The Collaborative International Dictionary of English v.0.48:

Harmattan \Har*mat"tan\ (h[aum]r*m[a^]t"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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sirocco \Si*roc"co\ (s[i^]*r[o^]k"k[-o]), n.; pl. Siroccos
   (s[i^]*r[o^]k"k[=o]z). [It. sirocco, scirocco, Ar. shorug,
   fr. sharq the rising of the sun, the east, fr, sharaca to
   rise as the sun. Cf. Saracen.]
   1. An oppressive, relaxing wind from the Libyan deserts,
      chiefly experienced in Italy, Malta, and Sicily.
      [1913 Webster]

   2. In general, any hot dry wind of cyclonic origin, blowing
      from arid or heated regions, including the desert wind of
      Southern California, the harmattan of the west coasts of
      Africa, the hot winds of Kansas and Texas, the kamsin of
      Egypt, the leste of the Madeira Islands, and the
      leveche of Spain.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

harmattan
    n 1: a dusty wind from the Sahara that blows toward the western
         coast of Africa during the winter