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The Ins and Outs of Fog

Occasionally fog can wreak havoc on the morning commute before burning off in the mid-morning hours. This is what happened yesterday with visibility dropping below an eighth of a mile at times.

6:30 AM 8/6/2019

Here is a viewer submitted photo of the fog:

Danny Murphy – South of Fairbank 6:40 AM 8/6/2019

This is known as radiation fog and is very common to eastern Iowa. Typically, the ingredients needed for this type of fog are: calm or light winds, a clear sky, saturated ground, and an increased relative humidity.

Increased Relative Humidity

The night before, a damaging line of storms rolled through which you can read about here and here.

The late evening rain lowered temperatures while the dew point remained relatively the same. In other words, the temperatures and dew point ended up very close together, resulting in a high relative humidity.

A high relative humidity is good for fog development because temperatures do not need to cool as much for the water vapor in the air to condense into liquid and develop a cloud.

Saturated Ground

The line of storms brought with it a healthy dose of rainfall after a dry month. Most areas picked up between 0.5″ and 1.5″ of rain. You can find a full list of rain totals here.

24 hour rainfall as of 8/6/2019

Even though the rain ended in eastern Iowa around midnight, there was enough rain on the ground to keep things saturated 5 or 6 hours later. A wet soil will continuously evaporate moisture into the air which will keep the dew point close to the temperature.

Light Winds

During the newscast, I described the atmosphere as a bowl of soup. If it isn’t mixed around from time to time, things kind of just sit at the bottom.

In terms of weather, the wind acts like a big stirring stick, helping to mix things around. If the wind is strong, the evaporating moisture from the ground will mix with drier air aloft. If the wind is absolutely calm, we could get a heavy dew or frost on the grass. A light wind will be just enough to mix some moisture a few feet above the surface but not into too dry of air. Light winds keep the dew point and temperature close together.

Winds on the morning of August 6th were in that sweet spot between 0 and 7 mph.

Clear Skies

The last main ingredient for radiation fog is clear skies. When skies are clear at night, we lose a maximum amount of radiation from the earth to space. That’s a fancy way of saying when skies are clear, temperatures cool off the most because clouds cannot trap that radiation (or heat) underneath.

The more temperatures cool, the quicker they can meet the dew point to develop fog. Skies were able to clear early in the morning, allowing temperatures to drop heading toward sunrise.


All of the ingredients came together Tuesday morning to develop some thick fog. When fog develops remember to take it slow and avoid using your brights. As always, avoid driving distracted.

Fog will burn off once the sun comes up and some solar energy can penetrate the clouds and hit the ground. Once the temperatures warm up, they slip away from the dew point and relative humidity drops. At that point, the air cannot sustain the fog.

Author Profile Photo

Brandon Libby


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