Patrick, It really has been unusual. It has been a great year for clear, dewless nights, but the smoke has been terrible for astrophotography. It has really been tough to see a clear sky filled with smoke every night. We need snow in Idaho to put out those fires! Tyler Tyler: A thin layer of cirrus cloud will act like a blanket and trap the heat on the ground so that the temperature doesn't fall below the dewpoint. This layer is often hard to even detect unless your trying to view galaxies through it. The moon, Jupiter, Saturn, Mars, and bright double stars can burn through it but detail in M101 or M97 just disapears. Data collecting like transit searches are protected by callibration against adjacent stars in the same field. Ground level dew is only part of the problem. I regularly view the U of Wyoming's sounding map website. It tells me the total water in the air all the way up. When less than 10cm the conditions are good. This year we have been consistantly of 20cm for weeks at a time. When it's this high you will always find a layer of air somewhere in the sky with a RH above 50%. That's the cirrus layer that kills the deep space viewing. DT