What Makes California California: More Than You Ever Cared to Know About Climate

California is certainly a special place.  A treasure trove of natural wonders and beauty, the state encompasses one of the world's hotspots of biodiversity.  But what makes California so remarkable, so rich in endemic plants and animals found nowhere else in the world?

There have been a couple of key factors at work in this mountainous West Coast state through the ages, shaping it into what it is today: one of those factors is its unique climate, and the other is its diverse topography and underlying geology.

Today, we'll take a look at California's very special climate.

A quintessential Californian landscape - golden, oak-dotted hills - inside the now-imperiled Del Puerto Canyon,
a hotspot for biodiversity, endemism and really neat geology in the central Coast Range.

The mild, Mediterranean climate of California is shared by only four other regions in the world: central Chile, the southwestern tip of South Africa, southwestern Australia, and the Mediterranean basin surrounding the Mediterranean Sea.  Mediterranean climates are characterized by hot, dry summers, and cool, wet winters; they experience essentially no extreme freezes.

In contrast to California's Mediterranean climate, typical summers across the rest of temperate North American are warm and wet, with ample rainfall, while winters are cold and dry, with plenty of snow but relatively low humidity.  The growing season in most of California (hopefully) begins with the first "spring rains" of November, when seeds of grasses and wildflowers begin to germinate, and continues through about May.  California's season of abundant growth essentially ends in May or June as rains stop, at a time when the rest of North America is just waking up to a gloriously green summer.  Not so in California.

Across California, organisms must cope with extreme conditions.  In many of the state's ecosytems, there is relatively little overlap between water availability and temperatures warm enough to sustain active growth.  (April is the peak window of plant growth in much of the state.)  To survive California's hot, dry summers, many plants enter summer dormancy, and some mammals go into estivation (basically summer hibernation).  California's true, quiet season of rest is not winter, but late summer, sometimes referred to as our "fifth season."

California's "fifth season," a season of dormancy before the first winter rains, as seen from the Great Central Valley looking west toward the golden hills of the Coast Range.

But why, exactly, does California, an 800-mile long chunk of land spanning 10 degrees of latitude (between 42 degrees north in Crescent City and 32 degrees north in San Diego) along the western edge of North America, experience a climate that is so different from that of the rest of the continental United States?


Before answering that question, let's pause to review a few key factors that play a role in the function of the earth's weather and climate.  If it's been a while since your last earth science class, read on!

• Tilt of the earth
• The earth is tilted on its axis at a 23.5 degree angle.  This tilt causes seasons, as the northern and southern hemispheres receive more direct sunlight at different times of the year.  You've probably seen a diagram that looks something like this:

My quick sketch of the seasons in the Northern Hemispshere

• Earth's rotation and ocean currents
• The  earth's eastward rotation causes what is known as the Coriolis effect, and is responsible for the westward ocean current at the equator, as well as the predominately clockwise currents in the northern hemisphere.  Wind patterns are also influenced by the Coriolis effect.
• Earth's wind patterns and atmospheric conditions
• Wind is caused by the uneven heating of the earth's surface and the resultant formation of high and low pressure systems.  (More on this in a minute.)  Air moves from areas of high pressure to areas of lower pressure; the greater the difference in pressure, the faster the air moves.  Air near the surface of the earth moves down and away from high pressure systems (clockwise in the northern hemisphere) and up and toward a low pressure system (counterclockwise in the northern hemisphere).  

Also remember:

• Air molecules move from areas of high pressure, where molecules are found crowded together at high densities, to areas of lower pressure, where the molecules are less dense and more spread out.
• Warm air is less dense than cold air, so warm air rises, forming areas of lower pressure.
• As air rises it loses pressure, the molecules spread out and air cools adiabatically (without the exchange of heat.)
• Cold air is more dense, so cold air sinks, forming areas of high pressure.
• Warm air has a greater capacity to hold moisture than cold air.
• Low pressure systems = storm systems.
• Water has a high specific heat capacity, which means it takes more energy to raise the temperature of water than it does surrounding land masses; water also holds heat better than air.  This allows large bodies of water to regulate air temperature.

And that was your crash-course in earth science for the day!  Now we can continue!


California's climate is influenced by two critical factors: it's proximity to the Pacific Ocean, and the presence of a semi-permanent high pressure system, called the North Pacific High, located off the coast of California.

The North Pacific High has been in its present location for the last few thousand years, contributing to California's geologically recent climate of warm, dry summers and cool, wet winters.  (Believe it or not, there was a time in the geologic past when this chunk of land now called California wasn't so dry!)  Situated as it is between the west coast of California and Hawaii, the North Pacific High reaches its northernmost extent and strength during the summer months, and is effectively positioned to deflect incoming storms from the north Pacific and Alaska, thus preventing storms - and rain - from reaching much of California throughout the summer.  This high pressure systems stays in place until sometime during the autumn months, when it weakens and moves just far enough offshore and toward the south to allow storm systems, generated by the Aleutian Low pressure system, to move across California from the northwest, bringing much needed winter precipitation.

