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May Gray flips to June Gloom in San Diego

Why do these two months traditionally bring overcast skies to San Diego?

SAN DIEGO COUNTY, Calif. — San Diego is widely known for its mild and dry climate and sunny weather by those that live here as well as those that visit.  While the sun does traditionally shine during most months of the year, May and June tend to bring a little different weather to the region.

What is May Gray and June Gloom?

The marine layer is a common weather phenomenon along the California coast. What makes May and June exceptional months for this overcast pattern is cold ocean water temperatures. 

The deep dips in the jet stream from areas of low pressure are not as intense as we transition from Winter and Spring to the Summer months. Stronger systems would clear out the cloud cover, we saw this the morning of May 31 as an area of low pressure exited east into Arizona, and there was a brief period of sunshine along the San Diego coastline. 

The sun intensifies during the Summer months due to a change in its angle. You'll see this reflected in higher UV index readings ranging from high to very high rather than more moderate readings in Winter. 

Additionally, the sun heats up the Earth's surface and air temperatures at a faster rate. The ocean absorbs heat. The marine layer forms when warm, moist air moves over cool ocean waters. The marine air of the Pacific Ocean is cooler than the air above it. Warm air above it holds more moisture than cold air. 

Air temperatures normally decrease with height. But due to the cold water, the air temperature increases with height resulting in temperature inversion. As a result, a strong cloud deck forms because the warm air traps the cooler air.

The air below the inversion is called the marine layer. This is the point in the atmosphere where the air is cooled and clouds form. Onshore flow pushes the cloud cover to the coastal mountain slopes and locks it in across the coastal basin -- areas west of the mountains. Visually, think about when you open a soft drink, it makes that hissing sound, and you see the fogginess that forms in the open space between the beverage and the cap. If you immediately put the cap back on the soft drink, the fogginess will linger. 

Why is May overcast in San Diego?

At the beginning of May, the ocean water was still cool with water temperatures still in the 50s and now the low 60s and climbing. The presence of upper low-pressure systems like the one that wrapped up the month kept the marine layer extended. The return of a weak ridge of high pressure will create a shallower marine layer, confining the cloud cover to the coast (stubborn to clear), as inland clouds lift aka evaporate. This will allow for more afternoon sunshine away from the influence of the ocean.

Why is June so gloomy in San Diego?

Transitioning into June Gloom, more cloud cover is expected due to another California staple, a coastal eddy aka a Catalina eddy. Due to the Catalina Islands, a wind pattern develops near this body of land. The air moves counterclockwise like a mini-low pressure system over the cool waters and eventually pushes stratus clouds in our direction. The development of low clouds and fog can linger throughout the day.

Looking ahead to June, overall temperatures for the month are leaning below seasonal with more clouds on deck and chances for seasonal precipitation, which is usually zero to a few hundredths of an inch on most days. 

Credit: KFMB

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