The California Sky Watcher: Understanding Weather Patterns and What Comes Next

SUMMER’S STABLE DROUGHT

Late night and early morning low clouds and fog, otherwise mostly sunny during the afternoons. High temperatures will range from the 70s at the beaches to the 80s on the coastal plain, dropping into the low 60s overnight. Mainly calm winds except for gentle afternoon sea breezes. No rain and little change in the long-range forecast.

The next day: ditto. The day after that: same again.

Millions of Californians are familiar with the ideally comfortable coastal California weather forecasting monotony that plays out day after day for months, especially from every May into September. Our trouble-free atmospheric mildness has been featured in art, poems, essays, books, music, and movies since visitors and settlers wandered and then poured into California. It resonates with those who celebrate it and is ridiculed by those who are energized by the erratic turbulence of weather extremes.

Our weather journey begins with these archetypal mild summer climates along the Southern California coast. As this chapter progresses, we will eventually travel north and inland across deep valleys and over lofty mountains to experience California’s more extreme and changeable climates, until we end our summer journey in the far northeast corner of the state.

Our iconic summer weather is revered from Santa Barbara (called the American Riviera by some) south into Baja. Along these coastal strips, you will find some of our planet’s mildest climates, which frequently live up to their stereotypes. Some of the larger populations of Native Americans enjoyed this comfortable near perfection for thousands of years before the great invasions and the evolution of California’s renowned beach and surf cultures. Our paradisiacal weather was shamelessly exploited in a twentieth-century culture increasingly dominated by advertising and image. It became a trope so ubiquitous, it was mocked into parody.

Those who have been fortunate enough to live along Southern California’s coast sense more than a degree of truth in the praise and the parody. After all, this was the weather that attracted folks ranging from plein air (open air) artists from around the world searching for brilliant light, to patients recovering from respiratory diseases. It was the environment that beckoned people who were escaping harsher climates in the US and beyond. Southern California coastal climates have always ranked at the top of the list of reasons why millions of people and a host of industries, ranging from agriculture to aerospace to technology to film and other entertainment, flocked to these coastlines.

The growth brought crowds that eventually jammed the roads and belched air pollution as the state’s population swelled toward forty million and its economy grew to become the fifth most powerful in the world. Bustling masses began squeezing the farms and renowned citrus groves out to more distant, less comfortable, and less desirable inland valleys. Consequently, these same mildest, sunny coastal strips are now some of the most crowded and expensive communities in the country. More recently, average wage earners have also been bumped out to those distant and more affordable inland locations, where seasons are harsher and the spent remains of exhausted sea breezes often dump the smog. This is also one reason why California has made great efforts to keep its beaches public and accessible to anyone who wants to sacrifice the travel time and make the effort to visit them.

FIGURE 1-1. The setting sun’s enduring longer wavelengths of red and orange briefly illuminate the bottoms of high, ice-crystal cirrus clouds. Millions of sightseers have witnessed similar spectacles from California beaches.

WHEN HIGH PRESSURE CONTROLS OUR WEATHER

Steadfast high pressure sums up the weather throughout most of California during summer. Such dense, heavy air sinks out of high-pressure systems, creating stable, fair weather. By contrast, because less dense air is lighter, it rises up within low-pressure systems. Such unstable air may expand and cool to its dew point, often resulting in stormy weather. (Dew point is reached when the air cools to reach its saturation temperature—100 percent relative humidity.) Because it spreads out of the highs and drains into the lows, wind always tends to blow from relative high-to-low pressure areas at the same altitude.

One of the most massive pressure systems in the world builds and then anchors itself over the northeastern Pacific Ocean, especially from spring through September. Dense, heavy air descends out of this resilient ridge of high pressure, blocking transient low-pressure systems from intruding into California. This North (or East) Pacific Subtropical High is California’s great protector and seemingly perpetual producer of stable air masses. It commands summer’s drought and often determines the extent of cooling sea breezes and severity of heat waves as it wobbles, shifts around, expands, and contracts farther out to sea one week and then nudges over our state the next. It sometimes earned the notorious ridiculously resilient ridge moniker (a term coined by climate scientist Daniel Swain) during our severe droughts. (In the most general terms, California has two types of droughts. Our Mediterranean climate is defined by each year’s regular summer-season drought. But when precipitation totals fall below the long-term average for a series of seasons or years, longer anomalous and exceptional droughts may develop.) As surface winds flow out from the high’s center, toward relatively lower pressure, they are veered to their right in the Northern Hemisphere by the Coriolis effect. The result is the North Pacific Subtropical High, a gigantic clockwise pinwheel of wind that dominates over the North Pacific Ocean. These surface winds turn toward the south as they drain out of the eastern side of the High (our West Coast). Cyclones (storms) spin winds in the same direction as Earth’s rotation, and anticylones spin in the opposite direction of Earth’s rotation; the North Pacific High is an anticyclone. And it is these clockwise anticyclonic winds that drive the California Current, that wide, cold stream of ocean water drifting from north to south along our coast. (During late summer, a very different subtropical Four Corners High may nudge in from the east, sometimes bringing moist summer heat.)

