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An arid environment has a high deficit of rainfall or snow. So there is much less humidity through the year than would be necessary for the demand for evaporation and transpiration. This is mostly due to the high temperatures and few storms that bring rainfall. Regions are classified as arid if their aridity index falls between 0.05 and 0.20.

Most deserts are semi-arid, arid or hyperarid, with the exception of Antarctica.