Weather Watch

Sorry, Bob Dylan — you do need a weatherman to tell you how the wind blows

A windy day in Morro Bay.
A windy day in Morro Bay. Special to The Tribune

Mike in Paso Robles asked me recently what causes the wind.

The straightforward answer is a difference in atmospheric pressure from one location to another. Nature never likes anything out of balance, and air will naturally flow from areas of high pressure to areas of low pressure. When that happens, it’s called the wind.

Think of an air leak from a bicycle tire (high pressure) flowing out to the atmosphere (low pressure).

The changes in air pressure are primarily due to uneven heating of the Earth’s surface by the sun and changes in speed and direction of the jet stream. The jet stream — typically a tubular ribbon of high-speed winds flowing in wave-like patterns for thousands of miles from west to east some 18,000 to 40,000 feet up — can carve out areas of low pressure in the atmosphere like a hot-steel spoon in a gallon of ice cream.

The strength or intensity of the low- and high-pressure systems combined with how close they are together will determine the steepness of the “pressure gradient” and the speed of the wind. Like a bicycle, the steeper the hill, the faster you’ll go. The steeper the pressure gradient, the stronger the winds. In 1934, winds above Mount Washington, a 6,288-foot peak in New Hampshire, reached a sustained speed of 186 mph with a peak gust of 231 mph due to an abnormally tight pressure gradient between an area of strong high-pressure and an intense low-pressure storm.

Bob Dylan once wrote, “You don’t need a weatherman to know which way the wind blows.”

So, Mr. Dylan, it would seem that if you have high pressure to the north and low pressure to the south, winds would then blow north to south.

However, this is not what happens. The eastward rotation of the Earth on its axis deflects the moving air away from its initial course in the free atmosphere. The free atmosphere lies above the frictional influence of the Earth’s surface.

In 1934, winds above Mount Washington, a 6,288-foot peak in New Hampshire, reached a sustained speed of 186 mph with a peak gust of 231 mph.

In the Northern Hemisphere, the air flow is deflected to the right from its expected path and to the left in the Southern Hemisphere.

The apparent force responsible for turning the wind flow is called the Coriolis effect, named after Gaspard Gustave de Coriolis, a French scientist who worked out its mathematics in 1835. Think of it this way: Even though all areas on the surface of the Earth make one complete rotation every 24 hours, the equator — the thickest part of the earth — must travel a little more than 1,037 miles per hour to go around the world in one day. As we journey north or south of the equator, the relative speed of rotation around the Earth decreases. At our Central Coast latitude, the speed is 850 mph.

It’s less than 1 mph at the North and South Poles. On smaller distances, there’s not much effect. Don’t believe what you might have heard about Coriolis making the water in a sink rotate one way as it drains in one hemisphere, the other way in the other hemisphere.

However, Coriolis force plays a significant role in shaping the Central Coast climate. As the northwesterly winds blow parallel to our coastline, the friction of the wind causes the ocean surface water to move. The Coriolis force turns the surface water to the right, or offshore, and causes upwelling along the coast as cold subsurface water rises to the surface along the immediate shoreline. The overlying air then becomes chilled, which contributes to coastal low clouds and fog. It’s not uncommon to see a 50-degree temperature gradient between the beaches and inland valleys during summer.

Atmospheric pressure along the Central Coast often shows a diurnal cycle — a daily rotation repeated every 24 hours. 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 creates 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 northeasterly (offshore) winds. Of course, many other factors can affect the winds. If these northeasterly (offshore) winds are strong enough, they can push the marine layer out to sea, leaving behind clear skies. They typically produce warm and dry weather.

Northwesterly winds, on the other hand, are referred to as onshore winds because they blow from the Pacific Ocean through our coastal valleys and over our mountains. They often produce low coastal clouds, mild temperatures and higher humidity levels. Southeasterly winds are sometimes referred to as prefrontal winds and are often a precursor to stormy and wet weather.

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Nearly 70 percent of the electricity PG&E delivered to its customers in 2016 came from greenhouse gas-free resources like nuclear (Diablo Canyon nuclear power plant) and large hydro. An average of 32.8 percent of its electricity in 2016 came from renewable resources including solar, wind, geothermal, biomass and small hydroelectric power plants.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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