This illustration shows how the Pacific High expands and the Aleutian Low contracts between winter (top) and summer (bottom). Storms tend to travel along the northern edge of the Pacific High. The red dot indicates the location of the Central California Coast.
By May, the water of the Pacific Ocean between California and Hawaii has become relatively warm (over 60 degrees F). This allows a large "bubble" of warm air, known as the Pacific High, to expand northward and become stronger.
By May, the Pacific High is growing in strength, and deflects most storms moving from the Gulf of Alaska so that they move eastward toward Oregon or Washington. Protected by the Pacific High, the Central Coast rarely sees rain after mid May. However, the weakened remnants of some low pressure systems may occasionally bring fog or drizzle to the area. Other low pressure systems pass just north of the Central Coast, bringing strong northwest winds to the area.
Note: The Pacific High is not just a summer phenomenon. It often forms during the winter months, but is centered fairly far south, and is relatively weak. A southerly flowing jet stream and strong winter storms may disrupt or dissipate the Pacific High for weeks at a time.
The northwest winds and upwelling that begin in March and ramp up in April become most consistent in May. By late April or early May, strong northwest winds and upwelling may become nearly continuous over the coastal waters within 10 to 20 miles of the coast.
Note: April may see even stronger northwest winds than May. However, the northwest winds are less consistent in April, because they alternate with southeast winds from storms that manage to push aside the Pacific High and reach the Central Coast.
May often sees northwest winds howling for days on end, bringing huge volumes cold water up toward the sea surface. This upwelling of cold water is strongest where points of land jut out from the coast, such as at Point Arena, Point Reyes, Año Nuevo, and Point Sur. From these "upwelling centers" the cold water flows far out to sea, as well as southward along the coast.
This illustration shows a plume of cold water (blue) known as the Davenport Upwelling Plume, which moves southward along the coast from the upwelling center at Point Año Nuevo, and across Monterey Bay
(Source: David Fierstein/MBARI)
Just south of Point Año Nuevo, for example, the Davenport Upwelling Plume
reaches maximum velocity and strength in May. After forming near Point Año Nuevo, this river of cold water flows along the coast northwest of Santa Cruz, then streams out across the mouth of Monterey Bay. Sometimes the Davenport plume hits the Monterey Peninsula. Other times it flows farther offshore and continues down along the Big Sur coast.
By mid May, upwelling plumes have affected most of the Central California coast, bringing the coldest water of the year to the waters of the outer continental shelf (10 to 20 miles offshore).
By early May, the Pacific High is deflecting most storms northward, away from the Central Coast. Thus, rain is unlikely. At the same time, the atmosphere over the ocean is still somewhat unstable and well mixed, while inland areas are still relatively cool. This reduces the chances that fog will form at the coast.
Far out at sea, the Pacific High has become so strong that it doesn't move much. This allows a stable layer of cool, moist air to form just above the ocean surface. This layer is typically 500 to 1,000 feet thick and may covering hundreds miles of open ocean.
Along upper edge of this "marine layer", the cool, moist ocean air comes in contact with much warmer air higher in the atmosphere. This causes water vapor (water in gas form) in the marine layer to condense into tiny droplets of water, similar to the condensation that form on the outside of a glass of cold water. In this case, the drops of condensation are so tiny that they float around, forming mist or high clouds.
Note: If you've ever flown or sailed across the Pacific in summer, you may have noticed a nearly continuous cloud cover over the ocean between California and Hawaii. This is the top of the marine layer trapped beneath the Pacific High. This continuous cloud cover not only makes it tough to do celestial navigation. It also hides the jet contrails that some sailors tried used for navigation when sailing from California to Hawaii in the days before Satnav and GPS.
As May progresses, the Pacific High expands, and the air over the coastal waters becomes more stable. This allows the marine layer to move toward shore, forming a layer from 200 to 2,000 feet thick over the coastal waters.
When moisture in the marine layer condenses near the top of this marine layer, it forms the "fog" that is the bane of summer visitors to the Central Coast (which is technically known as overcast, because it does not extend down to the ground surface). Occasionally, if the marine layer contains enough moisture, water will condense will occur all the way down to the ground, forming true "fog." This is most common along parts of the coast exposed to the west and northwest, such as the Monterey Peninsula.
The daily shift from fog in marine layer to afternoon sea breeze.
(Modified from NOAA/NCDC)
Because the marine layer is usually less than a thousand feet thick, it is typically blocked by coastal mountains, but flows inland through gaps in the mountains. This means that, as May turns into June, the coast gets lots of fog, but the land just inland from the coast receives more and more sunshine.
As the land warms up each day, so does the air above it. At the same time, the morning sun begins to cause the condensed water droplets at the top of the marine layer to evaporate, gradually clearing the fog that formed or moved inland during the night.
By afternoon, so much warm air rises from that land that it begins to pull cool air in from the coast to take its place. This can have two possible effects. If the clouds in the marine layer have completely evaporated, it will create strong winds blowing in from the coast. On the other hand, if the marine layer still contains a lot of condensed mist, the rising inland air can draw this "fog" inland from coastal areas.
Starting in May and continuing through July or August, the Central Coast is subjected to a constantly shifting battle between foggy, moist marine air and clear, dry inland air. The net result is that periods of clear, sunny, windy days alternate with cloudy days and light winds.
The way this usually plays out is that coastal areas will experience two or three days of warm, sunny weather, followed by a "surge" of very moist marine air. Such influxes of moist marine air often make their way up the California coast from the south, in a kind of "counter-current" flowing against the prevailing northwest winds. [Add photo]
Often an increase in the depth and persistence of the marine layer is caused by a low pressure system that slides past the Pacific High and reaches the Central Coast. By May, such systems rarely bring rain, but by lowering the atmospheric pressure, they allow the marine layer to expand vertically and carry much more moisture (see illustration).
This illustration shows how low pressure systems can lead to a deeper, more persistent marine layer.
(Modified from NOAA/NCDC)
During late May or early June, marine air often wins the atmospheric battle, and foggy mornings become the rule rather than the exception. During the period of "June gloom," overcast mornings may or may not give way to clearing skies in the afternoon.
Once an area becomes covered by a fully saturated marine layer, the winds usually become light or blow from the southwest. At the same time, upwelling ceases, and surface waters move toward shore in what oceanographers call a "relaxation event" (the opposite of an upwelling event).
One important result of the increase in fog and marine influence in late May and June is that strong northwest winds become less common near the coast. This causes upwelling to become more sporadic near the coast. On the other hand, strong northwest winds and upwelling may continue or even increase in June near the Farallon Islands and along the coast north of Point Reyes.