Picture this: It's about 20 minutes after takeoff, and you hear your captain make an announcement. "Ladies and gentlemen, we've reached our cruising altitude of 35,000 feet," they say. Or, maybe they say 36,000 feet, or 40,000 feet, or 33,000 feet. Cruising altitude isn't actually a fixed number —there are many factors that go into computing this for every flight.
That said, planes generally fly at higher altitudes —usually between 30,000 to 42,000 feet —because the air gets thinner the higher you go. When the air is thinner, planes can fly faster and more efficiently, using less fuel to maintain the speed required to develop lift.
However, the air must not be too thin. "Most commercial airliners have what we call a 'service ceiling' of up to 45,000 feet," says former pilot Dan Bubb, now a professor at the University of Nevada, Las Vegas. "The service ceiling is the point where the airplane can no longer develop lift, and gravity begins pulling it down to the ground."
Additionally, flying at higher altitudes allows planes to avoid most weather issues during the cruise phase of flight. Plus, there tends to be less turbulence at higher altitudes, according to NASA.
(An aside —flying at these high altitudes is why airplane cabins are pressurized. Even though the planes thrive in thinner air, the human body does not, so cabins are pressurized to a more comfortable level for the sake of their occupants.)
As for determining the specific cruising altitude for each commercial flight, aircraft dispatchers select the optimal flight path preflight. To do so, they consider factors like the specific aircraft and its weight, the wind and weather conditions, air traffic, and even the direction of the fight.
"Generally speaking, aircraft flying south, southwest, west, and northwest must be at an even altitude, like 36,000 feet. Aircraft flying north, northeast, east, and southeast must fly at an odd altitude, like 37,000 feet," says Bubb. "This enables air traffic controllers to safely space aircraft flying at different altitudes." Aircraft are allowed to pass within 1,000 vertical feet of one another, which is one of the reasons why planes stick to 1,000-foot increments for cruising altitude.
Then, once in-flight, the cruising altitude can change as conditions shift. For instance, if the pilots in one aircraft report turbulence at a specific altitude, those flying behind that plane may adjust their altitude to "find clear air," as you might hear over the loudspeaker.
The bottom line: As it goes with all things air travel, flying at a specific cruising altitude has everything to do with efficiency and safety.
