(WKBN) – The Valley has been impacted by dense fog over the past few days. Thursday, September 14 marks the third day of fog this week.

Why has there been so much fog this week?

There are three key factors in the formation of fog.

The first factor is a clear night, which is required for the formation of the fog itself. Clear conditions allow for the temperature to drop to the lowest possible value. What is the lowest possible value? The number is governed by the dewpoint temperature, which is famously named for the temperature at which dew forms.

This graphic shows the developmental stage of fog where cool air resides at the surface and warmer air is above.

When the temperature drops to equal the dewpoint temperature, water can then change phases from vapor to liquid, also known as condensation. Fog, similar to a cloud, is just a collection of tiny water vapor molecules.

The next requirement that is needed for fog to develop is a calm wind. If the wind is not calm, than any fog that tries to develop will be pushed away by the wind before it can become dense. This has happened at the Youngstown/Warren Regional Airport over the past couple of days. Here is a look at observations from the airport Thursday morning:

Weather observations from the Youngstown/Warren Regional Airport from 4:51 to 6:51 a.m. on Thursday, September 14, 2023 (Data courtesy of the National Weather Service).

First, notice that the air temperature and the dewpoint temperature are very close to each other. The air was near saturation for two hours Thursday morning, resulting in a relative humidity close to or at 100%.

The other main detail to notice is that the windspeed was at 0 mph for this entire period. This results in weather conditions of either “mist” or “fog” in the second to last column.

The last key factor is to have some moisture in the ground during the night. This usually comes from rain that has occurred earlier in the day. This area has had rain showers the past two days, so this has occurred.

All three of these factors have resulted in images like this one in Sharon over the past week:

Image of dense fog in Sharon, Pa. on Thursday, September 14.

Thankfully, once the fog clears out later Thursday, there will be sunny skies.