Seeing is the scintillation of the sky that causes star to twinkle.  As wonderful as twinkling stars are, bad seeing causes astrophotos to be less sharp.  Here is a link to a webpage called StormSurfing.  If you live in North America, this shows the current and future atmospheric jet streams.  This can help predict the seeing in your area.  The uncolored areas are least effected by the jet streams, the blue areas are lightly effected, and the other colors indicate bad seeing.  The webpage also allows you to explore other areas of the world, and other useful weather maps.