Voyager: Can May Gray be Predicted?

– Mark M., San Diego, Calif.

How well “May Gray” can be predicted depends on how far ahead we are forecasting. At forecast times up to a couple of days, the May Gray type clouds can be predicted fairly well at the coast, but predicting how cloudy an entire month or season will be is much more difficult. However although we can’t predict exactly how cloudy May will be, we can look at conditions in the ocean and atmosphere to get hints of what to expect.

First, let’s better understand “May Gray” and “June Gloom.” These terms refer to low clouds in the marine atmospheric boundary layer (or for short, marine layer) in coastal California. These are low stratus clouds, which means they are not puffy clouds (like cumulus), but rather clouds that form in a layer and create overcast, gray and often gloomy sky conditions. They form in the lower part of the atmosphere, in what is known as the boundary layer, which is influenced by the earth’s surface, in our case, the ocean. The marine layer often has a distinct top – the top of the stratus cloud. The air above the cloud top is stable, acting like a lid, because there is warmer air above cooler air and this makes it hard for air to mix across the marine layer.

Why is there warmer air above the cool moist marine air? This is due to large-scale circulation, like a very large circuit around the globe, in what are called Hadley cells. Near the equator, warm air rises. This air then moves poleward high in the atmosphere, then sinks around latitude 30˚, and once back near the surface flows towards the equator. The sinking air in this circulation, which is dry and becomes even warmer as it sinks, is critical to making the stable top of the boundary layer.

Right above the ocean surface, the air is moist and cool. Mixing will force some surface air upwards, resulting in cooling. If the cooling is strong enough, condensation will occur and a cloud can start to form. The stable top of the marine layer helps prevent mixing of dry air from above resulting in horizontally extensive low-lying clouds. In Southern California, atmospheric and oceanic conditions are optimum for these marine layer clouds to form and stick around the beaches frequently in late spring and summer, thus their common names, May Gray and June Gloom.

The temperature of the ocean surface – sea surface temperature or SST – influences marine layer clouds. Cooler than normal SST helps create more May Gray type clouds than normal. It also helps to make that cap on the marine boundary layer stronger and that cap keeps the stratus clouds from mixing up and dissipating into the warm dry air above. In addition, when SST is cooler than normal, and inland temperatures are warm, we can have stronger sea breeze winds blowing toward land. Over the last few months, SST off the coast of California has been slightly warmer than normal. This gives us a hint that we might have fewer than normal marine layer clouds this summer – but there is no guarantee.

For forecasting marine layer clouds on shorter scales of a few hours or days in the future, researchers can look at other conditions in the atmosphere and use satellites to see the full extent of the cloud deck. Two websites made by Scripps Institution of Oceanography, UC San Diego, researchers help better predict marine layer clouds on these short time scales. Find a prediction for morning marine layer clouds in San Diego that uses current information about the marine layer, winds, and humidity. Automatically match your uploaded real-time marine layer cloud photos with the satellite view from above. By looking at satellite images and viewing the clouds over a larger region, forecasters can better predict how the clouds at a given location will change in the near future.


– Rachel Schwartz is a fifth-year graduate student in the research group of climate scientist Alexander Gershunov

Related Image Gallery: Can May Gray be Predicted?

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