What is it about weather in Indiana that you have always wondered about? Does it relate to why tornadoes form more often in certain months than others? Or maybe you wonder why it frosts earlier in some parts of the state than others?

That’s the purpose of Weather Wise: answering those questions about Indiana weather that you always wondered about, but never got around to asking.

Ken Scheeringa, associate state climatologist, provides the answers. This month’s question explains why Hoosiers typically see more foggy days in the fall than during other times of the year.

Why do we have more fog in autumn?

CLEAR SKIES FAVOR FOG: You likely know clear skies toward evening favor frost, especially if it’s calm. Similar conditions in fall also favor fog formation in the early morning hours.

Scheeringa:  At this time of year, we seem to wake up to more foggy mornings in Indiana.  Daylight is growing shorter by the day, and this allows air temperature to cool for more hours before dawn.  Cool air cannot hold as much moisture as warmer air can. So water vapor in the air condenses and forms tiny moisture droplets, or “clouds on the ground.”  We call it fog.

What happens when the sun comes up?

Scheeringa: When the sun rises, its heat energy warms the air. As the air warms up, its water-holding capacity increases. The fog layer gradually thins and disappears. Many old-timers would say it "burned off," and the phrase is still used today.

Do other factors contribute to more fog in fall?

Scheeringa: Yes. Fog is also more common in autumn because overnight skies are more often clear and winds calm.  On windy nights, air mixes more completely at all levels. This prohibits the coolest air from hugging the ground and condensing into fog.  Air is typically coolest at the surface at dawn because daytime ground heat is being radiated away from the ground and back into the atmosphere as long-wave energy. 

What happens in late fall?

Scheeringa: Fog becomes less frequent by late autumn. Storminess generally increases into the cold season. Conditions become less favorable for fog formation as the calendar moves deeper into autumn.