A thin layer of haze hovers above San Francisco Bay on hot days like Wednesday and Thursday.

It's as if a massive Instagram filter is covering this vast body of water, creating gauzy, ethereal vistas.

Sometimes that filter is more grayish and translucent, and sometimes it's a yellowish brown, giving views a sepia tone.

Meteorologists say the haziness is essentially a result of pollutants, and the color depends on atmospheric temperatures.

These green house gases spewed from car tailpipes and factories often get swept into the Central Valley and into the South Bay and mixed higher into the atmosphere by ocean breezes on cooler days.

"But on hot days the wind has usually abated and we don't have anything to mix out the pollutants that are in the atmosphere," Meteorologist Jan Null of Golden Gate Weather Services explains.

What's more, hot weather is often caused by high pressure systems that act as a cap so air doesn't rise, Null adds. "You can also have a temperature inversion with warmer air aloft and that acts as a cap too. It all works together."

The nitrogen oxides and other volatile organic compounds in the air appear grayish at the start of a hot day when it's cooler, a precursor to smog. If things heat up, they get cooked by the sun, a chemical reaction occurs and they turn a little yellow, a little brown, forming smog.

The haze is hovering close to the ground on land as well, but is more visible over the bay because it's mixed with moisture.

"It's pollution and basically it's a visual indicator of why people should try to take carpool or public transit instead of driving to work," says Charley Knoderer, the supervising air quality meteorologist with the Bay Area Air Quality Management District.