What causes wind gusts over the ocean — and how do you predict them?

Bart Alford of Los Osos asked what causes wind gusts — especially out over the flat expanse of the world’s oceans without any obstructions except for the endless waves.

According to the National Weather Service, a wind gust is a “rapid fluctuation in the wind speed with a variation of 10 knots or more between peaks and lulls. The speed of the gust will be the maximum instantaneous wind speed.”

A good rule of thumb is wind gusts are typically about 1.8 times faster than sustained winds over an unstructured area.

If you move away from the Earth’s surface, wind speeds tend to increase. In fact, at the height of only 33 feet, the sustained winds often move twice as fast as at the surface because of less friction with the ground or water.

To get a better understating of a wind gust, let us first discuss what causes the winds?

The winds develop because of uneven heating of the Earth’s surface by the sun, which produces pressure differences.

Air will flow from areas of high pressure to areas of low pressure as the atmosphere attempts to achieve equilibrium. Think of an air leak from a bicycle tire (high pressure) flowing out to the atmosphere (low pressure).

In my daily weather forecast, I often refer to these differences in pressure as gradients. The steeper the pressure gradients, the stronger the winds.

So, if you have high pressure to the north and low pressure to the south, will the winds then blow north to south? Unfortunately, it is not that straightforward.

The Earth’s eastward rotation 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.

Airflow is deflected to the right from its expected path in the Northern Hemisphere and 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.

So, what causes wind gusts over the ocean? It is mostly turbulence, and here is why.

Turbulence is caused by many factors — such as solar heating of the Earth’s surface, which produces rising air currents (thermals) that cause uneven temperatures in the air column.

Wind shear, the difference in wind speed or direction both vertically and horizontally over a relatively short distance, and air flowing up, over and down mountains or through gaps, passes and canyons and produce severe turbulence.

In conditions like this, wind gusts can be many times stronger than the average wind speed. However, with much less obstructions, wind gusts are generally weaker over the ocean.

Turbulent flow is characterized by whorls of vortices of various sizes.

Lewis Fry Richardson, a British mathematician. physicist, meteorologist, psychologist and pacifist, first developed numerical weather prediction models. in 1922.

He wrote this poem:

Big whorls have little whorls

That feed on their velocity,

And little whorls have lesser whorls

And so on to viscosity.

Even today, modeling or predicting turbulent flow is one of the significant challenges of modern physics and, in turn, weather forecasting. We still do not have a straightforward way to model it.

Like water, the atmosphere is a fluid with eddies and swirls spinning about. When I write the weather forecast, I often wish a model could predict these unpredictable flows.

When I write the weather forecast, the first thing I do is to look at the wind speed data from the meteorological tower at Diablo Canyon Power Plant.

Even before I look at the wind direction data, I can often tell where the wind is coming from by the strength of the wind gusts.

Like an International Harvester semi-truck rolling down the Cuesta Grade, air from the higher elevations of the Santa Lucia Mountains flow downward along the mountain slopes toward the Pacific Ocean, pulled by the never-ending force of gravity.

These katabatic downslope winds are called Santa Lucia winds along the Central Coast.

As these winds pour through the rugged passes and canyons of the Irish hills behind the power plant, I’ve seen wind gusts as much as three times faster than the sustained winds.

If the winds are coming from the Pacific and blowing toward the land, the gusts are less than twice as strong.

Even a slight increase in wind speed can significantly increase the force applied to objects, such as the surface of the ocean, buildings or trees. For example, if you double the wind speed from 22 mph to 44 mph, you’re quadrupling the amount of force.

PG&E safety message

It is graduation season in California. Here’s an important reminder for the public about the safety risks of helium-filled metallic balloons.

If your graduation celebration involves balloons, make sure they are secured with a weight. Otherwise, they can float away and come into contact with overhead power lines, causing a public safety risk.

In the first four months of 2021, metallic balloons striking electric lines caused nearly 175 power outages in the PG&E service area alone, disrupting service to more than 107,000 customers. This is a 75% increase from the same time last year.

This story was originally published June 1, 2021 at 5:05 AM.