News - Local

Saturday, May. 30, 2009

John Lindsey: As a matter of fact, lightning can strike same place twice

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An upper-level low pressure system will approach California and become almost stationary immediately off our coast this week, producing a chance of lightning in the San Luis Obispo area.

Does lightning strike the same place twice? You bet. I took a time-lapse photograph in 1983 while on the bridge of the USS Trippe.

The photo shows the USS McCandless ahead of us being struck with multiple bolts of lightning while we moved through the Caribbean Sea.

Severe turbulence — updrafts and downdrafts — found in thunderstorms produces friction between rapidly moving ice particles and rain traveling in opposite directions.

This wipes off electrical charges, which produce an increase in pressure or voltage. The same effect can sometimes be experienced by rubbing your shoes across a wool carpet on a dry day and then touching a door knob. The shock you feel is the electricity discharged to the doorknob.

When electrical pressure or voltage becomes high enough, charges between parts of the cloud or between the cloud and the ground are released as lightning. Lightning occurs at all levels in a thunderstorm. The majority of lightning strikes never hit the ground.

However, aircraft flying several miles from a thunderstorm can be struck by the proverbial “bolt out of the blue.”

While in the Navy, our P-3 Orion — a maritime patrol aircraft — was hit by lightning over the Sea of Japan. The P-3’s outer skin is mostly made out of aluminum, and the electrical charge was passed through the aircraft’s skin and away from us. The metal skin was pitted with holes from where lightning had struck.

When lightning does hit the ground, it starts with a relatively thin “leader” stroke from the cloud followed immediately by a heavy return stroke from the ground.

Amazingly, a lightning discharge is incredibly powerful — up to 30 million volts at 100,000 amps — but over a short duration. The sudden and rapid increase in heat causes the air around the lightning bolt to rapidly expand, then collapse, causing the shock waves that we call thunder.

Just one bolt is more than six times hotter than the surface of the sun.

Lightning takes the path of least resistance. It tends to hit the highest places. Never stand under a lone tree in an open field or on top of a mountain during thunderstorms.

If you’re backpacking in the mountains and you feel your hair standing on end, get as low to the ground as you can and try to take cover.

On buildings, lightning rods allow electrons to stream off into the air or harmlessly to the ground. Wood structures and trees have high electrical resistance and can be heavily damaged unless grounded.

Lightning can also wreak havoc on electrical systems.

At Pacific Gas and Electric Co., the safety of our customers and crews in the field is top priority.

So, if you see a downed power line — whether it was by a lightning storm or other cause, assume it is energized and keep yourself and others away.

Call 911 immediately to report the location of the downed line, then call PG&E’s 24-Hour Emergency and Customer Service Line at 1-800-743-5002.

This week’s forecast

This morning’s marine layer will burn off this afternoon from the interior and coastal valleys, but will persist along our beaches.

The exception will be Avila Beach and Cayucos, which will see some clearing and warmer temperatures during the afternoon hours as the northwesterly winds mix out the coastal low clouds.

Today’s temperatures should range from the high 50s to low 60s at the beaches, 70s in the coastal valleys (San Luis Obispo), and the low 80s in the interior (Paso Robles).

An upper-level, cut-off low-pressure system is forecast to stall off our coastline late Tuesday through Friday.

This system will produce cooler temperatures and bring a threat of rain showers and possibly thunderstorms during this timeframe.

Exact location of the rain showers will depend on the location of the low pressure system, which at this time is almost impossible to predict.

Surf forecast

Increasing northwesterly winds will produce 2- to 3-foot northwesterly (320-degree shallow-water) seas (with a 4- to 6-second period) this morning, increasing to 3 to 4 feet this afternoon.

These 3- to 4-foot northwesterly seas will continue at this height through Monday.

Combined with these northwesterly seas will be a 3- to 5-foot westerly (270-degree deep-water) swell (with an 8- to 12-second period) this afternoon through Tuesday.

A 2- to 3-foot westerly (275-degree deep-water) swell (with an 8- to 11-second period) is forecast along our coastline on Tuesday, decreasing to 1 to 2 feet on Wednesday through Thursday.

Arriving from the southern hemisphere:

A 1- to 2-foot southern hemisphere (260-degree deep-water) swell (with a 19- to 21-second period) will arrive along our coastline on Thursday, building to 2 to 3 feet (with an 18- to 20-second period) on Friday through Saturday.

The longer-range models are still indicating a major storm developing off the New Zealand coastline early next week.

If this storm develops as advertised, it will produce a significant southern hemisphere swell event along our coastline on June 8, 9 and 10.

John Lindsey is a communications representative and meteorologist for Pacific Gas and Electric Co. in San Luis Obispo. E-mail questions to him at pgeweather@pge.com.