Termez is one of the hottest cities in Central Asia. Summer temperatures regularly reach 45–50°C, and spending long periods outdoors at archaeological sites in July or August is genuinely dangerous. The only sensible travel windows are spring (March–April) and autumn (September–October), when temperatures are warm but manageable and the light on the ancient sites is superb.
Winter brings cold nights and unpredictable weather but rarely the extreme cold of northern Uzbekistan. If you are combining Termez with Samarkand or Bukhara on a broader Uzbekistan trip, the spring circuit makes the most sense.