Does it matter what time of day you fly?

How your flight selection impacts your carbon footprint.

Oct 13, 2021

If you’ve looked at flights on Google recently, you may have noticed a feature that they recently rolled out: flight emissions. This new feature gives you an estimate of how much carbon your seat on the flight emits. It’s a nifty way to feel bad about taking a flight—don’t forget that it’s way better than taking a car—but it’s also a great way to compare and select flights that emit less than others. [1]

This flight, for example, produces 9% fewer emissions than similar trips between the same destinations. This number is by no means exact because there are infinite weather, air traffic control, and other effects that you can’t account for. But there are pretty good ways to get close. Google uses an algorithm that considers typical flight plans, airplane models, and time of year, among others, to predict emissions. The first thing that I assumed was that all connecting trips will produce more carbon than direct. In general, this is true but not always. This was a very interesting result to me. If you have to take off, climb, and land twice, you definitely would be emitting more. And that’s true, but on an aircraft level. Remember that airline emissions are calculated per passenger. So, if you’re connecting between a large hub, during peak travel hours, your two connecting flights are likely to be more full than your direct flight. The two flights would produce more total emissions, but because there are more people on the flights, it can offset. And if you do a quick search, it sometimes does.

But, alas, we must get to the point of the article: does it matter what time of day you pick your flight? The answer is actually yes, for a few reasons. Doing a quick search of flights between JFK and Boston, we see that the same flights, which last pretty much the same amount of time, produce different levels of emissions. Right off the bat in this example, upon expanding the flight info, we see that they’re on different aircraft. The top and bottom flights are on an Embraer 190, and the middle flight is on an Airbus A321. [1]

This aircraft difference explains the emissions difference immediately. Not because the A321 is more efficient than the E190, but because it carries twice as many people. The bigger A321 burns more fuel over the flight, but because it carries more people, the emissions per seat is less. All else constant, this trend generally holds.

That wasn’t a very interesting example, I want to get deep into the weeds of what goes into this, so let’s try another. Think of your favorite city in your head. Go ahead, take your time. Okay, got it? Great. Was it Newark? Obviously not, but we’re going to look at it anyway. Newark to Chicago is a pretty popular route, with dozens of flights back and forth each day. I did this search and checked the results and, sadly, the only differences are due to different aircraft. Sorry, no more screenshots.

Now I won’t argue too much with that, as differences in aircraft models will drive the largest difference in emissions, but there are some other factors in play. The first is air temperature. In general, hot air makes planes more efficient. One of the most important factors in engine efficiency is the temperature of the air inside. The hotter it is, the better the engine runs. (From an aerodynamic perspective, don’t tell any materials engineers that I said that) Additionally, the higher temperature, less dense air, means less drag felt by the aircraft. This makes it easier to move through the air so it burns less fuel. This means the most fuel-efficient flights will be when it’s hottest outside—flying in the afternoon is better than flying at night.

Before you go using all your PTO to travel, this result only holds for the portions of the flight when you’re at a low altitude. Up at a cruising altitude of 35,000 feet, the temperature is largely constant regardless of the time of day. This will therefore be most important on shorter flights, where takeoff, climb, and landing make up a larger portion of the overall flight.

Another con for midday flights is air traffic management. More people flying means more planes active at the airport. The more planes trying to take off, the more you’re going to have to wait on the ground for your turn. Especially on weekdays, most travel tends to happen during business hours. [2] Therefore, during these times, you would expect to be on the ground longer, burning fuel to keep the engines running at idle. Again, not a huge impact, but a considerable one.

So now we’ve come to a confusing conclusion—sometimes it’s better to fly at night but sometimes it’s better to fly during the day but it also depends on the route and the aircraft and weather conditions and… Before my head explodes let’s attempt to summarize: on weekdays, shoot for an early evening flight. This will maximize the air temperature while avoiding midday crowds. On weekends, opt for an afternoon flight to cash in on that good ole afternoon heat. A big, packed plane is better than a small empty one, opt for direct flights when you can (you already knew that), and try your best to consider the Google Flights emission ratings where you can.

Thanks for reading this week’s edition of It’s Not Rocket Science! I hope you’re having a wonderful Wednesday and I’ll see you next week.

It's Not Rocket Science

Discussion about this post