From earliest times, people have tried to understand the weather. Before computer models, surface charts, satellite images or modern meteorological reporting stations were available, people often relied on proverbs and folklore.
For instance, several readers have commented on the early migrations of tarantula spiders in the North County, which may mean a wet winter.
Or they’ve recited ancient meteorological lore: “When a dark halo surrounds the moon, the month will bring rain or gather clouds.”
Other weather watchers from Aristotle to French philosopher René Descartes tried to explain weather phenomena through the formation and lifespan of clouds, which is a much more reliable method of understanding weather. As any fifth-grader will tell you, clouds are part of Earth’s water cycle.
Clouds are the condensation of invisible water vapor on very small nuclei from dust, volcanoes, pollen, forest fires, pollution from cars and factories, salt from ocean spray or sulfite particles from phytoplankton in the oceans.
Clouds exist in the atmosphere in thousands of different forms and sizes, like so many different rocks on Moonstone Beach. That diversity seemingly makes it impossible for anyone to successfully classify and thus predict the weather.
A French scientist in the late 1700s and early 1800s, Jean-Baptiste Lamarck, proposed the first system for classifying clouds, but it wasn’t widely used.
A short time later, in 1803, English naturalist and amateur meteorologist Luke Howard developed a simple classification system.
Howard’s system used Latin words to describe three basic families of clouds: cirrus (“curl of hair”) cumulus (“heap or pile”) and stratus (“layer or sheet”).
In other words, he divided clouds into three types: hair, heaps and layers. Howard later went on to name any type of cloud that produced precipitation as nimbus (“cloud or violent rain”).
Howard later refined his cloud classification system and divided hair, heaps and layers into four primary cloud groups by the height of the cloud base above the Earth’s surface.
These classifications include:
High clouds are classified as cirrus, cirrostratus and cirrocumulus and form at altitudes from 16,000 to 43,000 feet at our latitude. They are primarily composed of ice crystals and can appear in a luminescent array of colors.
Middle clouds are classified as altostratus and altocumulus, which form at altitudes from 6,500 to 23,000 feet. They are primarily composed of water droplets. However, they can also be composed of ice crystals.
Low clouds are classified as stratus, stratus cumulus and nimbostratus, which form from just above the surface of the ground to 6,500 feet of altitude. When on the surface, they are referred to as fog. If the temperatures are cold enough, these clouds may also contain ice particles or snow.
And last but not least, clouds with vertical development are cumulus and cumulonimbus. As the air rises thousands of feet into the sky, it cools and releases tremendous amounts of latent heat.
This condition keeps the air rising inside the cloud and can trigger thunderstorms. These convective storms can contain areas of organized rotation a few miles up in the atmosphere.
If the conditions are right, these thunderstorms can spin out tornadoes.
These types of clouds can on rare occurrences burst into the stratosphere!
This week’s forecast
The Eastern Pacific high has moved far out to sea and will allow a series of unseasonably cold weather systems to pass over the Central Coast.
The first system in this series is a cold front that will pass over the Central Coast between 1 and 4 a.m. this morning with gentle rain and gentle to moderate (8 to 18 mph) southerly winds.
Total rainfall should be less than a quarter of an inch. This system is relatively cold, and snow levels are expected to drop down to near 4,500 feet.
Additionally, there are chances for thunderstorm development this afternoon over the mountains and eastern regions of San Luis Obispo County.
Heavier precipitation, small hail, lightning and gusty winds are possible with these thunderstorms.
However, most of San Luis Obispo County will see partly cloudy skies and cool weather with maximum temperatures only reaching the high 50s to mid-60s this afternoon through tonight.
Partly cloudy and slightly warmer conditions are expected for Monday.
A 1,007-millibar low-pressure system will approach the California coastline on Monday. The associated cold front will pass our area Tuesday morning with moderate rain and moderate to fresh (13 to 24 mph) southeasterly winds.
Rain showers will continue through Tuesday evening along with a chance of thunderstorms. Rainfall totals with this system should range between a quarter and a half an inch.
The average total precipitation for May in San Luis Obispo is 0.44 inches. The wettest May on record was 1998, which produced 3.41 inches. On May 22, 2006, a cold front stalled over the Central Coast and produced 2.76 inches of rain at the Diablo Canyon Ocean Lab.
It would be unusual not to have rain in May.
The Eastern Pacific high will move westward toward California and will give fresh to strong (19 to 31 mph) northwesterly winds and dry and warmer weather on Wednesday through the weekend.
Surf and sea report
This morning’s 3- to 5-foot northwesterly (290-degree deep-water) swell (with an 8- to 11-second period) will remain at this height and period through Tuesday morning, increasing to 4 to 6 feet on Tuesday afternoon through Wednesday morning.
Combined with this northwesterly swell on Tuesday morning will be 2- to 3-foot southwesterly (210-degree shallow-water) seas.
Increasing northwesterly winds will generate a 4- to 6-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) on Wednesday afternoon, further increasing to 6 to 8 feet with the same period Thursday through Saturday.
Arriving from the southern hemisphere:
A very large storm with hurricane force winds developed about 1,500 miles to the east of New Zealand. Southern hemisphere (190-degree deep-water) swell from this storm will arrive along our coastline on Tuesday at 1 foot (with a 25-second period).
This swell will further increase to 2 to 3 feet (with a 20- to 22-second period) on Wednesday and should peak on Thursday and Friday at 4 to 5 feet (with a 17- to 19-second period).
Coastlines with a more southerly exposure could see significant swell heights reaching over 7 feet.
Sunday’s seawater temperature report:
Seawater temperatures will range between 51 and 53 degrees through today, increasing to 52 to 54 degrees on Monday through Wednesday.
Increasing northwesterly winds will produce cooler seawater temperatures Thursday through Saturday.
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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for nearly 25 years. To subscribe to his daily weather forecast or ask him a question, email firstname.lastname@example.org.