An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. A number of aridity indices have been proposed (see below); these indicators serve to identify, locate or delimit regions that suffer from a deficit of available water, a condition that can severely affect the effective use of the land for such activities as agriculture or stock-farming.

At the turn of the 20th century, Wladimir Köppen and Rudolf Geiger developed the concept of a climate classification where arid regions were defined as those places where the annual rainfall accumulation (in centimetres) is less than Failed to parse (Missing texvc executable; please see math/README to configure.): R/2 , where:

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This was one of the first attempts at defining an aridity index, one that reflects the effects of the thermal regime and the amount and distribution of precipitation in determining the native vegetation possible in an area. It recognizes the significance of temperature in allowing colder places such as northern Canada to be seen as humid with the same level of precipitation as some tropical deserts because of lower levels of potential evapotranspiration in colder places. In the subtropics, the allowance for the distribution of rainfall between warm and cold seasons recognizes that winter rainfall is more effective for plant growth that can flourish in the winter and go dormant in the summer than the same amount of summer rainfall during a warm-to-hot season. Thus a place like Athens, Greece that gets most of its rainfall in winter can be considered to have a humid climate (as attested in lush foliage) with roughly the same amount of rainfall that imposes semi-desert conditions in Midland, Texas, where rainfall largely occurs in the summer.

Huckleberry "Huck" Finn is a fictional character created by Mark Twain, who first appeared in the book The Adventures of Tom Sawyer and is the protagonist and narrator of its sequel, Adventures of Huckleberry Finn. He is 12 or 13 years old during the former and a year older ("thirteen or fourteen or along there," Chapter 17) at the time of the latter. Huck also narrates Tom Sawyer Abroad and Tom Sawyer, Detective, two shorter sequels to the first two books.

Huck is the son of the town's vagrant drunkard, "Pap" Finn. Sleeping on door-steps when the weather is fair, in empty hogsheads during storms, and living off of what he receives from others, Huck lives the life of a destitute vagabond. The author metaphorically names him "the juvenile pariah of the village." The author describes Huck as "idle, and lawless, and vulgar, and bad," qualities for which he was admired by all the children in the village, although their mothers "cordially hated and dreaded" him.

Huck is an archetypal innocent, able to discover the "right" thing to do despite the prevailing theology and prejudiced mentality of the South of that era. The best example of this is his decision to help Jim escape slavery, even though he believes he will go to hell for it (see Christian views on slavery).