Hey, what’s with this marine layer?!?  Well, it looks like the notorious “June Gloom” has arrived a bit early this year.  Many pilots often wonder where it comes from.  Most of us have heard that the number one reason most non-instrument rated pilots become another accident statistic is continued flight into poor visibilities and/or low ceilings.  Let’s see what this coastal fog is all about…

Well, the fancy name for our California summertime coastal fog is “advection fog.”  The dictionary definition of “advection” is, “the transference of heat by horizontal currents of air.”  So a simple definition of advection fog is the formation of clouds at the surface caused by a moist (usually warm) mass of air moving over a colder landmass or water. 

California has a cold-water ocean current that flows South from Alaska just off its shore. This makes the ocean’s surface water temperatures near the coast colder than the surface water temperatures offshore.  In the summer months an area of high pressure forms over the northeast pacific.  This creates northwesterly winds for us in Southern California.  These winds as well as the sea breezes that keep the coastal beaches slightly cooler than the inland valleys in the summertime are the same winds that push cold, moist air onshore that can potentially create fog. 

As the offshore ocean air is blown by wind over the colder water, it gets cooled from below.  If the air is chilled enough to its dew point, fog is formed.  The stronger the sea breeze wind, the thicker the fog can get in altitude. But any more that 15 knots and the wind lifts the fog into a low layer of stratus or stratocumulus clouds. The average thickness of Southern California’s marine layer and coastal fog is usually no higher than 5000 ft MSL.  These summer winds can carry coastal advection fog quite a ways inland, but when it reaches warmer land, it tends to break up or lift into low-lying clouds that block out the sun’s warm rays.  When the fog is carried far enough inland where temperatures are significantly warmer than the coast, the clouds tend to “burn off” or evaporate.

But one of the big reasons for the month-long “gloom” along the beaches is because of a significant temperature inversion.  A temperature inversion is when the air temperature rises as you go up in altitude.  This in turn creates very stable air.  Hence, the lingering low stratus clouds hanging out along the coast all day long.  The next time you fly VFR-on-top, just above this marine layer, take notice of the significantly warmer temperature compared to the cooler surface temperatures under the clouds.

Pilots need to be extra careful when returning to airport destinations along the Southern California coast in the late afternoons because coastal advection fog can roll over an airport in a matter of minutes.  As the sun sets, the air temperature begins to cool.  If it cools to the airmass’ dew point, then one can expect mist and/or fog to reduce visibilities to less than VFR conditions.   Be sure to get a solid preflight weather briefing.  Have alternate airports in mind to divert to before your departure.  Also be sure to take advantage of enroute weather services like Flight Watch and FSS to check your destination’s conditions during cruise (if you are specifically concerned about fog, then be sure to check visibility, temp/dew point, and cloud cover).

Now with all of that said, I do want to remind you that it is fellow humans that are trying to predict what the weather is going to do.  This is not an exact science, so the TAF’s and FA’s are not always precise.  They do their best with the computer models and historical data that they have access to, but sometimes the weather does something completely unpredictable.  So as pilots, we should always expect the unexpected.  Try to have a plan for anything and everything that may not go as anticipated.  Safe pilots are always learning.  If weather is your weak spot, make it your goal to better understand weather in general so that you are not placing so much of your trust and responsibility on those men and women in the FSS that are really just relaying information to you.  I’m not saying run out to your nearest university and get a meteorology degree, but I am saying the more you understand about the one variable in flying that you have the least control over, the safer a pilot you can potentially be. 

If you want to learn more about weather or specifically advection fog, we recommend the DOT’s AC 00-6A “Aviation Weather.”  Go to any internet search engine and you’ll find endless information at your fingertips about many weather subjects.  By all means visit your local NOAA NWS forecast office and meet the meteorologists that create the Area Forecasts and Terminal Aerodrome Forecasts.  Our local office is right here in Oxnard!  The address is:

National Weather Service
Los Angeles/Oxnard Weather Forecast Office
520 North Elevar Street
Oxnard, CA 93030