Research programme. United Nations Environment Programme In the preparations leading to the 1977 UN Conference on Desertification (UNCOD), the United Nations Environment Programme (UNEP) issued a dryness map based on a different aridity index, proposed originally by Mikhail Ivanovich Budyko (1958) and defined as follows: where is the mean annual net radiation (also known as the net radiation balance), is the mean annual precipitation, and is the latent heat of vaporization for water. Note that this index is dimensionless and that the variables , and can be expressed in any system of units that is self-consistent. More recently in 1992, the UNEP has adopted yet another index of aridity, defined as: where is the potential evapotranspiration and is the average annual precipitation (UNEP, 1992). Here also, and must be expressed in the same units, e.g., in millimetres. In this latter case, the boundaries that define various degrees of aridity and the approximate areas involved are as follows: See also Climate classification Aridification Desertification References Huschke, Ralph E. (1959) Glossary of Meteorology, American Meteorological Society, Boston, Second printing-1970. McIntosh, D. H. (1972) Meteorological Glossary, Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p. Climatology Hydrology