North Texans have been experiencing erratic weather in the region as of late, and the most recent variable in the climate alphabet soup is fog.
Vaporized water clouds are considered dangerous for lowering visibility on the roads and causing travel disruptions, and this week’s fog was no exception.
The National Weather Service issued a dense fog advisory around midnight Wednesday until Thursday morning at 9 a.m. for parts of D-FW, warning people to slow down and use their low-beam headlights.
For portions of northeast Texas, including Dallas, Arlington and Denton, visibility went down to a quarter of a mile early Thursday morning, said NWS meteorologist Allison Prater.
“With areas of dense fog, visibility is going to decrease quickly …” Prater said. “It’s very low compared to normal times where you can see miles away.”
While the weather service has not issued an advisory since, foggy and cloudy conditions persisted Friday morning in downtown Dallas.
Here’s what causes fog and what drivers can do to stay safe during foggy weather.
What causes fog to form?
The Dallas area typically sees two types of fog: advection and radiation, and each one warrants different weather conditions.
This week, Dallas residents likely witnessed advection fog, Prater said. It is caused by warm, moist air flowing over a cooler surface, and usually does not involve cooler weather.
“When air that’s more moist, the warmer air is moved over cooler and drier air,” she said. “That warmer air is going to cool down and saturate — and that produces lower clouds.”
Over the last week, rain and some rumbles of thunderstorms have swept over the area, causing some amount of moisture and humidity in the air. Advection fog is most well known in San Francisco as one the city’s signature features.
Meanwhile, radiation fog forms overnight under a cloudless sky and calm winds. Fog begins to form when the surface temperature drops, producing especially dense clouds and potentially reducing visibility to near zero.
“With the light winds and the clear skies, we’re going to be able to get more radiation, or radiational cooling,” Prater said.
Radiation fog forms early in the morning and dissipates under warm sunlight. It is most prevalent during the fall and winter, according to NWS.
Other forms of fog include freezing, evaporation and hail, which aren’t nearly as common in Dallas.
How to drive safe in foggy conditions
During a dense fog advisory, the weather service warns drivers to be aware of road conditions.
Some other driving tips from the weather service include:
Use slow-beam headlights — never use your high-beam headlights which can cause glare
Keep distance between you and the car in front of you
Slow down and make sure you have plenty of time to get to your destination
When visibility is near zero, turn on your hazards and pull over if you can
