Understand your flight

Why do planes fly so high?

Jets cruise miles above the ground mainly because the cold air up there makes their engines more efficient. Inside, the cabin is pressurized to feel like a mountain town.

4 official sources, listed at the end Checked October 18, 2026
Photo: Harry Knight harryknight, CC0
In this article · 6 sections
  1. The short answer
  2. Higher is colder, and colder helps
  3. Weather still matters up high
  4. Why you can breathe normally
  5. Why your flight changes altitude
  6. Keep reading

The short answer

Jets fly high mainly because it's efficient. The FAA's handbook for pilots moving onto jets puts it plainly: the efficiency of a jet engine increases in the cold temperatures found at high altitude, and by flying high the airplane gets better fuel economy and speed.1

Higher is colder, and colder helps

The air several miles up is very cold. For a given engine speed and true airspeed, a jet engine burns less fuel as the outside air gets colder.1 That's a big reason airlines cruise high.

The handbook adds the trade-off pilots manage: at high altitude the engines may be near their limits and there's less spare thrust, so pilots plan maneuvers within what's available.1

Weather still matters up high

Altitude doesn't make weather disappear. Thunderstorms in particular are something pilots plan around, not through.4 For bumps in clear air, US forecasters publish turbulence forecasts at many levels, from low altitudes up to about 48,000 feet,3 and pilots use those, and reports from other aircraft, to pick a smoother level (see How do pilots know where turbulence is?).

Why you can breathe normally

At cruise, the outside air is far too thin to breathe comfortably. The cabin is pressurized, and US design rules require a cabin altitude of no more than 8,000 feet under normal operating conditions.2 That's similar to being in a mountain town. It's why your ears pop on the way up and down (see Why do my ears pop?).

Why your flight changes altitude

You may feel the airplane climb in steps or change level mid-flight. Pilots and air traffic control adjust altitude for other traffic, winds and smoother air, and the best level can change as the flight goes on. These are routine adjustments.

Keep reading

Quick answers

Why do planes fly at 35,000 feet?

Jet engines become more efficient in the cold temperatures at high altitude, so flying high gives better fuel economy and speed. The exact altitude for each flight depends on weight, winds, weather and air traffic control.

Is it dangerous to fly so high?

No. Airliners are designed for it, and the cabin is pressurized: under normal conditions, US design rules require a cabin altitude of no more than 8,000 feet, however high the airplane is flying.

Is the air smoother higher up?

Not automatically. Turbulence is forecast at many levels up to around 48,000 feet, and pilots use those forecasts and other pilots' reports to choose a smoother altitude.

Why does the plane change altitude during the flight?

Pilots and controllers adjust altitude for traffic, winds and smoother air, and the best level can change during the flight. Changing levels mid-flight is routine.

Sources

(4)
  1. FAA Airplane Flying Handbook (FAA-H-8083-3C), Chapter 16: Transition to Jet-Powered Airplanes
  2. 14 CFR § 25.841, Pressurized cabins
  3. NWS Aviation Weather Center, GFA help (turbulence forecast levels)
  4. FAA Aeronautical Information Manual, 7-1-27 Thunderstorm flying

Facts last checked against these sources: October 18, 2026.