For many pilots, summer means fly-ins, more flying and searches for the best $100 hamburger. It also marks the end of powerful jet streams and large, organized weather systems that cross the United States from one end to the other. By the time June rolls around, all that stuff shifts north and becomes a problem for our Canadian friends. But, that also gives them four months of mild summer temperatures, so dont feel too bad for them.So what exactly do we have in the United States? We can distill it down to three main features. First theres the large Atlantic Bermuda high, covering the eastern half of the country. Second is the cool Pacific high, which maintains a tenacious foothold on the West Coast. Finally theres the broad thermal low located between the Sierra Nevadas and Rocky Mountains, strongest by far over Arizona. Its driven mostly by intense solar heating.