FIGURE 1-2. Air tends to sink and circulate out of high-pressure areas and then flow into low-pressure areas.

As inland surfaces are warmed, the Earth transfers heat upward, causing air to expand, creating thermal low pressure near the surface. Summer winds flow off the cool ocean and toward these heated inland basins. As they are sucked onshore toward this inland void, the winds may be blocked by coastal mountains, siphoned into coastal valleys, or funneled through narrow canyons. These cool, heavy, dense air masses in coastal California are among the most stable on Earth. Resulting marine layers, often carrying the region’s famous coastal low clouds and fog (often called advection fog because it forms over the cold ocean and drifts inland), are trapped below summer’s high pressure. Stop to sense these feathery-soft summer sea breezes as they drift within marine layers that have settled and pooled in coastal valleys. You are feeling the molecules that make up the air itself as they rush by you, pouring out of high-pressure areas and draining into low-pressure areas.

FIGURE 1-3. It’s the middle of summer, yet a line of shivering visitors clamors to enter the Warming Hut at Crissy Field as the cold sea breeze funnels through the Golden Gate in the background.

FIGURE 1-4. Summer’s prevailing surface winds regularly blow out of the North Pacific Subtropical High, across the cold California Current, and toward the thermal low over our hot deserts.

Air parcels descending out of summer’s dominating upper-level high pressure are compressed and heated, settling like a cap above the sea breeze. The resulting warmer-air-above/cooler-air-below conditions may seem odd when you think of how temperatures usually become so frigid at higher altitudes. But the chill of that heavy, shallow sea breeze discourages surface heating down below and holds the marine layer near the surface. These inversions (see figure 1-6) along the coast help define our classic Mediterranean climate; they bolster our summer drought and stamp out any illusions that turbulent storms could loom on the horizon. Many of us climb up California hillsides to suddenly emerge out of the cool haze, fog, and low, flat stratus clouds into the clear, warm atmosphere at higher elevations. We are rewarded with a heavenly view through the dry, warm air with clear summer skies above, while the thick layer of cool, misty atmospheric cotton spreads below.

FIGURE 1-5. Particularly during spring and summer, cool marine layers are often trapped below regional inversions along the California coast.

POWER OF THE GYRE

Ocean water temperatures have direct impacts on weather patterns that can spread far inland through the western states and beyond. Our dominating North Pacific Subtropical High pushes out winds that churn the great North Pacific Subtropical Gyre, the colossal clockwise-spinning pinwheel of ocean currents that govern surface water conditions and air masses on the grandest of scales. Draw a line of latitude across every major continent and ocean at just above 30 degrees and you will find the same repeating patterns: magnificent ocean gyres transport warm waters from the equator toward the pole in the western oceans, along the east coasts of major continents. As the ocean water clock continues turning (subtropical gyres swirl clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere), cold water is transported along the west coasts of continents, from the poles toward the equator.

Hence the startling differences in climates when comparing the US East Coast to our West Coast. Contrast San Francisco with Richmond, Virginia, and San Diego with Charleston, South Carolina. Only their latitudes match. Sea breezes on those East Coast cities flow off of their warm Gulf Stream, or may be transported across the continent from the Gulf of Mexico. Summer air masses there are often warm, sticky with humidity, and unstable, leading to summer thunderstorms and heavy downpours. Furthermore, as prevailing winds and weather systems generally drift from west to east at these latitudes, they commonly carry relatively milder air off the Pacific and into the West Coast, but hotter summer and colder winter air masses from off the continent toward and over the East Coast. In California, cool, stable air masses usually must pass over the cold California Current before coming ashore.

FIGURE 1-6. This simplified map shows the North Pacific Subtropical Gyre that drives cold water down the California coast. It then turns to its right as a gradually warming current while it drifts toward Asia, sending warm water up along Asia’s east coast, and will eventually cool again as it drifts north. The pattern is similar in every major ocean basin. Those infamous garbage patches are trapped within the gyre. Source: NOAA Office of Response and Restoration.

