Rain factor

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The rain factor according to Richard Lang , usually called rain factor for short , is a key figure for describing the local climate and for delimiting climatic zones . It is formed as the quotient of the amount of precipitation and the temperature of a region. Information on the humidity and aridity of an area can be derived from the rain factor . The concept, which was first scientifically described by Richard Lang in 1915, is based on more modern concepts, such as the aridity factor , which relates precipitation to the saturation deficit of the air and evaporation .

Determination of the rain factor

The rain factor is still a simple method for determining the reference value for aridity and humidity. It can be used for the simple delimitation of climate zones and allows statements to be made about the water balance of an area.

When determining the rain factor, the mean annual precipitation (N, in mm) is related to the mean annual temperature (T, in ° C):

With a rain factor of less than 40, the area is considered arid, over 100 as humid. Altogether, building on Lang, a distinction is made between five defined moisture types: an area below 40 is described as arid, between 40 and 60 as semi-arid, between 60 and 100 as semihumid, between 100 and 120 as humid and above that as perhumid.

Background and development

Diagram of the development of soils according to temperature and humidity under the assumption of optimal formation conditions
( Richard Lang , 1915)

The rain factor was introduced in 1915 by the German geologist Richard Lang in order to relate the relationship between the dryness or humidity of an area with the typical soil formation . The humidity of the area increases with increasing precipitation; on the other hand, higher temperatures lead to higher evaporation. According to Lang, these two parameters are related to determine the moisture content of an area. He developed a climatic soil series from different soil types, which in his opinion develop in areas of different aridity. According to this, raw humus soils are found predominantly in Scandinavia , northern Russia and northern Germany in cool climates, while black earth and brown earth soils adjoin to the south and form the most important soil types in Central Europe. Yellow earth and red earth are found in southern Europe , the former in the cooler and more humid areas, the latter in the drier and warmer areas. The laterite is typical for the tropics in this sequence . The relation of the rain factors and thus the aridity and humidity with the soil colors was criticized several times without questioning the general influence of the rain factors on the soil formation. However, since soil formation does not only depend on the rain factors, but above all on the geological substrate, these were rejected as a classification for the soil types and only accepted for the assessment of moisture.

Lang's approach of rain factors to determine humidity and aridity has since been further developed by numerous scientists and generally related to the aridity of a region, while the aspect of specific soil formation increasingly faded into the background. Emmanuel de Martonne considered, for example, the number of days of precipitation, and Charles Warren Thornthwaite (1899–1963) developed a water balance model for agriculture for the US Department of Agriculture in 1942, based on the rain factor , in which evaporation plays a key role. In 1948 he introduced a humidity index (I h ) and an aridity index (I a ) and related the amount of precipitation exceeding potential evapotranspiration as excess water (s) with the water deficit (n and d) of the soil. He later combined the two indices to include seasonal factors and developed the I m index . On this basis, a finer system of aridity and humidity could be developed.

supporting documents

  1. ↑ a b " Rain factor " in the Lexicon of Geography, available at Spektrum.de; Accessed July 18, 2016.
  2. ↑ a b c d " Rain factor " in the lexicon of geosciences, available at Spektrum.de; Accessed July 18, 2016.
  3. ↑ a b Richard Lang : Attempt at an exact classification of soils in climatic and geological terms. International communications for soil science, Berlin 1915; Pp. 312-380. ( Full text )
  4. ^ A b Paul Schaufelberger: Forty Years Rain Factor Quarterly Publication of the Natural Research Society in Zurich 100, 1955; Pp. 194-201. ( Full text )
  5. ↑ Paul Schaufelberger: Why have the rain factors failed so often in soil science and climatology? Geografica Helvetica 13, 1958; Pp. 69–73 ( full text )
  6. ^ " Aridity factor " in the lexicon of geosciences, available at Spektrum.de; Accessed July 18, 2016.