Weather Watch

Observation systems help ensure safe landings

Near the end of a long cross-country flight, you discover that the marine layer has rolled in from the Pacific and covers the San Luis Obispo County Airport. As you make your final approach and descend into a soup of gray mist, an air of anxiety often fills the cockpit before you break through the cloud ceiling near the runway.

Don’t worry. Behind the scenes, airport controllers are working to ensure your safety. If the runway visibility range is too low for your particular aircraft, you’ll be directed to find another airport, probably many miles away. There are actually aircraft that can safely land with near zero visibility, but they are the exception and not the rule.

Not only does every airport have a visibility rating, but many airports have their own visibility rules and can close their runways at any time if the level of visibility falls below their minimums. Originally, runway visual range was measured by the aircraft controllers in the tower who calculated visibility based on landmarks such as runway lights. This method is still used today. However, most airports now use the Automated Surface Observing System (ASOS), which provides such critical information as wind, temperature, pressure, cloud-base height and runway visibility. These automated weather stations have become the backbone of weather observing throughout North America.

Former U.S. Air Force Command Airfield Manager Richard Howell, who took over the reins at the San Luis Obispo County Airport a few years ago, told me that another ASOS station may be installed at the Oceano County Airport in the future.

The instrument that measures visibility on the ASOS system is called a transmissometer. It consists of a laser transmitter and receiver that are separated at a fixed distance. The transmitter sends a laser beam through the air toward the receiver. The instrument calculates visibility by how much the laser beam is attenuated by the atmosphere. As the fog becomes denser, less light from the transmitter will find its way to the receiver, and the ASOS system will indicate lower runway visibility.

Airports also use an instrument called a forward scatter visibility sensor. It works like the transmissometer, except it uses infrared light rather than visible light.

The San Luis Obispo Airport ASOS weather station is operated by the National Weather Service and Federal Aviation Administration, and the current weather observations can be heard by calling 547-1260.

Today’s weather

Patchy coastal low clouds will develop along our coastline this morning. Otherwise, skies will be mostly sunny.

Today’s temperatures will range from the low to mid-70s along the southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach). The westerly-facing beaches (Pismo Beach and Oceano) will be a little cooler, ranging from the mid- to high 60s, while the northwesterly-facing beaches (Cambria, Morro Bay, Los Osos, Montaña De Oro and Nipomo Mesa) will range from the low to mid-60s.

The coastal valleys (San Luis Obispo) will reach the high 70s, while the North County (Paso Robles) will hit the low 80s.

The coastal stratus will increase along the beaches and into the coastal valleys tonight through Monday morning.

A dry cold front will cross the Central Coast on Monday and will produce a steep pressure gradient. This pressure gradient will bring strong to gale-force (25- to 38-mph) northwesterly (onshore) winds Monday afternoon through Tuesday. These onshore winds will mean cooler temperatures throughout San Luis Obispo County.

It continues to look like strong high pressure will build across the state Wednesday through Thursday. This condition will produce night and morning northeasterly (offshore) winds, sparkling clear blue skies and very warm temperatures. There should be just enough afternoon northwesterly (onshore) winds to keep things from heating up too much.

Nevertheless, temperatures along the beaches should reach the low 80s, with the coastal valleys (San Luis Obispo) hitting the high 80s. The North County (Paso Robles) should reach the low 90s during this period.

Another weather system will begin to affect our area Friday, with the return of fog and heavy drizzle and much cooler temperatures.

Surf and sea report

Arriving from the Northwest:

Today’s 4- to 6-foot northwesterly (310-degree deep-water) sea and swell (with a 7- to 12-second period) will continue at this height and period through Monday morning.

Strong to gale-force (25 to 38 mph) northwesterly winds along the California coastline will produce a 5- to 7-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 13-second period) along our coastline Monday afternoon through Tuesday.

A 964-millibar storm developed near the international date line Saturday with hurricane force winds. These winds transferred a tremendous amount of energy to the ocean in the form of seas. When the seas move out from under these winds, they become swells, longer-period waves that can travel for thousands of miles across the ocean.

A 26-foot swell (with a 17- to 19-second period) from this mid-latitude cyclone will reach at the SE PAPA marine buoy Monday night.

This buoy is moored about 600 miles west of Eureka. This swell will then arrive at the California NOAA marine buoy — 400 miles west of San Francisco on Tuesday at 20 feet (with an 18- to 20-second period). These two offshore buoys are referred to as the weather sentinels of the Eastern Pacific.

This swell will continue to travel toward the Central Coast along routes that mark out great circle tracks at about 25 mph.

This swell will arrive along the Pecho Coast on Tuesday night at 2 to 3 feet (with a 20- to 22-second period), peaking Wednesday at 9 to 11 feet (with an 18- to 20-second period).

Arriving from the Southern Hemisphere:

Another (190-degree deep-water) swell will arrive along the California coastline Thursday at 1 to 3 feet (with an 18- to 20-second period). This swell will peak at 3 to 4 feet (with a 16- to 18-second period) Friday.

Seawater temperatures will range between 56 and 58 degrees through Monday, decreasing to 55 and 57 degrees Tuesday through Wednesday.

Environmental note

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John Lindsey is a communications representative for Pacific Gas and Electric Company. He is a local weather expert. If you have a question, send him an email at