The USA is huge, and the best time to travel depends heavily on the region. A few rules of thumb make planning easier.
The Southwest
For the Southwest (Grand Canyon, Bryce, Zion, Monument Valley), spring and fall are ideal: pleasant temperatures, clear air, and fewer crowds than in peak summer. Midsummer is extremely hot in the deserts, temperatures above 100 degrees Fahrenheit are common, so get out early in the day.
High Elevations and the North
High elevation parks like Bryce Canyon stay pleasantly cool even in summer, but can be partly snow covered in winter. The North Rim of the Grand Canyon and many mountain roads are only open from about May to October.
California and the Coast
The coast is mild year round. San Francisco can be foggy and cool in summer, while the south around Los Angeles is almost always sunny.
Plan for Crowds
Summer vacation and holidays like the Fourth of July are the busiest and most expensive times. If you have flexibility, travel in May, September, or October for the best balance of weather, prices, and quiet.
