# Why does air move with the planet Earth **URL:** \ **Category:** Factual Questions\ **Created:** [November 12, 2021, 11:54pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260 "2021-11-12T23:54:22Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [November 13, 2021, 1:52pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/21 "2021-11-13T13:52:46Z")
> [@DesertDog](#): > > Actually, the critical word in **L\_J** ’s thesis is “accelerate.” Let me put this highjack (that admittedly I started), to rest. I don’t have a thesis. I asked a question in the OP. My post about acceleration was my crude questioning of Si\_Amigo’s assertion that gravity alone is enough to cause the atmosphere to rotate at the same rate as the planet. I don’t believe the planet is accelerating (as per my flatbed example) nor was that part of my original question. And I do readily admit I might be wrong about that as well. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 13, 2021, 5:06pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/22 "2021-11-13T17:06:46Z")
Really, the answer is the same as it is for why the Earth is rotating as a more or less single object instead of a sloshing mess of material. The tensile strength of the Earth’s crust would not be enough to keep it together as one piece. The Earth, as a system, comprising molten minerals, a crust of solidified minerals, surface water, loose surface materials, and the atmosphere, is rotating together, as a system. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [November 14, 2021, 2:06am UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/23 "2021-11-14T02:06:36Z")
Okay, look at it this way: Let’s say you start witt an Earth that isn’t spinning, then magically you rotate the planet to the speed it is rotating now. What happens to the air? Well, the air molecules interacting with the ground will start to move with it because of friction. So now you have a layer of air moving with the Earth. But that air is in contact with air molecules above it, and they eventually drag allng with the Earth. Repeat until the whole atmosphere is moving with the Earth. How can it be otherwise? How could you have some air standing still while lower air races by at hundreds of miles per hour? Given enough time, the entire atmosphere rotates with the Earth. Friction explajns all. Throwing a ball up and catching it doesn’t prove anything about rotation because the scale is too small, and because the friction with the atmosphere predominates. But if you do it at a large enough scale, you will in fact see the ball’s path curve because of the Earth’s rotation. This is known as the Coriolis effect. Fun fact: long distance shooters have to account for the coriolis effect all the time. And it’s complicated, because the size of the effect depends on the direction you are shooting and where you are on the planet. For example, in the northern hemisphere if you shoot north, the target has a lower rotational speed than you do, so your bullet will appear to curve to the right. Shoot south, and it appears to curve left. Some people call this the ‘coriolis force’, but there’s no force acting on the bullet - it’s traveling straight and the Earth moves under it, making it appear to curve. If you shoot east or west, there is no coriolis effect, but if you shoot in the direction of rotation the bullet will hit low because the target moved away as the bullet was teavelling towards it, so the bullet’s path is longer. And obviously if you shoot against the direction of rotation the bulket will hit high as the target is moving towards the bullet as it flies, reducing bullet drop due to gravity. Snipers who shoot more than 1,000 yards can miss a man-sized target due to the coriolis effect. Note that this has nothing to do with wind or friction, though. But both of those have more effect on the bullet. As an aside, It seems to me that with four long distance shots, two from each of two different latitudes, you should be able to calculate both the size of the Earth and its rotational speed. ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [November 14, 2021, 3:59pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/24 "2021-11-14T15:59:52Z")
> [@Mean\_Mr.Mustard](#): > > A slightly older, slightly wiser me decided that the earth’s movement would have been too subtle to demonstrate any effect of 10-year-old Mean Kid Mustard tossing a rubber ball five feet in the air. Also yes. There are a lot of forces acting…gravity, centrifugal (yes) acceleration, the movement of the air (driven by temperature differences and following dynamics laws such as coriolis acceleration)…but they are slow acting and changing on a long-ish time scale, and sometimes cancel each other out. You’d need very precise measurements to see these in action, even in a model rocket flying hundreds of feet into the air. Even then, the big forces such as gravity and the wind would dominate the results. Basically, we can get away sometimes with calling the surface of the earth an “intertial” frame, i.e. it’s “standing still.” Of course it isn’t, but the effects of its motion are pretty small. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 14, 2021, 4:47pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/25 "2021-11-14T16:47:28Z")
> [@Chronos](#): > > Of course the friction is tiny, but so what? That just means that it would take a long time for the air to (mostly) come to rest with respect to the surface… Maybe as long as a day or so? > > But it’s had four billion years. Even with only a very tiny amount of friction, that’s plenty. To expand on this, consider the motion of something on the Earth’s surface. The motion can be split into two components, horizontal and vertical. It’s moving at a _constant_ speed horizontally (tangential to the surface). Whereas In the vertical direction it is constantly _accelerating_ toward the center of the Earth to bend that horizontal motion around into a circle. Friction is only responsible for the horizontal component. Since the horizontal motion is at a constant speed, not accelerated, as @Chronos says friction can take as long as needed to bring a hypothetical “stationary” object on the surface “up to speed” - once it gets up to speed, it will just continue moving at the same horizontal speed as the surface. So there’s no requirement that friction be a _large_ force. It’s the vertical component that requires a constant large force to stop an object sitting on the surface from flying off in a straight (horizontal, tangential) line into space. And that constant vertical force is supplied by gravity. In fact, this relates directly to the way orbits work. An object in a stable orbit has a horizontal speed that is exactly sufficient that when gravity pulls the object down, it constantly “falls” just the right amount to bend the horizontal motion around into a circular path. And a stable orbit near the surface of the Earth (ignoring air friction) would need to be much faster than the horizontal speed of the surface of the Earth. That tells you that for an object sitting on the surface, moving at the same speed as the surface, the force of gravity is more than sufficient to keep that object sitting on the surface. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [November 15, 2021, 5:23am UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/26 "2021-11-15T05:23:58Z")
