Weather Watch

The greater the wind speed, the higher the wave will be

The earliest writings from ancient civilizations have told stories of huge waves damaging or sinking ships. Not even modern technology is immune from Neptune’s wrath.

During World War II, British Prime Minister Winston Churchill offered President Franklin Roosevelt the use of the Queen Mary and Queen Elizabeth ocean liners to transport large numbers of U.S. troops to the British Isles. At the time, the Queen Mary was the fastest ocean liner in the world and could outrun any German U-boat.

In late 1942, the Queen Mary with her thousands of cabins stuffed with soldiers set sail to Britain. Like sailors on submarines, soldiers were required to “hot rack,” or share beds on eight to 12-hour shifts to accommodate as many troops as possible.

On her journey across the Atlantic, the Queen Mary sailed into heavy seas and began to pitch and roll. Sure enough, motion sickness struck the thousands of landlubbers on board. Dehydration from vomiting became so severe many sought medical attention in the ship’s hospital.

As she continued across the stormy Atlantic, the liner was struck by a rogue wave several hundred miles northwest of Ireland. The gigantic wave caused the ship to roll within a few degrees of capsizing. A tiny bit farther and she would have taken more than 16,000 souls to Davy Jones’ locker, becoming the worst maritime disaster in history.

Later in the war, Adm. William “Bull” Halsey sailed his fleet right into the heart of a rapidly intensifying typhoon off the Philippines. Three destroyers — USS Hull, USS Monaghan and USS Spence — capsized and sank during this storm with the loss of 775 lives.

Such disasters brought into sharp focus the dire need for accurate wave forecasting. The Department of War recruited two oceanographers at the Scripps Institute of Oceanography in La Jolla, Harald Sverdrup and Walter Munk, to develop an accurate wave forecasting method. This eventually became the Sverdrup-Munk formula.

Their calculations used three parameters: wind speed, duration and the length of wind fetch (distance of water over which the wind blows).

The greater the wind speed, the higher the waves. For example, a 20-knot wind can generate a 5-foot wave, while a 70-knot wind can generate a 56-foot wave with the same wind duration and length of the wind fetch. Wind fetches in the Pacific can exceed 500 miles. The longer the duration of wind, the more of its energy is transferred to the ocean, resulting in larger waves.

As seas develop, they can reach at most a 7-to-1 ratio of wavelength to wave height. In other words, a wave with a 7-foot length can rise only 1 foot before it breaks. When the wave breaks, longer wavelengths develop, allowing the seas to increase in height over time. Deep-water waves in the open ocean can exceed 100 feet in height.

When the seas move out from under these winds they become swells, longer-period waves.

The longer the wavelength of the swell, the faster it will travel across the ocean. The wavelength is the distance from the crest of one wave to the crest of the next wave. The period of the wave is the time it takes for two consecutive crests to pass a fixed point.

The longer-period waves have greater amounts of energy and can travel longer distances. Shorter-period waves have much less energy and travel shorter distances.

Both sound waves and ocean waves are mechanical, and share many of the same properties. Think of a car driving through your neighborhood with its stereo blaring. The first thing you hear is the thump of the base. The low-frequency base is composed of longer period waves that can travel longer distances. You generally don’t hear the higher frequencies until the car is near you.

Longer-period waves, also called forecast forerunners, with periods of more than 25 seconds and wavelengths more than 2,000 feet, can easily travel across vast expanses of the Pacific. When forecast forerunners reach the National Oceanic and Atmospheric Administration marine buoys or the Coastal Data Information Program waverider buoys, such as the one off the shore of Diablo Canyon nuclear power plant, they can help determine and forecast the peak height of the wave event later on.

As a general rule, the average speed of wave trains traveling across the deep waters of the Pacific is about 25 mph per hour or about 600 miles per day. The speed of the swell slows as it begins to feel the bottom of the sea near the coastline.

The swell will transition from a deep-water wave to a shallow-water wave when its wavelength equals one half of the water depth. As the swell continues to move toward the shallower waters near the beach, its wavelength will shorten and its wave heights will increase until the wave breaks. Interesting to note, the wave period will remain constant.

Surfer lore will tell you, the highest waves come in the middle of the wave train. In the middle of the group, the wave crests and troughs are in phase with each other and add together for maximum height. This is the so-called seventh wave.

Years after World War II, wave forecasting accuracy was further improved by the Pierson-Neumann- James method that incorporated wave spectrum data.

Today’s forecast

This morning’s northeasterly (offshore) winds will produce crystal clear skies and overall great outdoor weather for January.

Today’s high temperatures will range from the 60s in the North County and the beaches to the 70s in the coastal valleys.

Temperatures will be cool Monday morning, followed again by mild to warm daytime temperatures.

Strong to gale force (25- to 38-mph) northwesterly winds along our coastline will produce cooler afternoon high temperatures and warmer overnight lows Tuesday. Also, marine low clouds and areas of fog will develop along the coast Tuesday night.

Clear and calm conditions are forecast Wednesday through Saturday with increasing chances for ground fog in the San Joaquin Valley. Beginning the following week, the storm track is expected to shift further southward, opening the door to a possible wet weather pattern.

Today’s surf report

This morning’s 4- to 6-foot northwesterly (315-degree deep-water) swell (with an 11- to 13-second period) will remain at this height and period through this afternoon.

A 960-millibar storm with hurricane-force winds developed off Russia’s Kamchatka Peninsula on Thursday.

A long period west-northwesterly swell from this storm will arrive along our coastline tonight at 3 to 5 feet (with an 18- to 20-second period), building to 4 to 6 feet (with a 17- to 19-second period) Monday.

Strong to gale force (25- to 38-mph) northwesterly winds along the Northern and Central California coastline will generate an 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 16-second period) Tuesday, decreasing to 6 to 8 feet (with a 7- to 14-second period) Wednesday.

A 4- to 6-foot northwesterly (300-degree deep-water) swell (with an 11- to 18-second period) is forecast along the Central Coast on Thursday through Friday.

Preliminary extended surf analysis: At this time, the surface charts and models are not indicating any significant storm activity over the Northern Pacific next week. Consequently, high-energy swell events are not expected through mid-January.

Seawater temperatures

Seawater temperatures will range between 54 and 56 degrees through Friday.

PG&E reminds customers to call 811 to have underground utility lines marked before any digging project. For more information about 811 and safe digging practices, visit www.call811.com.

John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for nearly 26 years. To subscribe to his daily weather forecast or ask him a question, email pgeweather@pge.com.

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