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Part 1 of 2: A Qualitative Explanation
The Coriolis force is a fictitious force that deflects large-scale motion on any rotating planet (including Earth).
To put it in simpler terms: if something is moving on a very large scale (e. g. an air mass moving from Mexico to Canada), then it will not move in the direction that you think it will move in. Rather, it will be deflected as it moves, due to the rotation of the earth. If it’s in the northern hemisphere, it will be deflected to the right; if it’s in the southern hemisphere, it will be deflected to the left.
The effect of the Coriolis force on a moving body is called the Coriolis effect. But “Coriolis force” and “Coriolis effect” are basically interchangeable terms.
A few examples would help to illustrate the concept.
Let’s say that you live in San Antonio, Texas, and you want to fire a ballistic missile such that it travels some 2500 km directly north and ultimately lands in Devil’s Lake, North Dakota. (Physics problems often seem to involve gratuitous violence and bizarre actions for which there is no apparent motive.)
However, if you fire your missile directly to the north, it will not land in Devil’s Lake, ND. It will probably land somewhere in Minnesota or Wisconsin. That’s because of the Coriolis force. We are in the northern hemisphere, so motion is deflected to the right. So instead of travelling due north, the missile will be deflected to the east and will land somewhere in Minnesota or Wisconsin.
Conversely, let’s say you live in Devil’s Lake, ND, and you want to fire a missile such that it travels directly south and ultimately lands in San Antonio.
Again, if you fire your missile due south, it will not land in San Antonio, as planned. Rather, it will be deflected to the west and will probably land somewhere in New Mexico or Arizona. Since this is still the northern hemisphere, motion is still deflected to the right, so instead of travelling due south, the missile will be deflected to the west.
In the southern hemisphere, it is the opposite: all motion is deflected to the left.
The Coriolis force is named after the French scientist Gaspard-Gustave de Coriolis, who derived the mathematical expression for it (see Part 2) in the 19th Century.
The Coriolis force matters only for large-scale motion. For small-scale motion, the Coriolis force is negligibly small. If you’re throwing a tennis ball across your back yard, you don’t have to worry about the rotation of the earth. It makes no significant difference. On the other hand, if you’re firing a ballistic missile over thousands of kilometers, then you have to take the Coriolis force into account.
There are two properties that affect the strength of the Coriolis force.
First, the Coriolis force is stronger at higher latitudes. The farther you are from the equator, the greater the deflection. So if you fire a missile from the equator to 10°N, it will only be slightly deflected, while if you fire a missile from 70°N to 80°N, it will be deflected to a greater extent.
Second, the Coriolis force is stronger when the motion is faster. That seems logical enough: if an object is not moving, then there’s no Coriolis force acting on it. But when it starts to move, the Coriolis force begins to act on it, and the faster it moves, the stronger the pull of the force.
The Coriolis force is an example of what’s known as a fictitious force. That means that it isn’t really a force; it just appears to be a force because you’re viewing the motion from a moving reference frame.
A common example of a fictitious force is when you’re riding in a car, and the car turns right, and you are pulled towards the left side of the car. There is not actually any force pulling you to the left. It’s just that the car is turning right while you (initially) were going straight, so it seems like there must be a force pulling you to the left …… even though in reality, there’s not. So you could say that from your perspective, there is a fictitious force pulling you to the left.
In the same way, if you fire a missile from Texas to North Dakota, and it gets deflected by the Coriolis force and lands in Minnesota, there was not actually any force pulling it to the east. It’s just that the earth was constantly rotating, and we live on the rotating earth, so from our perspective, it looks like there was a force pulling the missile to the east, even though in reality, there was not. That’s why the Coriolis force is a fictitious force.
In other words, the Coriolis force is an illusion. From our perspective, it looks like there is this force — the Coriolis force — pulling on the missile, but in reality, that’s just because we are living on a rotating planet. In reality, there is no such force.
But even if it’s an illusion, we will always have to deal with it, because we will always be seeing our environment from the perspective of our rotating earth. So even if it’s only a fictitious force, we still have to define it, understand it, and take it into account whenever we analyze large-scale motion.
The main reason why we care about the Coriolis force has nothing to do with people firing ballistic missiles for no apparent reason. Rather, the reason why we care about it is that it is extremely important for meteorology. If the earth were not rotating, the general circulation of the atmosphere would be very different than it is, and the general patterns of weather that we all know would also be very different. The effects of the Coriolis force include:
· The Coriolis force is the reason why there are three cells of atmospheric circulation in each hemisphere, rather than just one
· The Coriolis force is the reason why the trade wins move east-to-west
· The Coriolis force combines with the pressure gradient force to establish geostrophic balance
· The Coriolis force (through geostrophic balance) is the reason why the winds in a hurricane rotate counterclockwise around the eye (in the northern hemisphere)
· The Coriolis force (through geostrophic balance) is the reason why the winds in the upper troposphere tend to flow parallel to the lines of constant pressure.
Clearly, the Coriolis force is quite an important phenomenon in meteorology. In fact, it is one of the three major forces that affects the direction of the wind, along with the pressure gradient force and friction.
But my discussion of the Coriolis force has still left three questions unanswered:
1. Why does the Coriolis force deflect things to the right in the northern hemisphere and to the left in the southern hemisphere?
2. Why is it stronger when you’re farther from the equator?
3. Why is it stronger when the motion is faster?
In my next article, I will use math and physics to answer these three questions.
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