Aridity index

#As used by C. W. Thornthwaite in his 1948 climatic classification: an index of the degree of water deficit below water need at any given station; a measure of aridity.

It is calculated, independently of the opposite humidity index, as follows:

where d (the water deficit) is the sum of the monthly differences between precipitation and potential evapotranspiration for those months when the normal precipitation is less than the normal potential evapotranspiration; and where n is the sum of monthly values of potential evapotranspiration for the deficient months.
Thornthwaite puts the aridity index to two uses: 1) as a component of the moisture index; 2) as a basis for the more detailed classification of moist climates (perhumid, humid, and moist subhumid climates).

1. 
   See index of aridity.

• INDEX
• A
• B
• C
• D
• E
• F
• G
• H
• I
• J
• K
• L
• M
• N
• O
• P
• Q
• R
• S
• T
• U
• V
• W
• X
• Y
• Z
• 0
• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9

1. As used by C. W. Thornthwaite in his 1948 climatic classification: an index of the degree of water deficit below water need at any given station; a measure of aridity.