Re: [APML] 201XT- too hot outside?

[Glenn Shaw <gshaw@alaska.net>]

Greg Hartke wrote:
> 
> On Mon, 26 Jun 2000, Glenn Shaw wrote:
> 
> > Regarding star scintillation:
> >
> > In summer when the ground heats up by the sun, the air next to the
> > ground is heated and rises. The result is convection, which is
> > responsible for bringing afternoon cumulous clouds and afternoon
> > showers. The convection is more vigorous and deeper throughout the
> > atmosphere in summer.
> >
> > Scintillation of a star is caused by variations in the refractive
> > index of the medium through which the rays are passing. Since
> > refractive index is proportional to gas density, and since density
> > varies in convecting airmasses, scintillation is sometimes (not
> > always, but sometimes) more vigorous in summer than in winter.
> >
> > You can't take these arguments too far, however, because in highly
> > stable atmospheres, such as those over central alaska in winter, you
> > can get extremely serious scintillation caused by gravity waves that
> > propagate and build up in "waveguides" in the lower regions of the
> > atmosphere.
> >
> > So now that I have everybody confused, just suffice it to say that
> > "yes, its common to have poor seeing in the summer months".
> 
> Hi, Glenn,
> 
> Actually the seeing in most areas is normally much better in the summer
> than it is in the winter. The transparency is of course typically poor in
> the summer because with higher temperatures the atmosphere can hold more
> moisture which reduces transparency but as it turns out the nocturnal
> atmosphere is typically more stable and causes less twinkling of images.
> In a nutshell, here are some of the issues that need to be considered:
> 
> It's true that the ground gets very warm in the summer and creates strong
> turbulence via convection during the daylight hours. However, during the
> night the profile of virtual potential temperature (which is the relevant
> quantity: it's a combination of temperature, moisture mixing ratio (i.e.,
> specific humidity), and gravitational potential energy from altitude) in
> the lower atmosphere frequently evinces a temperature inversion in which
> the virtual potential temperature is increasing with altitude. (Strong
> convective transport requires that the virtual potential temperature
> decrease with altitude. Transport also takes place (in the opposite
> direction) when the virtual potential temperature gradient is in the
> inverted state but the transport coefficient in these situations is much
> reduced relative to that found when it is decreasing with altitude.) This
> shuts down most of the thermal transport (depending on how strong the
> inversion is) such that turbulence is created primarily via mechanical
> means (viz. that due to wind shear which is simply a gradient in the wind
> field) acting to mix the atmosphere. If the low level winds are light to
> moderate, the shear isn't creating much turbulence either and the lower
> atmosphere can be very steady. If you couple this state with light winds
> aloft (typically if the observer is well displaced from the jet stream)
> such that mechanical mixing is minimized, the seeing can be exceptional
> even if the transparency is really lousy. The keys are having a strong
> temperature inversion to turn-off thermal transport and light winds
> throughout the air column to reduce the shear. These are the two major
> mechanisms producing atmospheric turbulence and if neither are present,
> the seeing will be very good even if the transparency rots. These
> conditions are more likely to obtain in most locales in the summer.
> 
> I have little expertise in the area of gravity waves but they don't seem
> to have as much of an effect here in the lower latitudes since the seeing
> is typically much better down here in the summer. Alaska must be a
> different ballgame in that respect!
> 
> Greg Hartke
> Sykesville, MD
> 
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Well, Thanks for the lecture!! 

I teach graduate Atmospheric Thermodynamics at Univ of alaska.

Glenn

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