Unfortunately, I have to disappoint you here. Frame effects such as the
Coriolis and centrifugal forces don't cleanly explain weather patterns,
although they're certainly part of the story. Air is not as free to move as
an artillery shell, for instance. And even in the worst storms, air doesn't
move rapidly enough westward to get a net rightward deflection. |
However, after taking all the messy stuff into account, moving air in the
temperate zone of the Northern hemisphere still deflects to the right, and
moving air in the temperate zone of the Southern hemisphere still deflects to
the left. In general. On large scales. :) And this page will try to
explain how you go from "deflect to the right" to the way a low pressure
system curls around to the left.
Low pressure system in the Northern Hemisphere.
A low pressure system is some place where there's less air than average.
With less air above the ground, the pressure due to air is lower, yes? But
since there's more air elsewhere, that air will try to rush in to fill the
As the air rushes in, it will get deflected. Since we're looking at the
Northern hemisphere's temperate zones, that means it goes to the right. So
why doesn't a low pressure system spin around clockwise? Because the air
bending to the right is "herded" by the adjacent air also trying to come in
towards the center of the low pressure area. This ends up making everything
go around in a circle counter-clockwise.
In some cases, the air ends up going around in a circle so quickly that it
can't get any closer to the center of the Low...this is what happens in the
eye of a hurricane. And even without a hurricane, being near a Low tends to
involve a lot of air moving very quickly and spinning around a lot, so Lows
generally mean nasty weather.
High pressure system in the Northern Hemisphere.
A high pressure system, on the other hand, involves a place where there's a
lot more air than average, and it all tries to spread out to places
where there's less air. As the air rushes outward, it bends to the right in
the Northern Hemisphere, for an overall clockwise spin.|
Since air rushing outward picks up deflection and speed as it goes outward,
you don't see the kind of dramatic spin effects that low pressure systems can
give you. In fact, at the center of the High, air isn't moving very quickly
at all, hence high pressure being associated with nice weather.
I should note that the above is very basic, leaves out a number of
potentially complicating factors, and was written by someone (me) without a
deep background in meteorology. OceanMotion.org
has a more detailed explanation of these effects, with animations and
everything, for those who want to know more about the specifics of the
Coriolis effect when it comes to weather.
Back to the Main Coriolis force Page.