Taking a drive through the North County on a hot summer day, you can sometimes see vortexes of spinning dust rising up from the ground into the sky. These energetic swirls that look like small tornadoes are called dust devils.
In Australia, they are commonly called willy willies. A few American Indian tribes in the southwestern United States consider dust devils to be the spirits of dead ancestors dancing across the surface of the Earth. A dust devil is an atmospheric funnel through which warmer air moves upward in a circular fashion. Dust devils are not tornadoes and are generally much smaller and less dangerous. Still, they can range widely in size and strength.
These whirlwinds can reach more than 3,000 feet in height with winds of more than 60 mph and can last on the order of an hour or more. Typically though, they are short-lived and generally don’t extend much more than 100 feet into the air with winds around 25 mph.
It’s not uncommon to see several dust devils in the same area at once. In any case, you don’t want to be in its path or breathe the dust that it kicks up. An old Navy friend told me he was skydiving near Brown’s Field in San Diego when a dust devil picked his parachute up off the ground with him still attached.
Dust devils most often develop when you have uneven heating of the Earth’s surface, such as the interface between pavement and dirt roads or irrigated fields and dry land. Calm to light winds and clear and sunny skies are also important ingredients.
As the sun rises, it begins to heat the ground and the air immediately above it becomes warmer. As the air warms, it becomes less dense and, like a hot air balloon, begins to rise. When the updrafts of warmer air move toward the sky, the cooler air around it rushes in to fill the void.
This rush of cooler air can cause a spin and form a vortex. As the warmer air continues to rush upwards, it often elongates and narrows the dust devil. That causes it to spin faster, like the increased spinning of ice skaters as they bring their arms in toward their bodies.
The faster-moving air in the center of the dust devil produces lower air pressure and causes even more air to be drawn in. As the air rushes in along the ground, it picks up dust and debris, causing visible whirlwinds.
Dust devils are much more common than we originally thought. The eruption of Mount St. Helens in Washington spread a layer of volcanic ash over a large area. When the weather turned sunny, hundreds of dust devils could be observed as they picked up the light, powdery ash.
Dust devils have even been observed on Mars. NASA’s Mars Exploration Rover Spirit took a series of photographs of a dust devil on the Martian surface.
This week’s forecast
A weeklong cool trough of low pressure in the upper atmosphere aligned along the California coast, combined with a feeble 1,021-millibar Eastern Pacific high stationed about 900 miles to the west-northwest of San Luis Obispo, has produced a deep marine layer with extensive low clouds, fog and drizzle throughout San Luis Obispo County this morning.
Gentle to moderate (8 to 18 mph) northwesterly winds will develop along the coastline early this afternoon and the coastal stratus will retreat back to the beaches, then out to sea by late this afternoon.
A few mid- to high-level subtropical clouds will drift over the county today.
The northwesterly-facing beaches (Cambria, Morro Bay, Los Osos, Montaña de Oro and Nipomo) will range from the low to mid-60s, while the southwesterly facing beaches (Cayucos and Avila Beach) will range from the high 60s to the low 70s.
The coastal valleys (San Luis Obispo) will reach the low 70s, while the North County (Paso Robles) will only reach the mid-80s.
A subtle change in the weather pattern is expected Monday and Tuesday, as a high pressure ridge is forecast to retrograde west, over the desert Southwest.
While low marine clouds and mist are still expected to return to the coast and coastal valleys nights and mornings, daytime temperatures will gradually warm for all locations, especially across the North County.
Fresh to strong (19 to 31 mph) northwesterly winds will develop along our coastline Wednesday afternoon through Friday, producing a much greater amount of clearing along the beaches.
Also during this time frame, there is a chance for monsoon moisture from the remnants of Hurricane Greg to move north off the temperate mid-latitude Eastern Pacific, bringing slight chances for thunderstorm development in the mountains of San Luis Obispo County late Tuesday into Thursday, with higher likelihoods for the central and southern Sierra.
Surf and sea report
Today’s 2- to 3-foot northwesterly (300-degree deepwater) swell (with a 7- to 10-second period) will remain at this height and period through Wednesday morning.
Fresh to strong (19 to 31 mph) northwesterly winds will generate a 4- to 5-foot northwesterly (315-degree deepwater) sea and swell (with a 5- to 11-second period) Wednesday afternoon through Saturday.
Arriving from the Southern Hemisphere: Today’s Southern Hemisphere swell is arriving along areas of the Central and Southern California shoreline today, but its incoming direction is too far southeast (200 degrees) to have much effect along the coastline.
Today’s longer-range models are advertising a significant storm developing about 800 miles west-southwest of New Zealand on Tuesday and Wednesday.
If the storm develops as advertised, the highest-energy Southern Hemisphere (240-degree deepwater) swell of the season can be expected to arrive along the Central Coast shoreline Aug. 30 and 31.
Seawater temperaturs will range between 56 and 58 degrees through tonight, increasing to 57 and 60 degrees Monday through Tuesday.
They’re expected to decrease to 55 and 57 degrees on Wednesday through Friday.
PG&E is taking steps to enhance public safety partnerships with the first responders (police, firefighters and emergency medical technicians) who help keep communities safe.
To learn more, visit: ww.pge.com/mybusiness/edusafety/firstresponder.
Siren test Saturday
The San Luis Obispo County Early Warning System sirens will be tested Saturday. The sirens will sound twice — at noon and again about 30 minutes later — and will last three to five minutes.
During the tests, local radio and television stations will be conducting normal programming.
However, if you hear the sirens at any other time, go indoors and tune to your local stations for important emergency information and instructions. This program is sponsored by PG&E and the county.
John Lindsey, media relations representative for PG&E and local weather expert, has lived along the Central Coast for more than 25 years. To subscribe to his daily weather forecast or ask him a question, email him at email@example.com.