Why is it on foggy days, when the fog lifts, the sky is clear with no clouds?
They key in understanding the reason for this requires understanding the conditions leading to fog: high pressure, moist ground, light winds and clear skies aloft. This combination allows substantial cooling over the lowest part of the atmosphere in contact with the ground. That leads to condensation of moisture into water droplets from the damp ground (usually overnight)...but once the sun rises, the low clouds/fog burn off. Because the moisture is concentrated near the ground, there is little if any cloud cover higher up in the atmosphere....and that means clearing once the fog burns away.