[Paddlewise] Effect of Barometric Pressure on Tide Height: Debunking the Myth

A couple of folks have noted that during a storm (low atmospheric pressure)
that high tides run higher than normal -- the well-named "storm surge."
This surge can run several feet, as it has on many occasions on the Gulf
Coast.  Along with the huge waves generated by hurricane-force winds, the
storm surge causes massive damage to coastal areas.

Alas, it is not the direct effect of the low pressure that raises the height
of the tide by several feet.  Even in the center of the worst storms, when
the barometric pressure drops below 29 inches of mercury, the equivalent
rise in column of water is only a foot or so at the maximum  (density of
water is about 13.6 times smaller than that of mercury).  Normal sea level
air pressure is about 29.9 inches of mercury.  A low pressure of 28.5 inches
mercury (extremely low for the atmosphere at sea level -- about what you get
in the eye of a hurricane) only changes the tide height by 19 inches.

In short, it is a different effect, mainly the effect of the storm center's
winds, that pushes a bulge of sea water ashore along with the storm center.
Here is a pretty good description of the effect:
http://www.comet.ucar.edu/nsflab/web/hurricane/313.htm complete with
animation (takes a while to load).

Note that the highest levels of tide (worst storm surges) are to the right
of the eye of the hurricane (Northern hemisphere), and __not__ at the
location of lowest pressure.

On the coast of Oregon, storm surges are common, and run 1 to 2 feet.  When
the tide is at its monthly maximum, and a heavy rainfall is on us, the
combined effects near the mouths of rivers make for a lot of flooding in the
lowlands.

Just debunking the myth.

--
Dave Kruger
Astoria, OR

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