The Collaborative International Dictionary of English v.0.48:

Aridity \A*rid"i*ty\, n.; pl. Aridities. [L. ariditas, fr.
   aridus.]
   1. The state or quality of being arid or without moisture;
      dryness.
      [1913 Webster]

   2. Fig.: Want of interest of feeling; insensibility; dryness
      of style or feeling; spiritual drought. --Norris.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aridity
    n 1: the quality of yielding nothing of value [syn:
         fruitlessness, aridity, barrenness] [ant:
         fecundity, fruitfulness]
    2: a deficiency of moisture (especially when resulting from a
       permanent absence of rainfall) [syn: aridity, aridness,
       thirstiness]