The answer largely has to do with the season and our latitude.

Once a solid cloud deck forms it is difficult for the weak incoming sunlight this time of year to heat the earth’s surface and warm lower levels of the atmosphere, thereby dissipating the cloud deck. Also, longer nights allow the lower atmosphere to cool more. In summer, more intense sunlight heats the lower atmosphere allowing it to ventilate, or mix. This process typically will “burn off” fog and low clouds within a few hours of sunrise.

Vertical Relative Humidity Profile for Midway Airport (Image 1)

This past week provided an excellent example of why December skies are most often gray. Three vertical cross sections of the atmosphere over Midway Airport are shown. The first is relative humidity (RH). The dark blue/purple shade shows air that is nearly saturated. Note the shallow layer of high RH below 5 thousand feet. This suggests low clouds and perhaps fog.

Vertical Temperature Profile for Midway Airport (Image 2)

Image 2 is a what can be interpreted as a vertical temperature profile. Closely packed temperature lines on this depiction represent areas where temperatures increase rapidly with altitude and indicate the location of temperature inversions. Again, note these areas generally at or below 5,000 ft. This means that cool air near the ground has become trapped and stagnated by milder air aloft.

Vertical Wind Profile for Midway Airport (Image 3)

Finally, the wind profile in image 3 shows very light wind flow in the cool low-level air mass. With little mixing of the air, fog and low clouds trapped in this cool layer tend to just sit…sometimes for days.
Similar conditions are expected through this weekend.