Historical wind speed equivalents of the Beaufort scale, 1850-1950

The original Beaufort scale of wind force is a scale that does not present the actual velocity with which the air moves. It is merely a scale of wind conditions that was used by sailors to categorize the different sailing conditions. Over the years, the use of the scale became very popular. Unfortunately, due to the increasing number of types of sailing ships and eventu-ally the arrival of steam boats, exchanging and comparing wind force information became a problem.

Since 1850 many attempts have been made to express the Beaufort scale numbers into physical units by correlating the estimate Beaufort wind force numbers with the measured wind speeds by anemometers. The World Meteorological Organization (WMO) still uses the wind speed equivalents, or WMO Code 1100, of the Beaufort scale adopted in 1946, for operational use (OMI-CMI, 1947). Nowadays it is widely accepted that WMO Code 1100 results in systematic biases when compared to recent anemometer measured winds. Also converting the different anemometer units - i.e. knots or m/s - to Beaufort numbers could lead to unexpected results.

The purpose of this document is to give a historical summary and present metadata to the different equivalent scales developed and used in Europe during the period 1850 1950. The metadata which is often stored very fragmented or is hard to find, is very important to understand old measured wind speed data correctly. Special attention is given to wind speed equivalents used in the Netherlands and to the conversion of the different wind speed units.

Recent equivalent scales developed after 1948, are often officially adopted by the WMO for scientific use, but are not used for operational meteorology. Extensive reviews compar-ing these scales with the official WMO scale are not discussed in this report but can be found in various other publications (e.g. Verploegh, 1967; Kent and Taylor, 1997; Lindau, 2003).