What Causes June Gloom? A Scientist Explains
Tourists arriving in San Diego this month hoping for a vacation on the beach may be sorely disappointed. That’s because of an ugly blend of heavy clouds and cool weather that locals call "June Gloom."
The gloom is caused by the combination of cold water and higher pressure in the atmosphere, said Sam Iacobellis, a research specialist at Scripps Institution of Oceanography.
"We see this June Gloom and May Gray for a reason," he said. "That’s when the high pressure is pretty strong. This high pressure probably reaches its peak in June and July. And also the water in May and June still hasn’t warmed up to its summer temperature, it’s still pretty much on the cool side."
The cold water and high pressure act together to create what's called a "temperature inversion," Iacobellis said, where cooler air is found closer to the ground and air temperature increases with height. That's the reverse of what you normally find, he said.
Temperature inversions that hang around for awhile make clouds form, he said. But when the air pressure isn't as high, air can mix more, which means fewer clouds. As the sun heats up the air during the day, the warmer air mixes with the clouds and can help break them apart. That's why the gloom often fades away by afternoon.
While San Diego’s beaches have cold water during most of the year, Iacobellis said it’s only in May and June when the atmospheric pressure is high enough to trap clouds so they hover low over the city. That cloud blanket captures the cold air coming off the ocean to create the gloom.
"It’s two things acting together and May and June seem to be the time when we get the most amount of these clouds," he said.
Sea breezes that come from the water to the land also help keep the clouds along the coast, he said.
After June, the gloom should fade away. But if not, we may need to start using the names “Gray Sky July,” or even, as the science blog The Last Word On Nothing suggests, “Fogust.”