California is known throughout the rest of the country for its beach days and sunshine. But locals know once the calendar turns to May things change. May Gray is a term that locals use for the endless cloud cover that hovers over the coastal areas of California.

This phenomenon is caused by something called a “marine layer.” This occurs because the Pacific Ocean is chilly, which keeps the air just above the ocean cooler than the air higher in the atmosphere. This creates a temperature inversion - when cooler air is trapped below a layer of warmer air. The inversion produces a blanket of clouds over the coastal areas, but can sometimes be carried inland as well.

Sometimes May Gray can dissipate in the afternoon, once the sun rises high enough to warm the air. But some days, that marine layer can stubbornly stay in place, leading to a whole day of cloud cover.

The marine layer is one of the reasons why coastal areas, particularly in Southern California, are cooler than inland areas. The cooler air off the ocean acts like an air conditioner, keeping temperatures mild.

And if you think May Gray is bad enough, just wait until June Gloom rolls in.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.