When it comes to weather, air pressure is a weighty subject

Special to The TribuneFebruary 15, 2014 

Like the force of gravity which constantly tugs at all of us, the weight of the atmosphere is a force that most of us don’t think about very often. However, like gravity, air pressure has a profound influence on all of us.

For example, the seemingly perpetual ridge of high pressure over the West Coast that has produced one of the worst droughts in Central Coast history. Overall, the lower the atmospheric pressure, the greater chances for rain; higher pressure will produce drier conditions.

The weight of the atmosphere is often a difficult subject for many to get a good grasp on. But when it comes to weather, air pressure is the most important of all factors.

Imagine a 1-square-inch column of air measured from the top of the atmosphere down to sea level. It would weigh about 14.7 pounds!

This pressure, or weight of the air, is caused by gravity pulling the atmosphere toward Earth. Earth’s atmosphere becomes lighter with altitude.

Half of the atmosphere’s weight lies between Earth’s surface and an altitude of about 18,000 feet or 500 millibars. A millibar is a unit of air pressure used in meteorology.

Usually you don’t really notice air pressure because it’s being applied to you from all directions, but quick changes in altitude can affect your body. One local example in altitude change is driving over the Cuesta Grade, which is about 1,500 feet with air pressure at 13.9 pounds per square inch; you often feel your ears pop. This is caused by your inner ear trying to equalize with the outside air pressure.

Another example is when you fly across the country in a jetliner about 32,000 feet. The outside air pressure is about 4.3 pounds per square inch.

Atmospheric pressure along the Central Coast often shows a diurnal cycle – a daily rotation repeated every 24 hours – especially during the summer months. This cycle is caused by faster warming of the air mass over the inland valleys than over the Pacific.

As the air in our inland valleys warms up, it becomes less dense, producing a thermal low, which often produces increasing northwesterly (onshore) winds during the afternoon hours.

At night the opposite occurs, with the air mass over the inland valleys cooling faster and becoming denser. This condition often produces Santa Lucia northeasterly (offshore) winds. Of course, many other factors can affect the winds.

This condition is especially evident in the Central Valley of California. That is why there are so many wind turbines in the Altamont Pass in the east Bay Area; there is a lot of wind there.

Utility-scale wind farms are generally located in areas with average annual wind speeds of at least 13 miles per hour or greater.

As the cool and dense marine air rushes through the Altamont Pass to fill the void of the Central Valley thermal low, average wind speeds can range between 25 and 30 mph or greater during the summer months, producing a significant amount of clean energy.

Did you know that PG&E delivers some of the nation’s cleanest power? More than 50 percent of the electricity the company provides to customers comes from sources that are renewable and/or emit no greenhouse gases.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. He is president of the Point San Luis Lighthouse Keepers. If you have a question, send him an email at pgeweather@pge.com.

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