“Cod” means “mock” in the UK, as in “cod-Latin” being “mock Latin” (like spells in Harry Potter"), etc.
Or for that matter, being crushed while remaining in a centrifuge.
It was at the county fair, brat kids started a rock fight on the merry-go-round …
Here is an excellent video that talks about the Earth’s global atmospheric circulation. It starts very basic, the 6:50 mark starts talking about Coriolis, but the good part kicks in around the 9:30 mark. Watch there to the 16:00 mark, it discusses this very well.
(After that, it goes on to explain the monsoons in Asia and why they are so drastic compared to South America.)
Briefly, Coriolis is caused by the fact that the equator spins faster than the poles. This is simply due to the distance of the surface of Earth from the axis of rotation.
Cold air sinks (it is denser so heavier) at the poles and flows toward the equator. This southerly flow (in the Northern Hemisphere) is moving slower than the ground further south, so the air flows across the surface in a southwesterly direction. That is simply the ground moving faster than the air the further south you go - Coriolis in action. These form what are called the Polar cells, the air circulation bands around the poles.
Similarly, air is heated at the equator, it rises and flows toward the poles, e.g. north. The equator is spinning at 1600 km/hr, and as it moves northward, it overtakes the surface that is moving 1400 km/hr at the 30 deg latitude. Thus, the air circulates in bands around the equator to the 30 deg latitudes - these bands are called the Hadley cells.
As the hot air compresses around the 30 deg latitude, the airflow drops to the surface, and splits. Some airflow goes north, and is moving faster than the ground as it moves more northward, so the air flows from the west.
At around 60 deg latitude, the westerlies moving north meet the easterlies moving south. This forms what is called the Polar Front.
From your reference point, instead of talking about Coriolis acceleration, you can simply talk about the fact that the ground at the equator is moving at 1600 km/hr, and at 30 deg the ground is moving at 1400 km/hr. Air moving due north from the equator will circulate up in the troposphere going north where it drops back to the ground, it is moving faster than the ground below it. Then the air circulates in the Hadley cells back south and moves west as it goes south again. Some air moves at the surface north from the 30 deg mark, and that air is moving faster than the ground, ergo the winds flow east. It then meets up with southerly flowing cold air from the poles around the 60 deg latitude.
Oh, and one more thing, since the Earth’s axis is tilted, these zones move with the seasons.
Thank you Irishman, that really drives the point home about the basic circulation in the atmosphere. In the temperate latitudes, no forces are acting in the east-west direction. The air mass stays the same velocity, it is the Earth that slows down. In our lofty perch watching the world spin, we see the Earth and the air parcel at 30ºN are both moving at 1400 km/hr. The north-south forces push the air parcel along the surface up to Chicago at 45ºN. The air parcel is still moving at 1400km/hr, but the Earth there at Chicago is only moving 1100 km/hr. We can infer from our perch that the folks in Chicago feel a wind from the west at 300km/hr (Windy City indeed).
… wait a second … something’s obviously wrong …
So, take another look at the equations I posted above. Notice the “B” term I’ve called the pressure gradient force. This is gravity holding the air to the solid planet, though squishing the air down is a better description. Without the sun and without spinning, the air will even itself out so that at equal distance from the center we’ll have equal pressure. Let’s call this our equilibrium state. Spinning the Earth doesn’t change this, but adding the sun’s energy does. Moving the air parcel over a slower Earth causes a bit of a low pressure on one side and a bit of high pressure on the other, disrupting equilibrium. The force, from gravity, will push back from the high to the low. It is a gradient force, meaning the greater the departure from equilibrium, the greater the magnitude of the force acting on the air parcel to bring it back to equilibrium.
So we end up with a gentle 10 km/hr breeze instead of hurricane force winds because of the pressure gradient force pushing the air back towards the west. This force becomes very weak as we close in on equilibrium and its 0 km/hr wind. I want everybody to remember, for the air parcel over Chicago, there is a force component pointed north and the other component pointed west.
The Master’s article stops here, but I’ll briefly touch upon cyclonic motion. Storms spin counter-clockwise in the northern hemisphere. Looking at the force diagram at Chicago, I hope you get a sense of why this is. As the air parcel moves north, it has a west component that introduces a bit of a curl to the motion in the counter-clockwise direction. These large storms cover close to 20,000 sq km and over this large area, the sum of all friction tends to 0. A thunderstorm only covers around 200 sq km, thus giving a greater chance that friction will spin it clockwise, which spawns clockwise tornadoes. A 2 sq m bathtub is dominated by frictional forces, the water drains either way, just depends on the random currents over the drain at the moment it’s opened.
I haven’t counted, but just looking I’d say this entire thread is less than 600 words.
“fictitious force” is a bad term. “Pseudo-force” is only slightly better. “Pseudo-” anything is usually a bad term, since it’s better to call something what it is than what it’s not and then say it’s not that.
A much better term is “apparent force”. An apparent force is something that appears to be a force from a particular frame of reference, but is caused by a real force that’s more clearly seen from a different frame of reference.
It’s nicely similar to “apparent wind”, which is a common term used in sailing. When you’re not moving, you can feel the effect of the true wind. As soon as you start moving, the wind you sense is the vector difference between the true wind and your velocity vector. That’s the apparent wind. It’s a very important thing, because that’s what’s actually imparting a force on your sailboat: you set your sails for the apparent wind, not the true wind!
Likewise, the coriolis effect is an apparent force. It acts on you as though (in the Northern hemisphere) it’s tugging you to the left – and indeed, it is! But it’s not some special “left-pushing force”, it’s just a ramification of your motion on a rotating sphere.
Also likewise, centrifugal force isn’t a real force, but when you’re riding on a circus ride, it certainly feels like one. But it’s just inertia, experienced from a circular path.
It depends on what we’re doing. I’m handing the SDSAB the formula for Coriolis force when they’re heading to the roof of the Chicago Reader building so they know how far west of Milwaukee to aim the howitzer. When they let that bogie go, they’ll see it curl to the right. That’s fine if our objective is to have a pile of rubble where the Milwaukee City Hall used to stand.
However, our objective here is to explain cyclonic motion. Saying the air parcel doesn’t curl to the right as much as it should is just an added complication. It’s better to start in an inertial frame of reference where we can’t even observe the right-hand curl, just the curl to the left due to the pressure gradient force. It’s this curl to the left that gives us our counter-clockwise motion in the Northern Hemisphere.
We need to simplify where we can. There’s a couple of things I’ve cleverly been dancing around that defies any manner of simple explanation. As long as no one asks, I can save a lot of typing.
To be clear: As a linear force provides linear acceleration, so too does a torque provide curl (or a rotational force provides rotation acceleration).