitoring. In a world increasingly reliant on digital data, the enduring presence of the Class A pan is a testament to the value of straightforward, field-tested tools in science.