Wondering why there was a thick layer of fog over the city this
morning and afternoon? This fog is called advection fog. It
forms as a warm, moist air mass settles over the cooler Lake
Michigan waters. Hot and humid air (temps in the 90s, dew points
in the 70s) just west of Chicago comes over the cool Lake
Michigan waters (60s and low 70s) and forms fog. The near-
surface air is cooled to the high dew point (in the 70s) and fog
forms. Then, the lake breeze transports the fog from the lake
back onto land. The fog is concentrated at very low levels of
the atmosphere, with the tops of the taller buildings likely
experiencing temperatures in the 90s above the fog layer,
because the relatively cool air off of Lake Michigan is quite
shallow, the temperature is cooled to the dew point only at
near-surface levels. Given the strength of the sun and mixing of
higher temperatures just above the ground, the air temperatures
even near Lake Michigan should slowly rise above the dew point
and the fog will dissipate. What doe