Inversions and Inverse Barometers

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Well folks, there will not be any active weather for a long, long time. But there is always something meteorological that is interesting. Weather is like a fine wine...the subtleties are often the most rewarding. And a lot less expensive.

Take today. A high pressure area has been overhead, with relatively clear skies aloft and light winds. This time of year you know what that spells? Temperature inversions and fog.

Why? Under clear skies there is good infrared radiation cooling to space from the surface, allowing it to cool faster than air above. This produces an inversion in which temperature INCREASES with height. Want to see the effect in action? Look at the figure below from the Seattle Sand Point profiler near Magnuson Park in NE Seattle. In this figure you see temperature (in C) plotted with height for various hours (this is in GMT, 12 is 4 AM, 18 is 10 AM, etc). You can see a good inversion in the morning in the lowest 400m, which gets "burned off" when the sun finally got in to heat the surface. When the low-level air gets cooled to saturation we get fog.
Its just not fair...we get high pressure over us and it gets colder in the morning and some fog.

What about inverse barometers? I had number of emails and blog comments about the tide tables being WAY off last week--at some locations the water level was nearly TWO FEET higher than predicted! Here are two examples of this effect at Neah Bay and Astoria. The red line is observed and the blue is predicted. The green line is the difference.

Tide tables are normally pretty reliable, so what is going on? Fuzzy math? Nope...the answer has been the unusually low pressure this month. Take a look at the pressure at Seattle, Astoria, and Sea Tac in the plot below. From roughly the 15th to the 23rd the pressure was amazingly low...roughly 980 mb (mb is millibars, a unit of pressure). This is approximately37 mb below normal (1017 mb). So why is this important? It turns out that low atmospheric pressure results in a higher sea level. This is known as the "inverse barometer effect", with water level rising about 1 cm for every 1 mb drop in pressure (see the figure below). Thus, with a difference of 37 mb from normal, sea level would be 37 cm or 14.5 inches below normal. Thus, the extraordinarily unusual low pressure, something not considered in the tide models, resulted in the predicted water level height being much too low! There are also secondary factors, such as the sustained unusual winds offshore and higher than normal wave activity...but the pressure anomaly is the main driver.

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