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Comment by BonesJustice | Hacker News Reader

I’m a bit under the weather, so my comprehension may be lacking, but I think you’ve got your signs reversed (or your head/tail terminology flipped).

If tail wind (moving with the plane) were negative, then air speed would exceed ground speed. Intuitively, that doesn’t make sense. Air speed can only exceed ground speed if the wind is blowing in an opposing direction, i.e., the plane must travel faster against the wind to achieve the same speed relative to a ground observer.

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Wind is "named" after where it comes from. If you fly north, and the wind is a north wind, you fly slower. (This terminology is in line with what GP said.)

Then, ground speed = air speed - wind speed.

Thus, GP seems to have either the sign wrong, or ground and air speed mixed up (but not both :-)

> Wind is "named" after where it comes from.

This is absolutely true, but has always infuriated me because it seems needlessly contrary as compared to the way we speak about anything else travelling in a direction as read from a compass. Does anybody know why it's the case?

Probably because it matters where the air was before. Stuff like: north wind is could, south wind is warm, west wind is humid, etc

Doesn't that very much depend on where you are though? E.g., with respect to the equator, major bodies of water and landmasses, mountain ranges, prevailing weather conditions. Also perhaps on seasons.

I can understand that it might be just an ancient tradition originating from a particular part of the world, that we've turned into a convention, but it loses meaning when you go elsewhere.

It rather depends on where you are, but so does language. The words we use were cast locally. If you speak English some words were cast in the Ukraine, some in Germany, some in Rome, some in France, some in Britain, but the people living in those places weren't obliged to make the words globally meaningful.

If you go to Cape Town, a "north wind" presumably doesn't mean cold, because the area north of the city isn't colder than the city itself. This didn't matter to the people who first said "north wind" and I don't think it means much nowadays either. Just another little thing to watch out for in intercultural communication.

Can you name anything else named after where it's going, not where it comes from? I can't think of anything named like that, but I guess there must be some.

Within a marine context, current is typically given as direction to while wind and waves are direction from.

Or when you're out walking you might say that you're "heading north". Granted if you wanted to report your rough position relative to someone else you might say something like, "I'm 500m north of you."

No you are right. In case of tailwind, it is KIAS + WS whereas headwind is KIAS - WS

KIAS: Knots Indicated Airspeed (i.e. the speed at which air is hitting the pitot tube) WS: Wind speed

Slight, but important (if you don't want to run out of fuel) nit, indicated airspeed (KIAS) is not the same as true airspeed (KTAS).

To calculate ground speed (as required for navigation and fuel planning) you need true airspeed (not indicated) as well as wind direction and speed.

See some discussion here: https://www.quora.com/In-aviation-what-is-the-difference-bet...

Not so slight. At airliner altitudes they can differ by 50% or so.

IAS is what the pilots care about while flying because it’s what matters for aerodynamics (ie stall speed)

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