Source : Webster's Revised Unabridged Dictionary (1913)

Harmattan \Har*mat"tan\, n. [F. harmattan, prob. of Arabic
   origin.]
   A dry, hot wind, prevailing on the Atlantic coast of Africa,
   in December, January, and February, blowing from the interior
   or Sahara. It is usually accompanied by a haze which obscures
   the sun.

Source : WordNet®

harmattan
     n : a dusty wind from the Sahara that blows toward the western
         coast of Africa during the winter