Why is it Cold Outside the Aircraft but Not Inside?

Flying can be a fascinating experience, filled with unique phenomena that pique our curiosity. One common question travelers often ask is, why does it feel colder outside the aircraft than inside during flight?

Examples and Initial Impressions

Imagine boarding an airplane in Phoenix, Arizona, in the scorching summer of July. The ambient temperature outside might reach a blazing 100 degrees Fahrenheit (38 degrees Celsius). Most airlines use ground-based air conditioning units to ensure the cabin remains comfortable, typically around 80 degrees Fahrenheit (27 degrees Celsius). Once you're seated, the doors are closed, and the journey begins.

As the plane takes off and climbs to cruising altitude, a dramatic change occurs. The outside temperature drops dramatically. Even in summer months, it can plummet to well below -40 degrees Fahrenheit (-40 degrees Celsius) or even lower. Pilots carefully monitor these temperature changes to maintain a comfortable cabin environment. If the cabin feels a bit chilly, a flight attendant may ask the pilots to warm things up a bit. Yet, the outside temperature remains drastically colder, even in smaller private aircraft like Cessnas or Piper Cherokees.

When you land at your destination, the outside temperature might be much warmer or significantly colder, depending on the season. For instance, arriving in Buffalo, Minneapolis, or Albany in the dead of winter, the outside temperature can be much lower than the cozy cabin.

Elevation and Its Impact on Temperature

The key factor behind this phenomenon is elevation. As altitude increases, the temperature drops. This is why mountain regions with high elevations often experience snow well into the summer months, even where the base of the mountain remains warm.

The Science Behind Aircraft Temperature Control

Aircraft use a complex system of air circulation and conditioning to maintain a comfortable cabin environment. Here’s how it works:

Bleed Air: This is a key component of the pressurization and temperature control system. Engines compress intake air for combustion; some of this air is diverted and compressed, raising its temperature. This hot air is then passed through an air cycle machine (ACM), where it is decompressed, cooling it down. The cooled air then enters a mixing valve, where it is blended with pre-Air Cycle Machine (pre-ACM) hot air. Mixing Valves: On a hot day, the mix is mostly after-ACM cold air. On a cold day or at altitude, it is mostly pre-ACM hot air. Outflow Valve: This valve regulates the amount of air flowing overboard, maintaining cabin pressure and ensuring a constant flow of fresh air. It is never fully closed to prevent a buildup of stale air.

Understanding these principles helps explain why the cabin temperature can be regulated to maintain a comfortable environment, even as the outside temperature fluctuates dramatically.

Conclusion

Flying is an intricate dance of science and design. The temperature discrepancy between the inside and outside of an aircraft is just one of many fascinating phenomena you might observe during your journey. By understanding how the bleed air and cabin pressurization work, you can appreciate the finely tuned environment that makes air travel such a comfortable and safe experience.