China is a vast country with wide-ranging climatic conditions, so a decision about the best time to visit should be based on the regions you plan to tour and the kind of weather you enjoy.
Autumn
Normally, the most comfortable season of the year is early autumn (September to early October). During that period, temperatures are reasonable throughout China (about 50-72F) with a limited amount of rain. September for example is the only month in the year when the ancient and valuable paintings of the Beijing Palace Museum are displayed due to proper climate conditions.
Spring
Spring can also be delightful with the average temperatures roughly the same as in autumn (about 50-72F). The best way to deal with weather unpredictability is to wear layered clothing that will make you comfortable in both chilly and warm weather.
Summer
Summer (from June to the end of August) can be extremely hot with temperatures well above 72F, especially in the famous 'four furnaces' of China: Wuhan, Tianjin, Chongqing and Nanchang.
Summer is also a rainy season, so travelers should not forget umbrellas, light raincoats and rubber/plastic shoes.
winter
Winter can be incredibly cold especially in the north. Off season travel can also offer its rewards. For example, the Harbin Winter Ice Lantern Festival is quite charming.
Before deciding when to take a tour, it is worth checking carefully the China weather conditions of each city on the itinerary. Needless to say that the most attractive seasons to visit are also the most crowded. Regardless of the time of the year, a visitor will always be rewarded with charming scenery throughout China.
