Mosul has a hot semi-arid climate with very hot summers and mild winters. The best times to visit are spring (March to May), when temperatures are pleasant and wildflowers cover the Nineveh plains, and autumn (September to November), when the heat breaks and the days are clear and warm. Summer temperatures regularly exceed 40°C and can spike above 45°C in July and August — heat that makes outdoor exploration of archaeological sites and the Old City lanes genuinely gruelling.
Winters are mild by Central Asian standards: Mosul rarely freezes, though rain is common between December and February. The city lies far enough north that the winters are noticeably cooler than Baghdad. At any time of year, check the current security situation before planning a visit. Mosul is gradually returning to normal life and receives domestic visitors and foreign journalists, but the security context remains different from a typical tourist destination and the advice of your country's foreign ministry is worth reading carefully.