Weather 101: How Does Fog Form?

Weather 101

Fog formation is extremely common in Northwest Arkansas and the River Valley and can cause a nightmare for drivers.

Fog in Hogeye. Photo: Joe Young

Fog is actually a stratus clouds that forms very close to the ground when the air temperature and dewpoint temperature comes very close together and winds are calm or very light.

The dewpoint temperature represents the temperature at which saturation occurs. It also is the lowest value the air temperature can drop to at that time. To learn more about the dewpoint, check out Meteorologist Alexander Williams’s Weather Word Of The Week segment here.

Dewpoint is a measurement of moisture in the air. When the temperature reaches the dewpoint,
saturation occurs. In this scenario, saturation would occur when the air temperature falls to the low 60s.

An important thing to remember when looking for fog is calm or light winds. If the air temperature and dewpoint meet but the winds are strong, it will be harder for fog to form.

The left graphic below shows calm winds in multiple locations throughout Northwest Arkansas and the River Valley (occurred on October 21, 2020 at 5:54 AM). Fog occurred throughout the morning

The graphic on the right shows wind speeds in the region at 6:28 P.M. on October 22, 2020. In this scenario, fog would not form even if the temperature and dewpoint meet.

Also, evapotranspiration from vegetation can add moisture to the air. This is why you may see denser fog over a field compared to areas with a lot of roads. You can learn more about how evapotranspiration in a previous Weather 101 episode by clicking here.

Morning fog developing near tree in Northwest Arkansas. Photo: Amanda Edens

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