A
tornado is a local, short-term phenomenon that forms a funnel of air
typically about a hundred meters across. It forms on land, quite far
away from the equator, and never travels long enough (or far enough) to
cross the equator.
That, however, is not the end of the story (not by
a long shot!). While tornadoes are too short-lived to go anywhere,
there are several larger versions of similar storms (i.e., storms with
a spiraling wind flow) like tropical cyclones (names like “hurricane”
and “typhoon” are just local names of tropical cyclones), which do live
long enough and travel far enough (often several thousand miles over
the sea). The interesting question is; do they cross the equator?
Let’s
start at the beginning, with the birth of a cyclone. Sometimes during
the summer, the air near the earth’s surface heats up. Hot air is
lighter than normal air (that’s how hot air balloons go up), so it
rises. But this creates a “partial void”, a low-pressure region, which
attracts air from neighboring regions, which rush in to fill the
“void”. At this point, a second interesting effect kicks in.
The rotation of the earth, besides forming night and day, creates several other funny effects, one of which is the so-called Coriolis force.
Briefly told, this force moves all horizontally moving bodies to their
right in the northern hemisphere, and to their left in the southern
hemisphere.
Back to cyclones. If the low
pressure “void” talked about were in the northern hemisphere, air would
rush in from all directions to the low-pressure region. At the same
time the rushing air would be continuously deflected to its right.
Because air is moving in towards a point and also to the right, it’s
not hard to imagine that pretty soon it would be coming in in a
counterclockwise spiral. So the incoming air creates a counterclockwise
spiral, which, if it spirals fast enough, creates a storm called a
cyclone. Similarly, such a cyclone formed in the southern hemisphere
would spin clockwise.
So what does this tell us about the times a cyclone ventures across the equator?
Firstly,
to create a cyclone, the winds have to spiral fast enough. The Coriolis
force is weakest at the equator, and grows toward the poles. Near the
equator, it’s just not strong enough to create a cyclone, and so
there’s a region between (roughly) 5 degrees N and 5 degrees S where no
cyclones ever form.
Secondly, can a cyclone
formed outside that narrow belt cross over? As remarked before, the
Coriolis force is stronger away from the equator. So winds away from
the equator are spiraling faster than winds near the equator. This
creates a low-pressure region to the north (south) of cyclones in the
northern (southern) hemisphere (the so-called Bernoulli’s principle),
which pulls the cyclones away from the equator. So not only is the
Coriolis force responsible for creating cyclones, it also keeps them
away from the equator!
Thirdly, what if a
cyclone, against all odds, did cross the equator? If ever a clockwise
cyclone crossed over to the northern hemisphere, the Coriolis force
there would try to forcibly rotate it in the other direction. It is not
surprising therefore, that the Coriolis force would apply the brakes on
such a cyclone, which would slow down and ultimately peter out.
So
do cyclones ever cross the equator? It turns out that while some
cyclones in the Indian Ocean come very close to pulling it off, the
odds are ultimately too overwhelming, and they die out near the equator.
Cyclones
of the Atlantic and the Pacific, on the other hand, don’t even come
close. To summarize, it would be a safe bet to say that cyclones never
manage to cross the equator.
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