The Gaussen Index (or Bagnouls-Gaussen Index) or xerothermic index is a method of calculating and comparing

arid A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...

ity. According to

Henri Gaussen Marcel-Henri Gaussen (14 July 1891 in Cabrières-d'Aigues ( Vaucluse) - 27 July 1981 in Toulouse), was a French botanist and biogeographer. In 1926, he defended his thesis on "the vegetation of the eastern half of the Pyrenees", which laid ...

(French botanist and

biogeographer Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, ...

), a given period is said to be arid, when:

P <

. (P: total

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

millimeter file:EM Spectrum Properties edit.svg, 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British Eng ...

s over the given period, T: average

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...

over the given period) The resulting index number indicates the number of biologically dry days in a year for a given location (it therefore ranges between 0 and 365). The data includes not only

' but also

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily inf ...

dew Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at w ...

and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

of the air. In general, it is accepted that an environment is non-arid when the index is less than 100, semi-arid between 100 and 290, arid between 290 and 350, and hyperarid between 350 and 365. This index is very useful for the use of an , the latter always constructed on the scale model: 1° C = 2mm precipitation. Other indices such as the

Louis Emberger (Marie) Louis Emberger (23 January 1897 – 30 November 1969) was a French botanist and phytogeographer, at the University of Montpellier. Life Emberger was born at Thann, in Haut-Rhin, France in 1897, which was then part of German occupie ...

rainfall quotient (which is not unique) have been defined. However, the Gaussen index which is simple and precise is still preferable. Indeed Henri Gaussen defines precisely the 4 nuances of Mediterranean climate just against this index, while Emberger defines the level of humidity in a region of Mediterranean climate but does not support precisely this Mediterranean climate. The calculation does not reflect reality because it is based on averages. For example, according to the calculation, we find a total of 0 biologically dry days in

Lyon Lyon,, ; Occitan: ''Lion'', hist. ''Lionés'' also spelled in English as Lyons, is the third-largest city and second-largest metropolitan area of France. It is located at the confluence of the rivers Rhône and Saône, to the northwest of ...

, for 60 biologically dry days in

Marseille Marseille ( , , ; also spelled in English as Marseilles; oc, Marselha ) is the prefecture of the French department of Bouches-du-Rhône and capital of the Provence-Alpes-Côte d'Azur region. Situated in the camargue region of southern Fran ...

References

Bibliography

* * * {{cite journal , first1=Henri , last1=Gaussen, authorlink1=Henri Gaussen , first2=F. , last2=Bagnouls , title=Les climats biologiques et leur classification , lang=fr , trans-title=Biological climates and their classification , journal=

Annales de Géographie The ''Annales de Géographie'' is a French journal devoted to geography, first published in 1891. From the start the journal was an influential and respected academic journal. History The ''Annales de Géographie'' was founded in 1891 by Paul Vid ...

, volume=6 , number=355 , year=1957 , pages=193–220, doi=10.3406/geo.1957.18273 , ref={{harvid, Gaussen and Bagnouls, 1957 Climate and weather classification systems Meteorological indices Precipitation