January 27, 2010

Inversions and Inverse Barometers

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.


  1. The tide thing with the low pressure is amazing... is the opposite also true? If the pressure is really high are the tides lower than predicted?

    Presumably, most people wouldn't notice or care because lower tides don't equal flooding... but they could mean poor tide-pool formation, and maybe more interestingly, exposure of old shipwrecks?

    Just curious... and loving the "boring" weather. It's amazing to get to see some of the winter stars and planets... we've actually seen some meteorites in the past week as well. Can't think of too many winters when that's been the case, so it's an interesting change.

    My boss has been in Manila for the past ten days. He thinks my co-worker and I are lying when we say it's been sunny for at least part of every day that he's been gone. LOL

  2. We marked a predicted 13.2 tide on Jan 5. After your pressure/tide post last week, we marked a predicted 12.2 tide about 6" higher than that 13.2 mark. Very cool to observe this.

  3. On another note, last night there was a wide ring or "halo" around the moon, was that just refracted light from the moon? It was cool to see.

  4. Jayne,
    Go here to learn about moondogs, moonbows, and other such sights: Spaceweather.com

    It's also an excellent site... not as personable as Cliff is here, but lots of juicy information!

    For instance, the next full moon is BIGGER than a normal full moon... it's a perigee full moon, 14% larger than normal. And there have been polar stratospheric clouds observed at sunset a few times recently... photos here.

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  6. Where's the math, Cliff?
    According to your numbers above (15 mbars below normal) there would only be a 15 cm rise in sea level due to the "inverse barometer" effect. This is only 0.5 ft - just a quarter of the 2 ft difference between predicted tidal heights and observations. Also, look at 13 Jan - tidal heights are 1.5 ft above normal, but pressure is at 1005 mbar - only 8 mbars from "standard atmospheric pressure."

    Wind set-up is generally a much greater contributor to tidal anomalies. Wind set-up occurs when onshore winds "push" the water towards shore and cause the near-shore sea level to rise. (Actually, this is what happens in lakes and estuaries - in the ocean, the Coriolis force becomes important, causing Ekman transport towards shore, and set-up when winds are from the south for the West coast of the US). This also works in reverse, causing set-down, when winds are from the opposite direction.

    Similar effects occur during storm surges. The pressure surge is usually a good deal smaller than the wind and wave driven storm surge.

  7. I was fortunate enough to capture an image of the moon halo on Monday as some thin cirrus moved in. You can check it out here:

  8. i think that was my favorite of all of your appearances (can one appear on the radio?) on kuow this morning cliff.

    your laughter was contagious regarding the weather in april.

    great fun, thanks.

  9. jprinehimer,
    I wrote down 990 mb in error...should have been 980 mb The pressure was down to roughly 980 mb. Average pressure is about 1017 mb. the is a 37 mb or roughly 37 cm anomaly....14.5 inches. As you note, there are secondary factor...ocean currents driven by sustained anomalous winds and the impact of sustained high waves from the low offshore...cliff

  10. Is "inverse barometer effect" part of "storm surge", except in this case we had the low pressure without exactly a storm?

  11. In answer to JewelyaZ, yes you can see the inverse barometer effect for high as well as low pressure. There is a year-long study of this for Puget Sound at www.starpath.com/barobook

    the effect was known to 19th century mariners who had a saying "fog nipps the tide" implying that fog is often correlated with high pressure, which suppresses the tide

  12. Weather
    is a set of all the phenomena occurring in a given atmosphere at a given time. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.


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