Weather Watch

Inversion layers have ups, downs

Special to The TribuneAugust 25, 2012 

Like the tides, the ebb and flow of the marine low clouds along the California coastline is reassuringly timeless. At times the coastal clouds will surge into the North County and give relief from the summer’s heat. At other times “June Gloom” can persist along the coast for days on end.

If you ever wondered why this occurs, the answer lies in an atmospheric condition called the temperature inversion layer.

It may seem counterintuitive, but along the Central Coast during the summer months, a layer of much warmer and less dense air often rides on top of cooler air near the Earth’s surface, kind of like a layered ice cream cake.

We often think the higher you go in the sky the colder it gets. However, a weather balloon launched out of Vandenberg Air Force Base early Friday morning told a different story. As the balloon ascended through the atmosphere it indicated a gradual decrease in air temperatures up to about 1,700 feet in height; above this mark, the air temperature rapidly increased to 82 degrees in less than a thousand feet of altitude change. This is referred to as the temperature inversion or marine layer.

The height of the temperature inversion layer can vary from day to day. Along the Central Coast, it’s not uncommon to see morning temperatures in the low 80s on the mountaintops, while at the same time temperatures in the coastal valleys will be stuck in the mid-50s because of this temperature inversion.

Below the inversion layer, the air gradually cools as it rises and eventually reaches its dew point temperature. When this occurs, the moisture in the air condenses on microscopic particles and coastal stratus clouds are formed.

Above the inversion layer, the air temperature can rapidly increase, as much as 20 degrees or more. This area of increasing air temperatures with rising height is the region referred to as subsidence inversion. Within this region, the air sinks (subsidence) and warms by compression. As the air mass descends, not only does it heat up like a bicycle tire that’s being inflated, but it becomes quite dry, less dense, and very clear as the dew point temperature spread widens.

If the temperature inversion layer is shallow, say a few hundred feet, only the beaches will experience marine overcast. If the temperature inversion layer rises over 1,000 feet, many of the coastal valleys will see marine clouds surging overhead.

When the marine layer reaches toward 1,700 feet, parts of North County near Highway 101 will often get convective fog by way of the Salinas Valley or cool marine air will flow through the Templeton Gap. If the marine layer reaches above 3,000 feet, the entire county of San Luis Obispo can be covered by coastal clouds. The deeper the marine layer, the greater the chance for drizzle and even measurable precipitation to develop.

If the winds are calm, the beaches can remain overcast for most of the day. Generally when it’s calm, the warm and less dense air above the temperature inversion layer cannot penetrate into the cool and heavy moisture-laden ocean air underneath.

If the northwesterly (onshore) winds are strong enough, they can mix out the temperature inversion layer, leaving behind sunny skies. On rare occasions during the summer, northeasterly (offshore) winds can develop, driving the marine air out to sea and producing blistering hot temperatures along the beaches.

Various meteorological instruments, such as a weather balloon radiosonde or the Atmospheric Systems Corporation SODAR (SOnic Detection And Ranging) instrument installed at the Diablo Canyon nuclear power plant provide the depth of the marine layer.

Another method to estimate the depth of the temperature inversion layer can be found at the website sloweather.com.

Chris Arndt, an electronics and weather wizard, runs the website. He first installed a weather station at his home in western San Luis Obispo back in 1998 and a few years later put it on the Internet. Over the years, he’s added over 20 Davis Vantage Pro II weather stations ranging from the top of the Condor Lookout facility at 3,190 feet on Hi Mountain about 15 miles east of San Luis Obispo to the coast at the Point San Luis Lighthouse and areas in between.

Chris’s home weather station in western San Luis Obispo is at 310 feet above sea level. Chris installed a new station at the top of Prefumo Canyon at about 1,247 feet a few months ago. The station on top of Prefumo Canyon can be over 20 degrees warmer than Chris’s home station, indicating the temperature inversion layer lurking somewhere between. If the two stations are reporting roughly the same temperature, then you can assume the temperature inversion layer is higher than 1,247 feet. If it is higher, take a look at the Tassajera Peak weather station on the western portion of Cuesta Ridge at 2,775 feet on Chris’s website. Often, the temperature inversion layer will be below the station. If not, Condor Lookout on Hi Mountain is the highest station on the WeatherElement.com network. Chris always welcomes additional weather stations.

Today’s forecast

This morning’s temperature inversion layer should be about 2,000 feet. Consequently, marine low clouds have moved over the coastal valleys and should spread into parts of the North County by way of the Salinas Valley this morning. Areas of patchy fog and drizzle are expected along the beaches.

This coastal stratus will clear from the North County later this morning and from the coastal valleys by this afternoon. However, the relaxation of the northwesterly winds will result in persistent marine overcast with only partial afternoon clearing along the beaches. Today’s high temperatures will be the coolest of the week. Today’s maximum temperatures will range from the low 80s in the North County to the low 70s in the coastal valleys. The beaches will range between the low to mid-60s.

A dry cold front will pass over the Central Coast on Tuesday. At around the same time, a 1,031-millibar Eastern Pacific high will move southeastward toward the California coast and produce a steep pressure gradient. This pressure gradient will generate increasing northwesterly winds that will mix out the temperature inversion layer and give abundant sunshine along the beaches on Tuesday afternoon through Wednesday.

North County temperatures will rebound to above normal values by midweek as high pressure rebuilds. However, cooler temperatures are expected to return by Friday along with a deepening marine layer. No extreme heat or rain is expected through the forecast period.

Today’s surf report

The wind fields have shifted northward off the Northern California coastline, leaving behind a low height northwesterly swell. Today’s 2- to 4-foot northwesterly (310-degree deep-water) swell (with a 7- to 9-second period) will remain at this height and period through Monday morning.

Increasing northwesterly winds will generate a 3- to 4-foot northwesterly (315-degree deep-water) sea and swell (with a 5- to 10-second period) Monday afternoon, further increasing to 4 to 6 feet (with a 5- to 8-second period) Tuesday and will remain at this height and period through Wednesday.

A 3- to 5-foot northwesterly (310-degree deep-water) swell (with an 8- to 11-second period) will develop along the Pecho Coast on Thursday and will remain at this height and period through Saturday.

Arriving from the Southern Hemisphere

A large storm developed in the Southern Pacific last week and will generate a higher energy Southern Hemisphere swell later this week.

Swell from this intense storm will arrive along the Pecho Coast late Thursday at 1 to 2 feet (with a 23-second period) from 195 degrees. This Southern Hemisphere (195-degree deep-water) swell will build to 2 to 4 feet (with an 18- to 20-second period) on Friday and remain at this height but with a gradually shorter period through next Sunday. Wave heights in Southern California could reach well over 8 feet.

Seawater temperatures

The relaxation of the northwesterly winds have produced a strong northerly flowing current that has brought warmer water from the south.

Combined with a diminished amount of upwelling, it has produced much warmer seawater temperatures along the coastline.

Seawater temperature will range between 58 and 61 degrees through Monday, decreasing to 54 and 57 degrees Tuesday and will remain at this level through Friday.

• • •

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John Lindsey is a media relations representative for PG&E. He is a local weather expert and has lived on the Central Coast for more than 25 years. If you would like to subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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