Semnan has a dry continental climate: hot summers, cold winters, and a big difference between day and night. Spring and autumn are the easiest seasons for walking the old centre and making side trips into the foothills.
Summer heat can be draining, especially if you continue toward the desert. Winter can be sharp, but clear days are beautiful and the Alborz mountains north of town may be snow-covered. Bring layers if you plan to combine Semnan with Mahdishar or Shahmirzad.