However if you place a surface perpendicular towards the sun, as the windows of a car are, or a tracking solar panel, then the latitude is IRRELEVANT!
I completely got that. Really. As I said in my post:
Irrelevant, because we are comparing equivalent scenarios....A vertical pane of glass may expose more area for solar radiation to enter; but that is the true equally at the poles and in DC.
In other words, if you were to compare ANY situation you can dream of-- a greenhouse, a car, a solar panel-- in DC in the winter and the north pole in the summer, they will be very nearly identical, because:
In the northern hemisphere's winter, the sun is ~61 degrees south of DC (38 degree latitude + 23 degree sun tilt). In the summer, the north pole will be at a 67 degree tilt from the sun (90 degree latitude - 23 degree tilt).
I dont know how else to explain this. Where the sun is in the winter for DC is very nearly the same (in relative terms) for where it is for the pole in the summer-- ~65 degrees off of perpendicular. No scenario you set up will change it.
And for the record, summer in siberia is (according to wikipedia) 86 degrees-- which is a relatively mild summer day for DC. Its not remotely close to one of the hottest places; those would be between the 2 tropics (cancer and capricorn). Its also worth noting that Siberia is a huge area covering from 50 degrees north to 90 degrees north, so saying its temperature variation is huge doesnt say much. Some of its areas are halfway to the equator.