The Concorde's typical cruising altitude was between 50,000 and 60,000 feet (approx. 15,000 to 18,000 meters). At this height, the aircraft flew well above the "weather" and the subsonic traffic that cruises between 30,000 and 40,000 feet. From this altitude, passengers could clearly see the curvature of the Earth against the blackness of the space-edge sky. Cruising at Mach 2.04, the aircraft actually "grew" in length by about 6 to 10 inches during flight due to the intense kinetic heating of the airframe. Flying so high was not just for speed; the thin air at 60,000 feet was necessary to reduce drag, allowing the four Rolls-Royce/Snecma Olympus engines to maintain supersonic speeds efficiently while providing a remarkably smooth ride for the 100 passengers on board.