‘May Gray' overcast weather is typical in San Diego, but why?

San Diegans know a cloudy day at the coast all too well, especially during the spring and summer months. "Gray-pril," "May Gray," "June Gloom," "No Sky July" and "Fog-ust" are common phrases to describe the late spring and summer weather patterns.

Rachel Clemesha, a project scientist at Scripps Institution of Oceanography at UC San Diego, studies clouds.

"Some people love marine layer clouds, and some people, I don’t want to use the word hate, but very strongly dislike, so it’s kind of a spectrum," Clemesha said.

For those who love or hate them, there's much to understand about these stratiform clouds that are so commonly found on the California coast, hiding the sunshine.

"Here, we have the California current, so this water is coming from the north, from the polar region, and we have a lot of upwelling, so deep water coming up," Clemesha said. "So it’s really that contrast between that cool ocean and then that warm air above that helps promote these clouds.”

Several factors contribute to this weather pattern. Topography, upper-level winds and ocean temperature can play a role in an overcast day.

"The setup that we have that relates to these low-level clouds relates to the fact that we have low-level subsidence in the subtropics, so air rises at the equator," Clemesha said. "And then it sinks in the subtropics, so that’s the biggest circulation on the globe, and that’s what helps promote that warm air above our moist marine air.”

The rest, in part, has to do with the cool moist air above the ocean and the warm air that sits just above that, Clemesha says. Think of it as a lid on the lower part of the atmosphere.

“As there is mixing from the bottom, you can create a cloud as relative humidity reaches 100%, and then the top of that cloud is gonna be very horizontally uniform from the top because it can’t mix from the top," Clemesha said. "There’s this cap on it, so we call that a temperature inversion.”

According to Clemesha, how strong that temperature inversion is and how high or low that temperature is will determine how likely it is to clear out and how far inland those clouds can go.

For example, Clemesha shared on Wednesday that the temperature inversion at San Diego's coast was 8 degrees Celsius.

"That means there’s this lid and that’s how strong it is, there’s this 8-degree difference Celsius between that lower air and that warm air above it, but what’s really important is how high that temperature inversion is," Clemesha said.

Clemesha says that inversion tends to be higher in May and June, which can help these clouds settle in over inland areas. They can also be stubborn to clear at the coast as summer progresses.

"If you had the experience where you’re hot at your house, you say, 'Let's do a beach day.' You drive to the coast. It’s still cool and cloudy day," Clemesha said. "It’s probably a day we still have a temperature inversion, but it’s just lower.”

If you live in Southern California, you know exactly what she's talking about. What happens when these clouds mix out and we get clear? Blue skies with sunshine?

"As the air warms, there is more mixing that can occur as that air that is lower, closer to the surface rises up, and it’s warm and dry, so it helps mix out. We’ve then taken the saturated layer and added warm, dry air, and so you help dissipate the cloud," Clemesha said.