It is the hottest period in China. Jiuzhaigou will be cool and may not feel easy on the way. Visitors will be more since late July. You may go earlier that month. Jiuzhaigou is much cooler than Hangzhou, especially at night. The best way to Jiuzhaigou is by air. Jiuzhaigou is beautiful all year around, I think, well just cold in winter. The point is July is hot to visit Hangzhou.