Broadly speaking, the further south you go the warmer it becomes. With the exception of the Caspian watershed, both sides of the Zagros range, and that of the Orumieh Lake basin, the country has probably in no part a yearly rainfall exceeding 33 to 36 centimeters and throughout the greater part of central and southeastern Iran the yearly rainfall is probably under 15 centimeters.
Along the shores of the Caspian the average precipitation is from 1,200 to 2,000 mm. Along the Persian Gulf regions, in spite of meager precipitation, in certain seasons, the climate is very humid.
The regions along the mountainous parts of the country have milder summers and colder winters. In Tehran, in the central and southern Tehran in summer are hot, dry and stuffy, but you only have to make a short bus ride up to the foothills of Damavand to cool down by several degrees. But it is not humid, and the evenings are cool and refreshing. Winters in the capital can be very chilly, extremely so at night, although any snow usually disappears by early march. Showers are frequent between November and mid may, but rare in summer.
The central plateau of Iran is marked by hot and dry summers and sporadic rainy winters. The Desht-e Kavir, southeast of the capital, is harsh, inhospitable and very very hot in summer. Winters are not much better, and at night the temperature can fall well below zero. If any time of the year can be called pleasant in this salty wasteland, it would have to be between October and December. The Dasht-e Lut to the south is, if anything, even worse; almost completely devoid of water from any direction and the last word in extreme aridity.
In the far southeast of Iran, away from the Persian Gulf proper, temperatures are a little lower.
Summers are hot and dry, winters mild and dry. Up in Sistan conditions are harsh: the hot season lasts from April to November with an average temperature of 50 degrees centigrade; winter is equally unpleasant with extreme cold until March. Down in the south of Baluchestan, along the coast of the sea of Oman, the climate is similar to that of the Persian Gulf region, or even hotter, with strong winds in summer. There is very little rain throughout southeast Iran and frost would be a great novelty.
Spring and autumn are the ideal times to tour Iran, but summer or winter can be OK, so long as you do a little planning and take a few precautions. The northwest of the country is generally the coldest and among the rainiest parts of the country. The winters in Azarbaijan and Kurdestan can be severe: temperatures well rule between December and February and sometimes fall as low as -20 degrees centigrade. Snow frequently remains until early spring, or even later in the mountains.
Wind is undoubtedly the most unpleasant element especially from June in the east of the country. In Mashhad there are tales of wind from central Asia lasting for one hundred and twenty days, which in the middle of summer blow at up to 200 km/h. In all seasons, sudden gusts cause whirlwinds and sandstorms. But the coastal regions have quite a different climate. The Caspian coast is damp all year round and provides a pleasant contrast with the dryness of plateaus which are only a few hours away by road. But the temperatures are rarely excessive. Rain is frequent, vegetation is exuberant, as described elsewhere in this book, and the prevailing wind comes from the sea. The visitor should plan his/her wardrobe accordingly.
In all seasons, always have handy two indispensable items: a woolen pull-over and a pair of sun-glasses.
Altogether, the Iranian climate varies considerably from the rainy north and snowy northwest and west to the southern sunbelt, so take this into account as you pack your suitcase. In summer take lightweight and easily washed clothes of natural fabrics, a cardigan or pullover for the cooler nights, a pair of sunglasses and (only if you are male) a hat which will protect your face from the sun.
In spring and autumn take a sensible compromise, according to the conditions in the places you are going to visit. For men, a suit will only be necessary if you are travelling on business or planning to mix in the higher reaches of Iranian society; a smart jacket is useful but rarely essential. An umbrella may be useful in the Caspian provinces in the rainy season.
Tourists dress with a relative informality, though business visitors usually wear suits and ties for important meetings and banquets. By all means avoid ostentation.