Hey Reno, it has been a while for me here. Although the atmosphere thickness does defuse daylight more at northern latitudes, the loss is minimal enough where it does not have any real bearing on the size needed. Panels are deliberatly under rated on their outputs because the panels will degrade over time and many panels come with a 20 year power output warranty. My panels up here when oritentated right exceed there rated wattage when they come out of their box. The only time the output is really affected like at the Artic Circle is in the winter time which the sun is at its lowest and is under the horizion hence zero day light.
It would effect the total isolation hours you would get up here but then again so does everything else so the "loss" per say due to atmosphere thickness is minimal since the atmosphere is the thickest at sunrise and sunset and your array is typically not optimally oriented at that time of the day.
