Iran is a fairly large country so the climate differs pretty greatly depending on where you are within it.

This is a general climate map of the country of Iran. The green shows the mild climate along the coast of the Caspian Sea (represented by the blue color seen above the green band). The dark brown is the mountainous region and the yellow(ish) color is semi-desert and full desert climate.

In the mild Caspian climate, temperatures rarely fall below freezing. The area is generally humid. It never gets much hotter than approximately 85 degrees. Annual rainfall can vary between 26in and 67in when moving from east to west respectively.

In the rest of Iran, the climate is generally characterized by long, hot, dry summers and short, cool winters. In the summer, the sun is out almost all the time, with high humidity on the southern coast (where Iran boarders the Persian Gulf). Temperatures can reach as high as 102 degrees Fahrenheit, especially along the coast, causing danger of heat exhaustion.  Here is a general temperature graph for Iran. Note that to go from degrees celsius to fahrenheit, a rough conversion to use is double the number and add thirty. Or use this website.