When do schools call a foggy day? It’s rare, but here’s what to do when it happens

The science of fog

Fog limits visibility, delays air travel, brings danger to the roads, and makes things generally spooky. But, how does it form?
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Fog limits visibility, delays air travel, brings danger to the roads, and makes things generally spooky. But, how does it form?

The first foggy school days of the winter season have led some parents to wonder on social media: How bad does it have to get before districts will call a foggy day schedule?

The central San Joaquin Valley’s winter fog is hard to predict, according to Jerald Meadows, a meteorologist with the National Weather Service in Hanford.

“Generally speaking, across the country, fog is very predictable. But here, it’s a completely different animal, because we sit in a big bathtub,” Meadows said.

While the principle of how fog forms remains the same – the difference in air temperature and dew point temperature drops until water droplets form – the Valley’s air patterns and surrounding mountain ranges mean that it mixes differently.

“It sort of sloshes back and forth,” Meadows said.

Valley's Tule Fog (2)

Here’s a handy guide to some of the questions and misconceptions around foggy days, with more information available on the websites of Clovis Unified and Central Unified.

Foggy days are called at Valley schools to help buses avoid the morning commute through dense fog that leads to poor visibility and crashes. They’re not like snow days, no matter how much students might wish they were, as they only affect the transportation schedule. Students who don’t rely on the bus should come to school at the regular start time, where classes will proceed as usual.

Who makes the call?

Foggy days are typically determined by the transportation department. At Central Unified, for example, a morning supervisor will check dew points and temperatures and consult with district spotters “to give distance markers that correlate with the visibility in their area.” If visibility is less than 200 feet, a two-hour delay is called for buses.

Fresno Unified hasn’t called a foggy day in at least 24 years, district spokeswoman Vanessa Ramirez said, as the fog tends to mostly affect outlying areas instead of the city center. Rural districts are more likely to experience the dense fog that makes driving dangerous.

But at districts like Clovis and Central, which cover both rural and more densely populated areas, that can mean that some parts of the district are foggy when others aren’t.

“Please realize that foggy days are called on a Districtwide basis and, due to the size of the district (199 square miles), not all areas in the district have the same fog density,” Clovis Unified’s foggy day information reads. “Therefore, while it may or may not be foggy in your particular area, the decision to delay school is made in the best interest of the students Districtwide.”

Another complicating factor is what time the fog rolls in. To keep buses and drivers on schedule, and to get the information out to parents, most districts have to call a foggy day by about 6 a.m. But what looks to be a clear morning can turn into a foggy one later.

If buses are already on the road, most districts leave it up to individual drivers to determine whether to pull over and wait for visibility to improve. Additional calls are made as the morning progresses, and afternoon buses are expected to run on schedule.

But at the Southwest Transportation Agency, a joint-powers authority that serves 13 school districts in southwest Fresno County, a dense fog caused by the rural region’s irrigated fields can sideline both morning and activity buses.

Though there haven’t been any such foggy days this school year, the 2017-18 school year saw a handful, and previous years have had as many as 15, agency director Sandra Hoevertsz said.

The agency’s anonymous spotters relay visibility from different points throughout the agency’s 650 square mile coverage area, and a call is made according to each individual district’s schedule. What they’re looking for is visibility of 300 feet or less, which Hoevertsz said is like standing in the end zone of a football field and being able to see the goal posts on the other side. The information is gathered in a dispatch area that Hoevertsz refers to as the “9-1-1 room.”

If there’s even a little doubt, a delay is typically called.

What if a bus has to pull over?

When a bus does have to pull over, drivers are instructed to turn off their lights and use their best judgment to decide when to get back on the road, Southwest director of transportation Shelly Thomas said. The students on board typically take it in stride.

“They’re pretty calm. They all have iPhones,” Thomas said.

Thomas said that parents should check the agency’s website each morning for the most updated information.

Parents driving their children to school can also take precautions by checking the National Weather Service’s fog severity index, according to Meadows, which can estimate the day’s fog patterns the night before.

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Meadows said that the index should be taken into account when planning a trip to determine timing and direction. If you’re caught in the fog, you may be stuck there for a while.

“Around here, the fog will linger for hours,” Meadows said. “Delaying your trip is better.”