Journeywoman went to her travel library to see what guidebooks say about the weather in China. The following are excerpts from three guides that we use a lot here in the office, Traveler's Weather Guide, China Guide and The Treasures and Pleasures of China.

China Guide
"The best weather is May-June and September-October, but you could run into crowds and delays. You could also try late April. In November, go to Beijing first and then move south. Guangzhou, Guilin, Hong Kong and Hainan are wonderful in November and December."
(China Guide, Ruth Lor Malloy)

The Treasures and Pleasures of China
"Given the vastness of China, its seasons and climates vary depending on when and where you visit China. In general, we like to compare China to the United States and much of Europe. It's hot, it's cold, it's rainy, it's dry depending on where and when you visit the country. China experiences extreme cold and heat. Although there is no one ideal time to visit China, Spring and Fall, especially the months of April and October, are usually good times to visit."
(The Treasures and Pleasures of China, Ron and Caryl Krannich)

Traveler's Weather Guide
China is uniformly warm, humid, and rainy in the summer. In contrast, winter weather can vary considerably from city to city. Beijing, for example is very cold in the winter, with afternoon highs averaging only 34 degrees. Shanghai's January high is only 46. Hong Kong's tropical climate keeps it mild even in January, where highs are in the 60s. Rainfall in China is concentrated primarily in the summer months - July in particular should be avoided due to its high heat, humidity, and generous rainfall.

The best times to visit Beijing are in the early Fall and late Spring, especially May and September, with April and October almost as pleasant. Humidity, however, can be high from June to September. Shanghai is very pleasant in October and May.
(Traveler's Weather Guide, Tom Loffman)