Why Is It There?
Short answer: thermal wind. Does that help? No?
Then!
Long
answer: the poles are cold while the tropics are warm. Why? Well,
you know how the Earth is spherical rather than, you know, flat? What
happens is that near the poles, the Sun hits the Earth at a very low angle, so
the poles get a lot less light. Also, because of the inclination of the
Earth (which is responsible for seasons, by the way), the poles go six months
without sunlight at all, as the south pole in the figure is currently doing.
This results in a temperature difference between the air in the poles and in the
tropics and mid-latitudes, and while the temperature difference also involves
many other factors, and it is believed that in various parts of the past, this
temperature difference was much smaller, or perhaps larger, or whatever it is,
the main thing is that right now we have one and it's mainly because of the tilt
of the earth's axis.
Now, another
fun fact about the Earth that you probably already knew: the Earth rotates!
Crazy! Newton formulated some laws of physics back in the day, and one of
them, the first one, actually, said that an object at rest or traveling at a
constant velocity will continue to do so unless acted upon by a force, and the
second goes on to describe how that happens (the force produces an acceleration
proportional to it in the same direction, etc.). Consider an object at
rest at the surface of the Earth. Since the Earth is spinning, the object
isn't really at rest; it's actually going around in circles very quickly.
If we pretend that the Earth is not moving, just spinning, the speed of an
object "at rest" at the equator is about 465 m/s, which is about 1040 mph.
That means that Newton's laws don't really work the way you expect them to work.
We have to introduce what are called fictitious forces, and then things
will move the way we expect them to. Basically, in an ideal space (called
an inertial frame), if I push you forward, you will go forward. If I don't
push you at all, nothing will happen. This is assuming that you're not
moving by yourself, of course. In a rotating frame, like in a
merry-go-round, if I don't push you at all, you will still go flying out!
This is assuming that the merry-go-round is spinning VERY quickly, of course.
The thing that makes you go flying out is just inertia, but to someone who's
actually sitting on one of the horses on the merry-go-round, we call this
mysterious effect the centrifugal force. It is a fictitious force.
It does not exist. It's a computational tool only, used in rotating
frames. The picture to the left is from
XKCD.
To understand it quickly, consider a heavy ball on a string that
you're spinning above your head. The ball feels itself pulled away from
your hand, but in reality, you are pulling the ball towards your hand, therefore
making it spin. That is the centripetal force. The ball,
however, just feels a strange force pulling it away. That is the
centrifugal force. On the Earth, that pulls us away from the ground, but
away from the ground is the direction opposite gravity, so we automatically
adjust for it in our calculations, and we don't feel it at all because to us,
it's just part of gravity. Because of the centrifugal force, gravity is a
bit weaker than we would expect by universal gravitation. The centrifugal
force is therefore not very important to us in this context. There are
some other forces as well, but the important one is the Coriolis force.
On a merry-go-round in the night
Coriolis was shaken with fright
Despite how he walked
'Twas like he was stalked
By some fiend always pushing him right.
-- David Morin
The Coriolis force has nothing to do with toilets.
NOTHING. Seriously. You may have heard that toilet flushes flow
counterclockwise in the northern hemisphere and clockwise in the southern, and
then which way do they flow on the equator. If you think that, you're
confusing toilets with hurricanes, which are different.
Toilets.
Hurricanes.
DIFFERENT.
The problem with the Coriolis force is that it's weird.
Very weird. It's very weak compared to the other forces we feel in our
day-to-day lives, mostly since it's only noticeable over large distances, and it
pushes things to the right of whichever way they're going. (In the
southern hemisphere, they push things to the left.) This is rather
counterintuitive, since usually we don't expect things to be deflected to the
side, but in the case of large-scale flows in the atmosphere (and in the oceans,
too), this is the deal. Whatever the direction of the flow, there's
another force pushing it to the right. Now, in real life, there isn't one,
right? I mean, the Coriolis force is fictitious like the centrifugal
force! Consider this. Pretend that there's a record player spinning
in front of you. Now pretend you're throwing a little ball (the kind of
ball that doesn't damage things it hits, let's say) over it. Just over it,
not at it. It'll go in the way you expect it to go, right? Just
straight? Now, pretend that you're seeing this from the perspective of the
record player, or better, sit on a spinny chair, keep spinning around while
looking up, and get a friend or enemy to throw a ball over your head.
You'll notice that it takes a curved path from your spinning point of view!
This is the Coriolis force!
Why is this
important to us? Well, when there is a difference in pressure, there is a
force (a real one, natch) associated with it called the pressure gradient force.
Pressure is, in a sense, how much "stuff" is over a particular point, so if
there's more "stuff" over one point than another, there will be a force trying
to fix that. When there is high pressure near low pressure, there will be
a force from high to low trying to get the "stuff" to equilibrate itself.
As it turns out, the cold air over the poles also has lower pressure than the
warm air over the mid-latitudes, so there's a force from the mid-latitudes to
the poles, pushing the air that way. But does the air go that way?
NO.
As you can see from the diagram, once stuff starts going from
high to low, it gets deflected to the right. The flow will have a constant
velocity and just go straight only when the forces are balanced; that is, when
the pressure gradient force pushing from high to low pressure exactly balances
the Coriolis force. Since the Coriolis force pushes to the right, the
resulting flow is to the right, in the diagram to the right. For large
enough motions, air will travel with the low pressure centers to the left, so in
a hurricane, with a low pressure eye, flow is counterclockwise. In the
southern hemisphere, all of this reverses. The reason for why this happens
is outside the scope of this webpage, but you can look it up or, alternatively,
think about it while walking places. But as I said, the cold air over the
poles has lower pressure than the warm air over the mid-latitudes, so air will
tend to flow counterclockwise around the north pole -- the sense of the jet
stream!
Because of the temperature difference between the two sides of
the jet stream, there is another thing that happens that is rather too
complicated to explain without math, but I'll try anyway. Where the
temperature is sloping (you can see this clearly on the picture
here; look at the white lines), there is wind
shear, partly as a result of the various weird effects of the Coriolis force.
Wind shear, in this case, is where the wind at the surface is different from the
wind higher up. The wind increases PERPENDICULARLY to the direction of
temperature change. In our case, this temperature change is generally
north-south, so it's the east-west wind that increases as you go up. What
happens, then? As you go up, the westerly (that's FROM the west) winds
keep increasing in that region with the temperature difference until, finally,
the temperature difference changes and the winds start decreasing. That
maximum is where the jet stream happens, at around 12 km altitude, right over
the spot with the temperature difference. This is called thermal wind.
The Coriolis force is weird enough that all sorts of unexpected things happen,
and this is one of them. The thermal wind is responsible for the existence
of this wind maximum, essentially a jet through the high air over a region of
rapid temperature change. Read this paragraph a few times. That's
where the jet stream comes from.
Now you can impress beautiful members of the desired sex at
parties, and also hopefully understand a bit about the weather and the
atmosphere.
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