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Comment by dghughes | Hacker News Reader

I see people talking about this but they all seem to have a hard time understanding air speed and ground speed.

The way I like to explain it is imagine a person walking up an escalator. Your total speed is your walking speed (air speed) plus the escalator speed combined (ground speed). Just walking up regular stairs would be like an aircraft flying with no wind.

It can also happen in reverse. If you walked down the up escalator your walking speed (air speed) may be equal to the escalator. Your total speed (ground speed) may even be zero. Aircraft leaving London going to NY were fighting the wind their ground speed was reduced.

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Also why it's still a subsonic flight even with a ground speed greater than the speed of sound (~1200 km/h).

Edit: as explained in the article, hum.

Speed of sound is 767 mph. Without the wind, they were at 801 mph, which is still well over the sound barrier. I still don't understand.

They weren't at 801mph airspeed. The article didn't make it clear exactly, but:

"The Boeing 747-436 reached speeds of 825 mph (1,327 km/h) as it rode a jet stream accelerated by Storm Ciara."

Ie max ground speed 1327km/h. And:

"The jet stream reached speeds of 260 mph (418 km/h) on Sunday morning, according to BBC Weather."

So if their max ground speed coincides with max wind speed and the wind was perfectly tail, their air speed would have been closer to 909km/hour.

The article says it's "slower than 801 mph", but sadly doesn't specify how much slower. The ground speed was 825 mph, so if Ciara is so impressive, I'd expect it to give a bigger boost than a measly 24 mph. The article doesn't specify the unassisted airspeed, unfortunately. Considering the jet stream was moving at 260 mph, naive math would suggest the air speed was 565 mph.

...which is about the commercial cruise speed of a Boeing 747 (570 mph), so your naive math may be pretty close ;)

As an aside, the speed of sound varies depending on pressure and temperature. At the cruising height of an airliner it's typically significantly less than the speed at 0m/20 degrees. In fact at 35,000 feet it's usually closer to 660 mph.

When you get close to the speed of sound in air, all sorts of weird aerodynamic effects happen. Because it varies, the term "mach" is used to denote 1x the speed of sound (regardless of what the pressure or temperature is) - so an aircraft flying at mach 0.85 is at 85% the speed of sound for that height and temperature.

The only thing that matters to an aircraft at altitude is the airspeed.

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