How the Jet Stream impacts astronomical “seeing”

“Seeing” refers to the the amount of “blurring” caused by the earth’s atmosphere — most apparent in a telescope using higher magnifications.  Astronomers refer to this atmospheric turmoil as “bad seeing”.  Seeing well at high magnifications is only possible when the air is steady.
Excellent seeing means, at high magnification, you will see fine detail on planets.  In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished.
Bad seeing is caused by atmospheric turbulence combined with temperature differences in the atmosphere.  The turbulence may be caused by ground level winds as well as higher level air movements created by the fast flowing, meandering jet stream.  When the jet stream is overhead, seeing is generally poor.
Bad seeing can occur during perfectly clear weather.  Often good seeing occurs during poor transparency.  It’s because seeing is not very related to the water vapor content (transparency) of the air.