Why all the gray skies?
It's a reasonable question with a fairly complex answer that we find ourselves asking yearly when they show up and stick around for a few months.
Known as May Gray and June Gloom, the period’s a sign of transition from cool winter weather to scorching summer temperatures.
And to understand what’s going on, first we’ve got to turn to our ocean, which is where the gray skies, or the marine layer, come from.
The air high up in the atmosphere is warming at a rate much faster than the air directly over the ocean itself, which is holding on to those cool winter temperatures.
When warm air masses show up over the ocean, the air cools, condenses as it reaches its dew point, and creates low level stratus clouds or the gray you see in the sky.
Those clouds then push inland, reflecting sunlight back into the atmosphere, away from the ground, keeping things much cooler than they’d be without them.
This scenario is especially common in spring when hot air develops out over the desert, which helps pull the air in from the ocean towards the land. This onshore flow helps the marine layer creep further inland.
Hop in a plane, head a few thousand feet above the coastline, and stick your hand out the window, and you’d feel warmer temperatures than those experienced on the ground.
The warmer air resists mixing with the cooler air below it, acting as a sort of blanket, or what’s known as a temperature inversion. That not only keeps the low level clouds around, but often keeps pollution from escaping, leading to all sorts of air quality issues.
These clouds are more common along the coast, but can stretch to the inland valleys. Our geography influences how far the marine layer manages to creep, with our sizable mountains sometimes holding it back from reaching all of the way out to the desert.
It’s one of the reasons why you may have a cool gray 60-degree day in Santa Monica, while Palmdale’s sweating through a 90-degree scorcher.
Depending on your location, sunrises this week will look like one of these photos. If you are above the deep marine layer (1), or in the deep marine layer (2).
— NWS San Diego (@NWSSanDiego) May 23, 2023
Here's a link to the Area Forecast Discussion: https://t.co/lPx3JhcABu#cawx pic.twitter.com/p33lTbaiHo
The inversion tends to be the strongest in the morning, but eventually the marine layer does burn off.
That happens throughout the day as the sunlight heats the tops of the clouds and that condensed moisture evaporates. But that takes a lot of energy, so it makes sense that in the cooler areas along the coast, the gray might stick around a bit longer than, say, our inland areas further from the water that warm faster.
By August our water temperatures off the coast are warm enough that the inversion layer isn’t as strong, letting the moisture burn off and mix faster than during cooler months.
