Wednesday, November 2nd, 2022 will go down as one of the foggiest days in recent memory. In many locations, the visibility was close to 0 miles throughout the night and well into the morning hours of Wednesday.
Now, fog is not an uncommon phenomenon in the Valley, but fog lasting into the afternoon hours is not common. Why was the fog so bad this past week? Well, today I will dive into the science of fog.
First, there is some background reading that you can do to help with this explanation. A couple of months ago I wrote about the different types of fog and how they develop.
Fog requires two main ingredients
The development of fog requires two main ingredients: moisture at the surface and calm winds. There had been some showers across the area on Monday and Tuesday morning. Furthermore, the clouds during the day on Tuesday kept the ground damp and prevented the water from evaporating during the day. The first ingredient was in place.
The second ingredient is harder to achieve and can be difficult to predict. The winds during the night Tuesday and Wednesday was at 0 mph at multiple times during the night. Below are the wind observations from the Youngstown/Warren Regional Airport starting at 06 UTC Tuesday (2 a.m.) and ending at 12 UTC Saturday (8 a.m.).
Notice that there was a long stretch of time on Wednesday where the surface wind was 0 mph. This is crucial for the development of fog. If the wind continues to blow overnight, it brings down warmer air from above the surface which does not allow the temperature to cool as effectively to the condensation temperature.
Calm winds are easier to achieve at the surface due to surface friction from the ground, trees, and buildings which slows the wind down. However, as the sun comes up, stronger winds will usually mix down to the surface and evaporate the fog.
The only issue on Wednesday is that there were no stronger winds above the surface. Below is an image of the wind speeds at a air pressure of 700 millibars (or around 10,000 feet above the surface of the Earth) at 8 a.m. on Wednesday morning. Notice that the winds are around 2 knots (or about 2.5 mph)! Those are extremely weak winds for that level of the atmosphere.
In fact, the average wind speed at this height during early November is around 30 MPH and a wind speed of 3 MPH might be one of the lowest wind speeds ever recorded at 700 millibars for this time of year.
In conclusion, a couple of factors combined to produce very dense fog on Wednesday and Thursday mornings:
1) Moisture left over from rain showers on Tuesday morning persisted into the morning Wednesday
2) Anomalously calm winds above the surface of the atmosphere allowed the dense fog to develop and persist throughout the day on Wednesday
All of these factors combined to produce sights such as this one from Wednesday morning.
