Why? The nights are long and there is lots of time for the earth to cool off as it radiates infrared energy to space. But you need one more thing...clear or nearly clear skies...which still occur now, but happen less often in the middle of the winter. Why? Clouds intercept infrared radiation from the surface and emit some back down to the surface, thus reducing the cooling. So temp drops on clear, autumn nights while the dewpoint, the temp at which dew (or fog) forms, generally remains near constant.
Take a look at the plot of temperature and dewpoint at the top of the UW Atmos Sci building... temp fell through the night, but still hasn't reached the dewpoint. No fog at the UW yet. But look at the other plot..at Sea Tac Airport... The temp dropped and dewpoint rose a bit...and they met....and there is fog there.
On clear nights, the lowest temps are often near or just after sunrise...so fog can form very late sometimes....
Today should be very nice, particularly after the fog burns off...and it still looks like a weak front will make its way across us tomorrow evening...so tomorrow should also be good as well.
