Climate classification, particularly that of Köppen, has been commonly used in disciplines such as geography and ecology. However, specialized systems have been developed for viticulture. The first scientific study of viticultural climates was that of the French researcher A. P. de Candolle in the mid 19th century, a time when reliable climatic data were just starting to become available. De Candolle observed that budbreak in European vineyards corresponded closely with the dates on which average mean temperature reached 10 °C/50 °F. He proposed that useful heat for vine growth and ripening could be measured by the amount that actual mean temperatures exceed 10 °C. Therefore, a summation of the excesses for all growing-season days would give a measure of the total usable heat for the year.