city difference. The implication is that
if one were to match the velocity differences on the two scales, then
the Coriolis force, or maybe the displacement, would suddenly be the
same in each case. Alas, it would not be so (unless other things were
?The traditional response to what you have suggested is that you have
confused two things, a spatial scale and temporal scale. As usually
presented, the Coriolis force (being a force) produces a result (such
as displacement) over time. The spatial scale (how far it is across
something, or how far an object travels) does not even appear in the
equations. There is much to commend this approach. Within this
context, it is not the spatial scale that produces the effect (for a
given force), but how long the event lasts. A sink drains quickly (not
much time for a small force to produce a significant displacement); a
missile or the air in a hurricane takes much longer to traverse its
territory (a much longer time for a force to produce a significant
?Bad Coriolis FAQ?
Ascher Shapiro of MIT demonstrated this in 1962 with the following experiment.
?All this was demonstrated way back in 1962 by one Ascher Shapiro, a
researcher at the Massachusetts Institute of Technology. Shapiro
filled a circular tank six feet in diameter and six inches high in
such a way that the water swirled in a clockwise direction. (Remember,
now, that Coriolis forces in the Northern Hemisphere act in a
?Shapiro then covered the tank with a plastic sheet, kept the
temperature constant, and sat down to read comic books or whatever
scientists do while they wait for their experiments to percolate. When
he pulled the plug after an hour or two, the water went down the drain
clockwise, presumably because it still retained some clockwise motion
?On the other hand, if Shapiro pulled the plug after waiting a full 24
hours, the draining water spiraled counterclockwise, indicating that
the motion from filling had subsided enough for the Coriolis effect to
take over. When the plug was pulled after four to five hours, the
water started draining clockwise, then gradually slowed down and
finally started swirling in the opposite direction.?
?Do bathtubs drain counterclockwise in the Northern Hemisphere??
A more technical explanation can be found here.
?In a kitchen sink, of course, speeds and time scales are much smaller
than hours and miles. Water rushing down a drain flows at speeds on
the order of a meter per second in most sinks, which are themselves
less than a meter wide. Qualitatively, there doesn't seem to be much
chance for deflection. Quantitatively, putting these numbers into
Equation 1 results in an estimated change in rotation of only a
fraction of a degree per second, and a very small fraction at
that...less than an arc-second (1/3600th of a degree) per second over
the course of the entire draining of the sink, ignoring additional
effects caused by conservation of angular momentum and the like. Under
extremely controlled conditions, this can cause water to flow out of a
container counter-clockwise in the northern hemisphere and clockwise
in the southern hemisphere, but your kitchen sink is not so
controlled. Things like leftover spin from filling the sink (even when
the water looks still, it's rotating slowly for a long time after it
seems to stop), irregularities in the construction of the basin,
convection currents if the water is warmer or colder than the basin,
and so forth, can affect the direction water goes down the sink. Any
one of these factors is usually more than enough to overwhelm the
small contribution of the Coriolis effect in your kitchen sink or
bathtub. Research in the 1960s showed that if you do carefully
eliminate these factors, the Coriolis effect can be observed.?
"Getting Around The Coriolis Force"
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