April–May and September–October are the only comfortable months. Spring brings wildflowers and is the best time for Merv and Gonur Depe; autumn is dry and clear. Daytime temperatures sit in the 20s°C, evenings are cool.
Summer is punishing — daytime temperatures regularly reach the high 40s°C with no shade on the Merv site. Visiting the ruins in July or August is, in a real sense, dangerous; if you have no choice, start at first light and be off the site by ten. Winter is cold, with snow possible, and the surrounding desert can be bitter. Most tour operators run April through October.