Picture this, April 28 -- 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.

Here's why airplanes fly at a cruising altitude of 30,000 to 42,000 feet

Airplane flying a...