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

[Greg Hartke <ghartke@clark.net>]

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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