MARINE LAYER MADNESS

Summer marine layer temperatures near the coast will usually resemble the ocean temperatures where they originated. Fog and low clouds are more common along the north coast because those summer air masses have been in contact with colder water (average water and air temperatures are at least 10°F cooler near the Oregon border compared to the Mexican border), keeping air temperatures closer to their dew points for longer periods. Imagine the puff of fog when you breathe out on a cold winter night: that’s your warm, moist breath suddenly cooled to its dew point, or saturation temperature. When coastal (advection) fog streams through the frigid Golden Gate and piles up along north coast slopes, it has condensed in soggy, saturated air blown in from the cold and watery embrace of the California Current. During the day, the low clouds and fog may attempt to drift inland with the sea breeze, above land surfaces warmed by the sun. But the tiny water drops usually evaporate (or burn off) into warmer air that has gained greater capacity to hold its water vapor. As land surfaces get even hotter, the fog and low clouds appear to retreat back over the cool ocean. On some summer afternoons, sea breezes strengthen and the fog races off the cold ocean as if it were attacking the continent, only to quickly evaporate and disappear over warmer land. Farther south, the air more frequently warms above its dew point, causing the low clouds and fog to clear and making way for those iconic, sunny Southern California summer days. Those same land surfaces will cool off relatively quickly after sunset, allowing the fog and low clouds to spread inland, often covering coastal valleys by late night. The sun will start its warming and burning-off process again during the next morning; those repetitive summer weather forecast chants echo along the California coast during late spring and summer months.

FIGURE 1-7. When you hike up through the coastal sage and above the cool coastal (advection) fog and stratus clouds, you will find yourself above the inversion layer and in the warm, dry air above.

Within drier air masses, the atmosphere may not get cold enough to reach saturation. But if the air remains calm on these clear nights, you might observe smaller-scale examples of saturation, condensation, and evaporation cycles when Earth’s surface becomes colder than the air. The first surfaces to cool to the air’s dew point are usually those that heat up faster and cool off faster, such as metal. Scientists refer to these objects as having low specific heat or heat capacity. A car exterior is a good example. When nighttime air comes in contact with these much colder surfaces, it loses its capacity to hold water vapor and becomes saturated (as it often does over our cold ocean). Condensation occurs, depositing liquid water on those colder surfaces. As surfaces continue cooling overnight, the dew may spread to cover more objects, especially on the cold ground. The same process deposits water from the air onto the outside of the cold glass that holds your icy drink. During California’s long seasonal droughts, this early morning moisture can become a lifesaver for many plants and the animals who pause for a lick here and a sip there before the dew evaporates into the warming day.

On a grander scale, these round-the-clock cycles orchestrate similar alterations within moist coastal air masses. During warmer afternoons when air temperatures rise far above dew point, evaporation occurs into air that has a greater capacity to hold water vapor; this air with lower relative humidity feels drier. Overnight temperatures cool closer to the air’s dew point, raising relative humidity again toward 100 percent (saturation). And so we see a sort-of daily do-si-do, featuring temperature and relative humidity as the oscillating dancers: when one goes up, the other goes down, and vice versa. Even though the low clouds and fog may retreat each day and return each night during this dance, the marine layer remains in place throughout most summers, often spreading several miles inland in those regions where mountain barriers don’t block it. These low marine layers also contribute to milder temperatures, with their blankets of moisture that filter solar radiation during the day and then trap infrared radiation from escaping at night. A comfortable combination of cool afternoon sea breezes and nighttime quilts of insulation conspires to produce California’s mildest coastal climates.

Summer’s sea breezes and marine layers will also fluctuate within longer cycles that may last for days or weeks. When high pressure strengthens and moves over us, it squashes the fog toward the Earth’s surface, limiting it to the immediate coast for days. When high pressure weakens or moves away, marine layers can invade inland valleys and thicken to a few thousand feet. Dense, low clouds may then spread from the coast all the way up against mountain slopes. The changes can be particularly dramatic following inland heat waves, as upper-level high-pressure cookers suddenly weaken or retreat, opening the door to sudden and more powerful cooling sea breezes.

When I was a kid growing up in Santa Ana, I hated the marine layer. I saw it as a dull gray confining slab, obstructing the expansive sky that I wanted to view. And when it finally cleared to haze, I often couldn’t see very far through the smog. Perhaps it reminded me of my confinement to a place where I didn’t want to be. By contrast, clear skies represented a window to the world I wanted to explore. I wouldn’t be the first kid with such an imagination and craving for adventure. That’s probably why I learned to love warm or unsettled weather. No marine layer, and you can see all the way to the mountains and any significant clouds or weather that might be popping up near the horizon.

