Understanding Wind and Waves

Understanding Wind

The relationship between the wind and the waves is crucial for boat skippers to understand. It is so significant that a new classification system was created to guide them, incorporating both wind speed and the corresponding wave conditions. This system, known as the Beaufort Scale, was developed in 1805 by Admiral Sir Francis Beaufort of the British Navy. It serves as a guideline for what one can expect under certain conditions and acts as a weather classification system.

When considering the Beaufort Scale, remember that it is based on open sea conditions. This means the open ocean, with no land masses to block the wind. This is referred to as unlimited fetch. Fetch is the ocean or lake surface area over which the wind blows in a relatively constant direction, thus generating waves. If you are 1/2 mile offshore with hills, trees, or buildings close to the shoreline, and the wind is coming from the direction of the shore, the Fetch is the distance from the shore to your boat, so about 1/2 mile. On the other hand, if the wind is coming from the open ocean, then you have “Unlimited Fetch”.

The strength of wind on the Beaufort Scale is referred to as “Force“. Force 0 indicates calm seas, while Force 12 indicates Dorothy and Toto are flying past!

The Beaufort Scale

The Beaufort Scale enables you to estimate wind speed by observing the state of the sea surface. The condition of the sea, including the size of the waves and the presence of white caps, foam, or spray, is mainly influenced by three factors:

1. Speed. The higher the speed of the wind, the greater the disturbance.
2. Duration. Disturbance increases the longer the wind blows at a given speed until a maximum state of disturbance is reached. In other words, the longer the wind blows, the bigger the waves get, to a certain point.
3. Fetch. This is the length of the stretch of water over which the wind acts on the sea surface from the same direction.

For a given wind speed and duration, the longer the fetch, the greater the sea disturbance. If the fetch is short, such as a few miles, the disturbance is relatively small, no matter how great the wind speed or how long it has been blowing.

Aside from just wind speed, temperature is also a factor in creating waves. Warm air (which rises) moving over water has a less acute angle of attack on the surface than does cool air (which sinks). A cold front moving across open water will create much steeper waves and hence create breakers sooner than a warm front moving at the same speed.

A change in wind direction over existing waves can create confusion and lead to larger waves. For example, if a wind has been blowing from the northeast over an open body of water for three days and then suddenly shifts to blowing from the northwest, new wavelets will form within the existing wave system. The energy from both systems will combine, resulting in larger waves.

When a wave system encounters a current, two scenarios can occur. If the wind and the current are flowing in the same direction, the waves tend to smooth out, resulting in long swells. Conversely, if the wind and current are moving in opposite directions, it produces steeper and more aggressive waves.

Why Knowing How Waves Are Made Is Important

You can determine several things from waves.

One way to determine a boat’s speed is by observing the waves it creates. This method applies to displacement vessels, such as keelboats, but not to planing vessels like speedboats. Displacement boats continually push a significant amount of water as they move. The heavier the boat, the deeper the trough it carves through the water.

Additionally, the physics of waves tells us that as a wave’s speed increases, its length also increases. As a boat’s speed rises, the number of waves it generates diminishes until the boat becomes caught between the crest and trough of a single wave that it has created while moving through the water.

Understanding Waves

The first principle to keep in mind about waves, particularly in the open ocean, is that they can be unpredictable. While this statement may seem paradoxical, it highlights an important aspect of nature. There are fundamental physical factors that characterize a “normal” wave, but various other influences can affect their behavior due to the dynamic forces at work. Nevertheless, gaining a deeper understanding of wave formation and movement can be immensely advantageous for boaters in their daily experiences. This knowledge can enhance safety and improve navigation skills on the water.

Three factors make up waves:

• Wind speed
• The length of time the wind has blown
• Distance of open water that the wind blows over (Fetch)

All of these factors work together to create waves. The more significant each of the variables in the equation, the greater the waves.

A fourth factor, depth of water, comes into play when very near shore. As the depth of the water reduces, water is pushed up, amplifying the wave. This is the cause of the larger swells and rollers you see on the beach. When cruising off the coast of New Jersey, we were about a mile offshore in 20-25 feet of water and were encountering 4-5 foot swells. We went another mile farther off shore where the water depth was over 50 feet, and the swells were only 2-3 feet. Sometimes, moving into deeper water can reduce the effect of waves.

Waves are measured by:

• Height (from trough to crest)
• Length (from crest to crest)
• Steepness (angle between crest and trough)
• Period (length of time between crests)

Waves do not all come in uniform heights; rather, they follow a systematic pattern with varying sizes. To classify wave height, we determine the significant wave height, which is the average of the highest one-third of the waves in a given system. This is the measurement used by weather reports to specify wave heights. Once we have the significant wave height, we can easily calculate the theoretical average height, the height of the highest 10% of waves, and the overall largest waves in a specific area. Mathematically, this is simply a matter of arithmetic based on predetermined ratios.

Rather than whipping out your calculator when getting knocked around by waves, just remember that about 1 in every 6 waves is usually larger than average.

TYPES OF WAVES

Waves take their time to develop; they don’t spontaneously erupt from the ocean. It takes a certain speed of wind to blow over a certain distance for a considerable length of time to create lasting waves.

There are three different types of waves that develop over time: Ripples, Seas, and Swells.

• Ripples appear on smooth water when the wind is light, but if the wind dies, so do the ripples.
  
• Seas are created when the wind has blown at a given velocity for a while. They tend to last much longer, even after the wind has died.
  
• Swells are waves that have traveled away from their origin and are not affected by local wind conditions; in other words, they are seas that persist long after the wind has died down.
  

Understanding swells can be somewhat perplexing, particularly when you consider that waves don’t truly travel through water. Instead, the water moves predominantly with the current, which represents a distinct aspect of water’s behavior. To visualize this, picture two individuals at either end of a long, sturdy rope, gracefully undulating their arms in synchronized motion. As they do so, waves ripple along the length of the rope, creating the illusion of movement from one end to the other. Yet, the fibers of the rope remain stationary, fluctuating only in an elegant up-and-down motion. This phenomenon mirrors what happens with water waves. 

The velocity of a wave is measured by the time it takes for a crest to pass a specific point, like an imaginary line marked on the ground beneath the rope. Although water particles exhibit some movement within the wave, it’s essential to grasp that the water itself does not travel along with the wave. Instead, the energy of the wave transfers through the water, creating the mesmerizing dance of swells in the ocean.

Waves can be further described as Non-Breaking and Breaking.

Non-breaking waves are often referred to as “normal” rolling waves.

Breaking waves are waves whose bases can no longer support their top and collapse. Depending on the size, this can happen with considerable force behind it—5 to 10 tons per square yard—enough force to crush the hull of a boat. When the ratio of steepness of a wave is too great, it must break. This happens when a wave runs into shallow water or when two wave systems oppose and combine forces.

Understanding Your Limits

By combining an understanding of Wind and Waves, you can determine both your safety range and comfort level. Having pre-set limits and sticking with them can improve your safety and overall enjoyment on the water. If you decide to make an exception, it is done with careful consideration and a backup plan.

Using tools like Windy, and Predict Wind will let you see both the anticipated, wind and waves. These Apps also show you the expected direction of the wind, waves, and current so that you can make your decision with all the information you need. If you want to check wave heights and periods on a specific route, check out the Free Marv’s Weather website for current wave heights, as well as predictions for many popular routes along the East Coast and in the Gulf of (America/Mexico).

Hint: Check out our Article on Weather Apps For Boating to learn more about these valuable tools!

Do you have preset limits for wind speed or sea state? What are they?

Let us know in the comments below!

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