Sun-blocking clouds are trapped in place. The added clouds have allowed temperatures to cool early Thursday afternoon. An inversion implies there is a reversal in the way temperatures are distributed, cooling with height. Temperatures are actually warmer one mile up above.
Winds are light and are needed to increase to mix the atmosphere up bringing milder, drier air down to the surface.
A great view of how the inversion forms is captured below. Moist air (snow melting) is added back to the air & trapped as low clouds. The view from Lebanon in Boone county early Thursday morning and again Thursday afternoon.
Also note the sky camera over Indianapolis late day. Very little wind is blowing, proof by the steam/smoke stacks emissions steady rise. The moist air is already cutting visibility - eventually the fog that forms overngiht.
The result is a chilly afternoon. When the sun was out Thursday we reached 38° but fell back to 33° by 4 PM as clouds thickened.
Cloudy, hazy and eventually foggy conditions will develop overnight. Be patient, Friday may open gloomy but should brighten by early afternoon. With brighter skies in the forecast comes milder afternoons. High temperatures this weekend are forecast or exceed 50°.