That’s why, if you’ve watched a rocket launch, it might – especially the big ones – start out vertically but soon pitches over to more or less horizontal. [![](https://img.youtube.com/vi/zsJpUCWfyPE/hqdefault.jpg "STS-129 HD Launch") ](https://www.youtube.com/watch?v=zsJpUCWfyPE) Not how the altitude and down range callouts pretty well match at first, but the former gets left behind pretty quick. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [November 15, 2021, 6:49am UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/27 "2021-11-15T06:49:59Z")
> [@DesertDog](#): > > That’s why, if you’ve watched a rocket launch, it might – especially the big ones – start out vertically but soon pitches over to more or less horizontal. Right, the launch vehicle for a spacecraft or satellite has to do a lot more work to attain the necessary speed for a typical low earth orbit than to attain the necessary altitude. It’s also why suborbital flights, like the first phase of Jeff Bezos’ Blue Origin venture, can be accomplished with much lower power launch vehicles, and they attain a significant fraction of the altitude of a typical LEO satellite but only about 8% of its velocity. The Mercury-Redstone launch vehicle that launched Alan Shepard into suborbital flight produced 350 kN of thrust; the Mercury-Atlas (Atlas LV-3B) that launched John Glenn into orbit in essentially the same capsule produced a maximum 1,517 kN of thrust. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 15, 2021, 12:10pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/28 "2021-11-15T12:10:25Z")
> [@Mean\_Mr.Mustard](#): > > I actually tried the ball-tossing experiment as a young-un. I was disappointed to find that the ball dropped neatly back into my hand. > > A slightly older, slightly wiser me decided that the earth’s movement would have been too subtle to demonstrate any effect of 10-year-old Mean Kid Mustard tossing a rubber ball five feet in the air. > > That still seems plausible to me. Is it correct? > > mmm I think that if the earth were perfectly flat, and moving at a constant speed (or not moving at all), then a ball launched vertically would land at its launch point no matter how high you launched it. However, for a spherical earth that rotates, and assuming no atmosphere at all, and further assuming a truly vertical launch vector - that is, the ball is confirmed to be moving precisely vertically at the moment it leaves the ground - I don’t think a ball can land exactly at its launch point. The higher you launch the ball, the larger the discrepancy becomes. This can be illustrated with an extreme case: imagine for example launching a ball vertically from the equator with so much speed that it takes 12 hours to fall back to earth. During its flight, the ball is on an elliptical orbit that passes through the earth; if the minor axis of this orbit is small enough, then 12 hours after departure the ball will hit the earth less than 90 degrees east of its original departure point (while the launch point will have moved 180 degrees around to the far side of the earth from where it was). If the minor axis of the ball’s orbit is large enough, then 24 hours after departure it will impact the earth 180 degrees from where it was launched (while the launch point will have completed one full spin of the earth and returned to its original position). ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 15, 2021, 12:29pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/29 "2021-11-15T12:29:47Z")
> [@Machine\_Elf](#): > > However, for a spherical earth that rotates… Aren’t you just describing an extreme example of the Coriolis effect? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 15, 2021, 12:52pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/30 "2021-11-15T12:52:06Z")
> [@Riemann](#): > > Aren’t you just describing an extreme example of the Coriolis effect? It turns out yes, a particular case of it for vertically launched objects: > **[Coriolis force | Eötvös effect](https://en.wikipedia.org/wiki/Coriolis_force#E%C3%B6tv%C3%B6s_effect)** > > The practical impact of the "Coriolis effect" is mostly caused by the horizontal acceleration component produced by horizontal motion. > There are other components of the Coriolis effect. Westward-traveling objects are deflected downwards, while eastward-traveling objects are deflected upwards. This is known as the Eötvös effect. This aspect of the Coriolis effect is greatest near the equator. The force produced by the Eötvös effect is similar to the horizontal component, but the much larger verti... > …objects traveling upwards (i.e. _out_ ) or downwards (i.e. _in_ ) are deflected to the west or east respectively. This effect is also the greatest near the equator. Since vertical movement is usually of limited extent and duration, the size of the effect is smaller and requires precise instruments to detect. Whatever you want to call it, the slightly older, slightly wiser version of the young @Mean_Mr.Mustard was right: > [@Mean\_Mr.Mustard](#): > > A slightly older, slightly wiser me decided that the earth’s movement would have been too subtle to demonstrate any effect of 10-year-old Mean Kid Mustard tossing a rubber ball five feet in the air. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 15, 2021, 9:45pm UTC](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260/31 "2021-11-15T21:45:12Z")
> [@Machine\_Elf](#): > > However, for a spherical earth that rotates, and assuming no atmosphere at all, and further assuming a truly vertical launch vector - that is, the ball is confirmed to be moving precisely vertically at the moment it leaves the ground - I don’t think a ball can land exactly at its launch point. Strictly speaking, it can, but it’d have to go clear around the whole planet to do it. [Previous page](https://boards.straightdope.com/t/why-does-air-move-with-the-planet-earth/954260.md?page=1)