I never know how to dress for a plane trip. No matter how warm it might be where I board the plane, it always seems chilly inside the cabin. As I was shivering on a recent flight - they were all out of blankets - I got to thinking about how the cabins of airplanes are cooled. It's not as simple as you might think.

Let's take as an example a commercial aircraft flying at 30,000 feet. To keep the air fresh inside the cabin, air from outside the plane is brought inside. But air at that altitude has only about thirty percent of the pressure at sea level, so the air has to be pressurized before it's vented to passengers. Now when air is pressurized, it heats up, which is a good thing in this case because air at 30,000 feet is very, very cold to start with, perhaps -40^oF. It turns out, however, that once the pressure is high enough for the air to be breathable, the air actually has heated up too much. As a result, the pressurized air then has to be cooled before being vented to passengers.

So it may seem strange, but it's true: though the air outside a high-flying aircraft is brutally cold, it still must be air conditioned before you breathe it in the cabin of the plane.