The northernmost part of Iran lies at approximately latitude 40º north, about the same as Philadelphia in the United States, and its southernmost part around latitude 25º north, about as far south as Miami. The diversity of climate between Philadelphia and Miami, however, is not nearly as extreme as that to be found in Iran, which varies dramatically from region to region and according to elevation. On the plateau, aridity combined with high elevation produces a very rigorous continental-type climate, with great variation in temperature between seasons and even between day and night. At Tabriz (elevation 4,400 feet), in the northwest, the temperature falls as low as –13°F in winter and soars to 104°F in summer. At Mashhad (3,300 feet), in the northeast, it drops to –18°F in winter and rises to 108°F in summer. Other major cities are far enough south to avoid these extremes in winter. Tehran (3,800 feet), the capital, has an average yearly temperature of 64°F: winter brings a good deal of snow and average lows of 29°F in January; summer is extremely hot and dry, with an average high of 97°F in July. The extremes vary from a record low of 9°F in January to 108°F in July. Isfahan (5,150 feet), in the center of the plateau, has a yearly average of 62°F, with average lows of 29°F in January and highs of 95°F in July.
Of course, areas in the high elevations are extremely cold in winter, while some of those in the south can be blisteringly hot in summer—daily highs over 122°F are not uncommon in Irânshahr (2,200 feet). Spots in the Dasht-e Lut can claim to be among the hottest on earth, with temperatures often over 134°F and reported measurements as high as 156°F.
Off the plateau, the climatic regime is quite different. The Caspian areas are much more humid and also milder in temperature, rarely falling below freezing in winter in the lower elevations and with summer highs around 93°F. In the Khuzestân plains, the temperatures at Ahvâz range from highs of around 68°F in January to 118°F in July. The British traveler Percy Sykes noted that the temperature at nearby Shustar in June 1896 measured 129°F in the shade day after day. 1 Along the Persian Gulf, humidity hovers near 100 percent, and daily highs range from 68°F in winter to 106°F in summer.