Since air moves down and away from high pressure systems, swirling in a clockwise direction in the northern hemisphere, the presence of the North Pacific High also contributes to the prevailing west and northwest winds experienced in California (with the exception of some special conditions in Southern California, like the Santa Ana winds.  The story is a little different down there.)

So, why does the North Pacific High hang out off California's coast, not somewhere else?

Let's dive a little deeper.

The sun is the ultimate source of energy for life on earth (apart from some unique chemosynthetic organisms), and it is also the driving force behind earth's weather.  Year round, the sun shines with the greatest strength on the tropics, low latitudes along the Equator.  Far less solar energy reaches the poles, and the earth manages to compensate for this temperature imbalance by redistributing heat through a series of atmospheric circulation patterns.

As the sun heats the earth's surface with its full force at the Equator, air warmed by the sun rises, creating a band of low pressure around 0 degrees latitude.  As the warmed air rises, it moves north and south toward the poles.  Also, as warm air rises, it cools, condenses, and falls as rain, causing the tropics to be wet and rainy - but warm - year round.  As the cooler air moves toward 30 degrees north and south latitude, it begins to fall, becoming more warm and dry as it descends.  These falling air masses create areas of high pressure in a belt around the earth, resulting in warm, sunny weather.  Over oceans, these falling air masses are manifested as high pressure systems, like the North Pacific High and the comparable Bermuda High over the Atlantic Ocean.  Over land, masses of descending air are responsible for creating the warm, dry conditions that result in desert ecosystems.  Around the world, deserts are found around 30 degrees north and south latitude for this reason.  Take a look at the map below and notice that the world's major desert regions, illustrated in yellow, are almost all situated around 30 degrees.  (Also note the Gobi desert of Mongolia and China, which lies in the rain shadow of the towering Himalayas and Tibetan Plateau.  More on rain shadows in part two of this series!)

Worldwide distribution of major biomes.  Notice how the world's deserts are distributed, for the most part,
around 30 degrees north and south latitude.
From Encyclopedia Britannica, Inc. 

For more on atmospheric circulation cells and global climate, please read on.  To skip straight to the bit about California specifically, jump down a couple of paragraphs below the next diagram!

Low latitude atmospheric cells between zero and 30 degrees are called Hadley cells; Ferrel cells occur between 30 and 60 degrees, and Polar cells are found above 60 degrees.  As relatively warm air continues traveling poleward from 30 degrees, it runs into cold air moving down from the poles around 60 degrees.  This front of converging warm and cool air creates a band of low pressure that extends around the globe at 60 degrees.  As warm air rises over cool air, the rising air masses repeat the process of cooling, condensing into clouds and falling as precipitation.  On the map above, notice which regions are situated around 60 degrees north latitude: notoriously rainy places, like Alaska and Scotland.

Of course, this is an oversimplification and there is much more that goes into local climate than just atmospheric conditions; ocean currents, prevailing winds and topography interact with each other in complex ways, all playing significant roles as well.

For a thorough understanding of these atmospheric phenomena, including prevailing winds, I am forever indebted to my university biogeography professor, Dr. Ann Kohlhass, who had us draw out detailed diagrams for every comprehensive exam.  And I still remember them!  Here's a sample (drawn just today, not back in college!)

The A+ achieving diagram, circa my college days:
Atmospheric circulation cells (on the left), belts of low and high pressure, and direction of prevailing winds (the curved arrows).

Okay, back to California.

California's proximity to the Pacific Ocean also has a great effect on its climate.  Remember that water has a high specific heat capacity, which means it takes longer to heat up or cool down than surrounding air or land.  This has a moderating influence on land masses adjacent large bodies of water, such as the Pacific Ocean.  While both daily high and low temperatures, as well as seasonal highs and lows fluctuate widely in California's interior, the temperature remains relatively stable along the coast nearly all the time.

Along the coast, the average temperature really only changes 10 or 15 degrees Fahrenheit between summer and winter, and throughout the year average daily temperatures may only fluctuate 15, maybe 20 degrees Fahrenheit between daytime high and nighttime low temperatures.  For example, during the winter (more like November through April) in Monterey, daytime highs hover around 60 degrees Fahrenheit while nighttime lows drop into the mid-40's.  During the warmer months, average highs reach around 70 degrees Fahrenheit, and lows are around the mid-50's.

In contrast, temperatures fluctuate widely in the Sierra Nevada mountains, and even more so east of the Sierran crest in the arid Great Basin Desert.  Here, in the cold, dry, high desert region, water is scarce.  Without the moderating effects of water, the difference between the summer maximum and winter minimum temperatures can easily span 80 degrees Fahrenheit or more, reaching temperatures of 100 degrees or more in the summer, and dropping well below freezing, in winter.  Daily temperatures can easily fluctuate 30 or even 40 degrees between the daytime high and nighttime low, reaching the upper 90's during summer afternoons, before dropping into the low 50's at night.

