The cold isn't why your flight is cancelled.

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With much of the country in the midst of a deep freeze, travelers may be wondering how cold it has to be for the severe weather to foil upcoming travel plans. But it isn't the cold that's a problem for most aircraft.

After all, commercial airplanes can cruise at an altitude of nearly 40,000 feet, where temperatures hover around -70 degrees Fahrenheit.

Jet fuel freezes at around -40 degrees Fahrenheit, but it will work just fine as long as it’s kept above that temperature on the ground. Once the plane is moving, the fuel is heated as it passes through the engine — and crew members carefully monitor fuel temperatures during flight.

Related: What Should I Do if My Flight is Canceled Due to Weather?

In fact, airplanes are more efficient in low temperatures, as cold air is denser than warm air. That typically means shorter, faster takeoffs and better performance. There is one major challenge with cold weather, however, and that's the accompanying ice and snow.

“If ice or snow builds [on] critical surfaces, it will change the pattern of airflow over those surfaces,” Seth Laskin, a former aircraft deicer at Philadelphia International Airport, told Travel + Leisure.

“Wings, for instance,” Laskin said, “are very specific shapes — and even a little bit of frost can decrease the efficiency of the wing, leading to longer takeoff rolls and a higher stall speed.”

That’s why airplanes must be carefully cleaned and protected during inclement winter weather. In the event of a snowstorm, for example, deicers can treat an airplane’s wings with a special, heated anti-freeze mixture.

Related: How Airplanes Stay in the Air

After a wing is clear of accumulation, it can be sprayed with another anti-freeze product designed to sheer off at take-off — and in the skies, jet wings are heated with bleed air from the engines.

At that point, Laskin noted, snow won’t accumulate because of the speed.

While small planes, like private jets, may not be equipped to deal with extreme icing conditions, commercial jets can typically fly fast enough to avoid build-up during flight. Certain aircraft may also be outfitted with anti-icing features like heated leading edges on the wings and pores that secrete glycol.

Of course, cold and wet winter weather can still lead to flight delays and cancellations.

Even though airplanes can fly just fine when they're properly deiced, that process can take some time (sometimes an hour or more, depending on the size of the aircraft and rate of accumulation). And severe weather can also impact a number of other critical factors at the airport, like tarmac conditions and air traffic control visibility.

WATCH: Homemade Cold Remedy

As Winter Storm Grayson brings blizzard conditions across the Northeast, more than 3,000 flights have already been cancelled. As the Chicago Tribune reported, severe weather can keep ground crew, like baggage handlers and ramp workers, from spending more than 15 minutes at a time outside, and the equipment used to pump jet fuel can freeze, leading to delays in refueling aircraft.

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