▁result ▁from ▁their ▁surrounding ▁ge ography . ▁For ▁example , ▁rain - block ing ▁mountain ▁ranges , ▁and ▁distance ▁from ▁o ce ans ▁are ▁two ▁ge ographic ▁features ▁that ▁contribute ▁to ▁desert ▁ar id ity . ▁ ▁Rain - block ing ▁mountain ▁ranges ▁create ▁Rain ▁Sh adows . ▁As ▁air ▁r ises ▁and ▁co ols , ▁its ▁relative ▁hum id ity ▁increases ▁and ▁some ▁or ▁most ▁mo ist ure ▁ra ins ▁out , ▁leaving ▁little ▁to ▁no ▁water ▁v apor ▁to ▁form ▁precip itation ▁on ▁the ▁other ▁side ▁of ▁the ▁mountain ▁range . ▁ ▁Des ert s ▁occup y ▁one - fif th ▁of ▁the ▁Earth ' s ▁land ▁surface ▁and ▁occur ▁in ▁two ▁bel ts : ▁between ▁ 1 5 ° ▁and ▁ 3 5 ° ▁latitude ▁in ▁both ▁the ▁southern ▁and ▁northern ▁hem is pher es . ▁These ▁bands ▁are ▁associated ▁with ▁the ▁high ▁solar ▁intens ities ▁that ▁all ▁areas ▁in ▁the ▁trop ics ▁receive , ▁and ▁with ▁the ▁dry ▁air ▁brought ▁down ▁by ▁the ▁desc ending ▁arms ▁of ▁both ▁the ▁Had ley ▁and ▁Fer ell ▁atmos pher ic ▁circul ation ▁cells . ▁D ry ▁wind s ▁hold ▁little ▁mo ist ure ▁for ▁these ▁areas , ▁and ▁also ▁tend ▁to ▁ev apor ate ▁any ▁water ▁present . ▁ ▁Many ▁desert ▁e cos ystem s ▁are ▁limited ▁by ▁available ▁water ▁levels , ▁rather ▁than ▁rates ▁of ▁radiation ▁or ▁temperature . ▁Water ▁flow ▁in ▁these ▁e cos ystem s ▁can ▁be ▁thought ▁of ▁as ▁similar ▁to ▁energy ▁flow ; ▁in ▁fact , ▁it