July 09, 2013

Boring Weather Reveals Hidden Weather Signals

The weather of the past week has been BORING.  Day after day with the western lowlands warming into the mid-70s to low 80s, no storms, modest winds, lots of sun, and no rain except for thunderstorms east of the Cascade crest.  Some low clouds in the morning.

No wonder the Weather Channel is avoiding us.

The visible satellite imagery at 7:15 PM Tuesday shows some offshore clouds and a few convective showers over the eastern Cascade slopes and southeastern BC. 

But boring can also be revealing, exposing weather phenomena normally obscured by active weather.

Let me prove this to you. Question.  If the large scale weather pattern is static with no storms or weather features approaching or leaving, will surface pressure change?

The answer is yes.   Lets take a look at the surface pressure variations at Seattle Tacoma Airport for the last three days (the top panel is sea level pressure, next precipitation, and then solar radiation).  The precipitation was obviously flat-lined, but look at pressure--you see those wiggles?

On each day, pressure tends to be highest around 18Z (Zulu or Greenwich Mean Time), which is 11 AM PDT.  Pressure tends to be lowest around 00 Z (5 PM).

In fact, careful analysis reveals that pressure tends to be highest around 11 AM and 11 PM (with the 11 AM being highest), and lowest around 4 PM and 4AM, with 4 PM having the lowest pressure.  This is what is called a semidiurnal pressure signal and it true most locations around the world.  This signal gets stronger near the equator.

I did a paper on this subject once and here are the average pressures through the day for various locations around the world.  You see the semidiurnal pressure variation?  They are bigger in Miami than Seattle (Denver is large because of mountain effects I won't get into here).

 The signal can be a few mb (hPa), with typically a large pressure drop in the early afternoon between the primary max and min pressures.

So you ask, why is pressure varying during the day?  Is is heating at the surface during the day and cooling at night?

It turns out the answer is no...most of the pressure variations is due to an atmospheric tide in the upper atmosphere.

You know about tides.  They occur because of the attraction of the moon and there are two highs and lows.  Here is an example for Milford, CT for one day.  Two highs and two lows.
You can see why this is through this schematic:  the attraction of the moon actually causes two areas where the oceans bulge up and bulge down.
So how does this help explain the pressure variations at the surface?  Well, it turns out the heating of the sun results in a similar tide in the upper atmosphere, with two highs and lows.   The heating tide changes the density of the air, producing the semidiurnal pressure cycle.   You will be proud to know that a some of the research that unraveled upper atmospheric tides were done here at the UW!

Seattle School Board Race

I strongly support Sue Peters for school board and I hope you will too.  She has been a strong advocate of better math in Seattle Public Schools and I have been extraordinarily impressed during my interactions with her.  She has a very impressive educational resume, including serving on a number of district committees and task forces, has a respected education blog, has been an active volunteer in local schools, and was a founding member of the Seattle Math Coalition. Sue is supported by FOUR of the current school board members and has a long list of endorsements by local organizations.  The Seattle Times, the supporter of corporate education philosophy, endorsed her opponent...another sign that Sue is on the right track.

Check out her web site at: http://suepeters4schoolboard.org/http://suepeters4schoolboard.org/
Please vote for her in the primary and support her campaign.   


  1. O K Cliff,

    I think we need more detail on this atmospheric tide thing. Is the gravitational force of the Sun and moon "pulling" the atmosphere like it does the water? And does the heating of the atmosphere coincide with the atmospheric tide?



  2. I agree with Paul. You carefully explain the water-tide phenomenon, but leave the solar pressure-tide to the reader. After you state that it is not due to solar heating of the air.
    Please help.

  3. The water and air tides occur due to the difference in gravitational force on the earth by the moon at the center of the earth and the near and far sides of the earth. The force on the center (of mass) of the earth keeps the moon in orbit. The force is greater nearer to the moon and less farther from the moon. This causes the water and air to flow towards the moon on the near side and away from the moon on the far side relative to the center of the earth. So, most of the flow is really on the skinny sides toward the near and far sides.

  4. Hi Cliff! I was wondering if there's any word yet on what kind of winter we may have...El Nino, La Nina, or Neutral? This summer, so far, has been great!


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