Near the end of a long cross-country flight, you discover that the marine layer has rolled in from the Pacific and covers the Santa Ynez Airport.

As you make your final approach and descend into a soup of gray mist, an air of anxiety 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 such landmarks as runway lights. That method is still used today.

However, most airports now use the Automated Surface Observing System, or ASOS, which provides such critical information as wind, temperature, pressure, cloud-base height and runway visibility.

Those automated weather stations have become the backbone of weather observing throughout North America.

Most ASOS stations are located at airports. However, there are a few that are not at airfields, like the one in Central Park in New York City.

ASOS stations are also installed at the Santa Maria Public Airport, aka Capt. G. Allan Hancock Field, Lompoc Airport and Vandenberg Air Force Base.

The instrument that measures visibility on the ASOS system is called a transmissometer and 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, and 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 Santa Ynez 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 805-686-8903.

The Lompoc Airport number is 805-735-3075, and the Santa Maria Public Airport line is 805-928-0384.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Follow him on Twitter @PGE_John.

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