Southern Californians can usually set their watches to the arrival of May Gray and June Gloom every spring.

But why are morning skies so reliably cloudy this time of year and not others?

It has to do with the marine layer, said Rachel Schwartz of the Scripps Institution of Oceanography.

The marine layer is a low-lying mass of hazy, moist air that forms over the ocean and sometimes gets blown ashore at night.

This effect is especially strong in May and June when a wind pattern known as the Catalina eddy is at its peak. During this period, the marine layer can be swept as far inland as Palmdale.

Schwartz said in May and June, those marine layer clouds get trapped over the coast because of a strong ceiling of warmer air.

“They literally hit a top in the atmosphere that kind of acts like a lid," she said. This lid, she says, keeps the gloom from dissipating as usual, causing it to create a uniform grey across the sky.

This lid-like system of warm air rises at the equator and sinks around Southern California, creating this atmospheric ceiling effect.

In the winter the presence of this warm air is not as strong in our region and is usually broken up by storm systems, so it can’t trap the gloom as easily. Later in the summer, the ocean water heats up, so conditions aren't right for the marine layer's hazy blanket.

Schwartz says it takes just the right conditions to create a sustained cloud cover.

“They kind of align in May and June.”

There are only a handful of other places in the world that have the right combination of characteristics to see a yearly spring gloom. Those places include the Canary Islands, Peru and Namibia in Southern Africa.

So even though it may seem gray and ho-hum, these gloomy mornings are a pretty unique thing, atmospherically speaking.