Sunday, January 13, 2013

Which way does the wind blow?

The other evening at dinner we were discussing where one would site another large telescope at SAAO, if one were to do so.  I quipped that it should be on the SE corner of the plateau by the 74" because that's where the wind comes from.  I got a hearty laugh and a scoff, but I was not completely kidding.  In my experience the wind is from the SE much more at night than it is during the day.  And many times I've seen westerly winds during the day quickly give way to SE winds soon after sunset.

Thanks to SALT's ELS database we have the numbers to show how crazy my idea might be.  A query of the ELS shows that we have wind data dating all the way back to late April 2008. Thus we have millions of data points spanning almost 5 years.  In my queries, I defined night as between 20:00 and 05:00 and day as between 07:00 and 18:00.  This provides over 700,000 nighttime measurements and nearly a million daytime measurements.  Here are the resulting histograms:

So I'm not completely crazy! The most probable wind direction at night is ESE.  Fully 1/3 of the time the wind at night is coming from between E and S and quite concentrated between E and SE.  However, 45% of the time the wind is coming from between SW and N so westerly winds are still favoured at night, but not by nearly as much as during the day.

This nearly 180 degree change between night and day helps explain why SE winds tend to bring bad seeing. During the day the sun-heated ground creates convection that couples the surface air with dry, westerly, upper-level winds.  However, once the heating goes away, the surface layer decouples and allows cooler air from the southeast to flow in.  The friction between the westerly upper level winds and easterly surface winds creates turbulence which in turn degrades seeing. On nights like tonight when the upper and lower level winds are both from the SE, the seeing can actually be pretty decent. Unfortunately, nights like this are not the norm so my idea ends up pretty crazy in the end.  

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