Generally
speaking , The climate is dry tropical in most parts of the plateau
between the Caspian sea and the Persian Gulf. It is bracing and
mild in the valleys, and rigorous in winter in the northwest and
higher valleys. With the exception of the Caspian coast, where
the average rainfall is heaviest, clear days are the general rule.
little or no rain falls between the end of April and the beginning
of November. Spring ( April to May ) and autumn ( late September
to late November ) are good seasons for visiting Iran .Summer
is pleasant in Tehran and on the Caspian coast.
Avoid the southern coast from Abadan (west) to Bandar Abbas (
East ) in the 
height
of summer, and the middle of winter. for spring and autumn visits,
lightweight suits and dresses will be comfortable. In winter,
temperatures occasionally drop as low as 5c and a topcoat will
be needed.