Today, I appreciate the marine layer. I’ll take 75° over 99°, especially when I have to live, work, and play in such weather. I remarked to a friend the other day that our air near the coast was so soft, it was like being surrounded by fluffy pillows. Millions of others have noticed, too, and residing in that soft, fluffy pillow air along our coasts has become exorbitantly expensive.

Nippy summer breezes and thin low clouds and fog along the coast may create an illusion that the sun’s rays are harmless, or not even making it to the surface. But almost all of the incoming ultraviolet radiation sneaks through these thin veils of chilly maritime moisture, burning the shoulders and noses of forgetful locals and those unsuspecting visitors more familiar with the immediate and intense heat along the Atlantic shore. You might blame ocean currents for these differences between East Coast and West Coast summers.

The cold California Current differs dramatically from the south-to-north-flowing Gulf Stream, which carries relatively warm ocean water up along the East Coast. The Gulf of Mexico and Gulf Stream water temperatures can soar above 80°F (27°C) by late summer. Warm, muggy, and therefore unstable air forms over the warm currents off the southeastern US. By contrast, late-summer water temperatures may finally peak at only about 70°F (21°C) along the Southern California Bight; they often remain in the 50s north of Pt. Conception, even into the warm season. (Pt. Conception teams with Pt. Arguello to form that massive elbow protruding into the Pacific, effectively separating Northern and Southern California coastal waters. The Southern California Bight refers to the more protected coastline trending southeast from there to the Mexican border.) Such cool ocean currents keep our summers not only cool but relatively calm, compared to a state such as Florida, which is the thunderstorm capital of the US.

FIGURE 1-8. Temperatures remain near 70°F (21°C) in the air and water on this beach near San Onofre, even though the sun is high on this summer day. Typical low stratus clouds are burning off.

BLOCKING THE SEA BREEZE

Air masses flowing off our cold ocean are modified as they move inland. Traveling from the cool beaches to overheated inland valleys, you will experience some of the most extreme summer afternoon temperature gradients in the world, occasionally exceeding 1°F per mile.

Topographic barriers represent major sea breeze blockades. This is why parts of Long Beach downwind of the Palos Verdes Peninsula’s hills may run 10° warmer on summer afternoons, compared to nearby beach cities. When strong high pressure builds overhead and squashes the marine layer closer to the surface, world-famous Malibu and Topanga beaches often experience 70°F (21°C) summer afternoons, while less than 10 miles inland and over the mountains, Woodland Hills and Calabasas are roasting in temperatures well over 100°F (38°C). Pick your favorite adjacent coastline, mountain range, and inland valley from Mexico to Oregon and you may discover similar temperature contrasts, especially throughout the summer.

Around the Bay Area, you will find startling temperature gradient examples across Marin and San Mateo Counties. Traveling from the beaches on the western sides of their mountains to the inland (leeward) sides during summer afternoons, you will often transition from wintery dull gray to summer’s sunny warmth. The world’s bustling epicenter of technology— San Jose and the Silicon Valley—was built in a basin, the Santa Clara Valley, protected by the Santa Cruz Mountains from direct hits of maritime air masses. This leaves the valley with consistently warmer and sunnier summer afternoons compared to the cold coastlines from San Francisco to Monterey Bay. Travel farther south on the inland side, more distant from the cooler San Francisco Bay air masses that may have swirled through the Golden Gate and down into the Santa Clara Valley. Where orchards once relied on the regularity of sunny summer weather, you will encounter increasingly higher summer afternoon temperatures through the sprawling suburbs that have replaced farms all the way south past Morgan Hill and Gilroy.

Majestic, rugged ranges rising along the far north coast are also formidable barriers to shallow sea breezes and marine layers. Summer heat can rule surprisingly close to the coast, within narrow valleys and canyons that become sheltered from sea breezes as they meander inland through steep mountain barriers. For the foggiest summer, find a cabin on Cape Mendocino or the Lost Coast, both of which stick out into persistent, frigid fog banks. Patches of coast redwood forest (or places where they grew before they were logged) are usually found in areas with the most consistent summer fog. Where the fog goes, the redwood grows—the cool coastal mist not only decreases evapotranspiration (the total amount of water evaporating from plants and other surfaces) in these plant communities but also adds more than 10 inches of fog drip to the forest floor during summer’s drought months. Heavy rains take over during winter, nourishing the natural range of coast redwoods and other mixed coastal forests from Big Sur north past the Oregon border. But you won’t find redwoods growing in natural stands far inland and distant from the fog