1.) It is my understanding that all pilots must set their altimeters to 29.92 (pressure altitude) above 18,000 feet. However, as planes in Class A airspace fly into different areas of pressure, the altimeters should change (read higher or lower) depending on the pressure of that area. Therefore, are planes usually constantly ascending or descending as they move into different areas of pressure above 18,000 feet?
2.) Why was 18,000 feet chosen as the floor Class A airspace?