Moral of the story (and in some ways all you need to know about California's climate): pack warm clothes for summer camping trips in the Sierra and along the coast!  While the interior may be hot, the coast and high elevations are not!

The nicest, clearest, warmest days on California's central coast are often during the fall and winter months.

The fact that the Pacific Ocean is so cold off the coast of northern and central California is extremely important for our climate as well!

Okay, so, why is the ocean so cold here??

There are a couple of key factors at work, keeping the waters off California's coast between 50 and 60 degrees Fahrenheit (10-15 degrees Celsius) year round.  The first factor at work is the California Current, which transports cold water south from Alaska and the North Pacific down the west coast of the United States.  The direction of the current is, basically, a product of the earth's rotation and the Coriolis effect, which causes the clockwise swirl of currents in the northern hemisphere.

The second factor we have to thank for our chilly water is, ultimately, the wind.  Prevailing winds blow from the west and northwest, away from the North Pacific High.  (Prevailing westerlies are found all around the world between latitudes of 30 and 60 degrees, due to the atmospheric conditioned outlined above.  See my handy diagram!)  More specifically, longshore winds, which blow from northwest to southeast along the coast of California, cause a phenomenon called upwelling.  As surface water is blown away from an area by strong winds, water from deep below is drawn up to replace water at the surface.  Water from deep beneath the surface is cold, and loaded with nutrients from all the decaying organic matter than sinks to the sea floor and builds up over time.  (Aside: just off the coast of California, quite near the shore in Monterey Bay, is the nearly 12,000 foot-deep Monterey Bay Submarine Canyon.  Upwelling in this area makes it one of the most biologically productive regions in the world.)

The situation is a little different in Southern California.  South of Point Conception (the boundary between central and southern California) is the Southern California Bight, a long, gradual bend in the coastline that essentially forms a large, very open bay, dotted with islands.  In this region, the cold California Current is kicked off the immediate coast and replaced by a northward countercurrent, bringing warmer water up the coast from Baja California.  These two currents mix around the Channel Islands, resulting in the occurrence of species with both northern and southern affinities, and a high amount of biodiversity.

A shot looking west across coastal scrub toward the Pacific, taken from a hill above La Purisima Mission in Lompoc, near Point Conception.  Notice the marine stratus layer (coastal fog) drifting over the land.

California certainly wouldn't be California without fog and a discussion of California's climate wouldn't be complete without it.  Coastal ecosystems, especially coast redwood forests, are highly dependent on fog for much of their water.  Our coastal climate is heavily influenced by this thick marine stratus layer, which is also essentially a product of cold water, upwelling, and ultimately, wind.

Because upwelling creates a zone of especially cold water along the coast, the result is a temperature gradient: surface water decreases in temperature closer to land.  As the sun rises over the Sierra Nevada in the east and begins to warm the air inland, the warmer air rises, effectively drawing cooler air inland from over the ocean.  As air moves across the surface of the Pacific, helped along by prevailing westerlies, it hits the region of colder water near the coast and is cooled, often to the point that water vapor condenses out as fog.  The greater temperature difference inland during the warm summer months is the reason that the marine stratus layer is most common in the summer.

Cold Pacific waters also create an inversion layer in the atmosphere, where air temperatures increase, rather than decrease, with an an increase in altitude.  Typically, air cools as it rises from the surface into the atmosphere.  But in the occurrence of a temperature inversion, air near the surface of the land or sea is cooled by the cold surface, and actually rises in temperature as it increases in altitude and gets farther from the cold surface, until it reaches the top of the inversion, at which point it begins to cool again.  The Pacific is so cold that this inversion layer is fairly deep, resulting in a high marine stratus layer of fog.

To put everything I've just said all in perspective, consider that the most essential underlying reason we have majestic forests of coast redwoods along the coast of California is the wind, which is itself a product of the sun acting in concert with the tilt and rotation of the earth.

Another quintessentially Californian landscape: a coast redwood forest, shrouded in sustaining, life-giving fog.

One more related note, since I am a native of California's Great Central Valley:

Inversion layers are also responsible for the dense tule fog that the Central Valley experiences in the winter months.  The valley lies in a cold air basin, where cold air sinks into the valley from surrounding hills during the night, creating an inversion layer.  On calm and clear windless nights, often after rain when there is enough moisture on the ground and in the air, the cold temperatures near the ground cause the moisture in the air to condense into fog.  This layer of fog can persist for days, or even weeks, self-perpetuating by preventing incoming solar radiation from heating the ground.  Only wind will drive it away.  Aside from waiting for wind, tule fog can be escaped by heading up, into the foothills surrounding the valley and above the inversion layer.  It is a really neat experience to drive through miles of dense fog, begin to climb the hills and then quite suddenly pop through the clouds and emerge into brilliant sunshine above!

Stay tuned for part two of this series, where we will look at the second main factor that makes California so wonderfully California: geomorphology!


Sources:
Ecosystems of California (edited by Harold Mooney & Erika Zavaleta)
A Natural History of California (Allan A. Schoenherr)
NOAA.gov
Notes from college!