1. Why Waves Arrive Even When the Wind Stops: Wind Waves vs. Swells
When you stand by the sea, the wind might be almost non-existent. Still, large waves come rolling in slowly from the offshore direction. The Japan Meteorological Agency divides waves into two types. Waves generated by the wind blowing on the sea surface are called "wind waves." Waves that remain after the wind has stopped developing them are called "swells" [1].
The large waves hitting a beach where the wind has died down are likely swells. Swells are waves that have traveled from winds blowing in distant places, such as typhoons or low-pressure systems [3]. This raises a question: how does wind from far away grow these waves so large?
2. How Strong, Long, and Wide Winds Make Big Waves
Imagine blowing air with a fan over the water in a bathtub. If you fan it gently, only small ripples appear. Educational materials from the University of Wisconsin list three conditions for waves to grow large [2]. First is wind speed; energy transfers to the wave only if the wind moves faster than the wave crest. Second is the duration the wind blows. Third is the fetch, the distance over which the wind blows without changing direction significantly.
A bathtub hits its walls quickly, but storms at sea can blow over wide areas for a long time. That is why large waves grow inside storms. But what happens to these waves after they leave the storm?
3. How Waves Sort Themselves, with Long Ones in Front, After Leaving a Storm
Inside a storm, long and short waves are mixed together. However, once they leave the storm area, long waves move to the front and short waves fall behind. This is because longer waves travel faster [2].
Think of it like trains: if a local train and an express train leave the same station, the express arrives first. However, unlike trains, a wave’s speed is determined by its length. The separated waves become smooth and regular swells. According to the Japan Meteorological Agency, in sufficiently deep water, the wavelength is proportional to the square of the period [1]. This means waves with longer periods have longer wavelengths and travel faster.
Explanations from the National Weather Service in Hawaii give an example: wave energy from strong winds in a distant low-pressure system off Russia moved southeast toward Hawaii, becoming the swell there [4]. As waves travel far, they weaken while their height decreases and their period lengthens [3]. Even so, high swells can be observed thousands of kilometers away. In Japan, people have long experienced these distant swells as high waves around typhoon season.
4. Doyo-nami: High Waves That Arrive Before a Typhoon
From summer to autumn, high waves often roll onto Japan's Pacific coast. People in the past called these waves "Doyo-nami" [3].
The Japan Meteorological Agency explains that typhoons during this season circle around the Pacific High before approaching Japan. Therefore, the swells arrive in Japan before the typhoon itself does [3]. For surfers, swells created by distant storms can be the source of good waves [6]. However, on days with high swells, danger increases both at the water's edge and in the sea. Please do not go near the sea on such days.
Now, the swells that arrived from far away change shape again near the shore.
5. Why Waves Slow Down and Grow Taller in Shallow Water
On a road, when the car in front slows down, the cars behind bunch up, and the distance between them shrinks. A similar thing happens to waves. When swells enter shallow water, the waves slow down, but their period does not change. Because the next wave still arrives at the same time interval, the distance between waves (wavelength) becomes shorter, and the waves grow taller [4]. This is called shoaling. However, how much they grow depends on the location and water depth.
It is important to remember that waves are not cars. The water itself is not traveling forward; the wave shape is. The National Weather Service in Hawaii suggests a rule of thumb: breaking wave height is about 2 times the offshore swell height [4]. This is a guideline for Hawaii and does not apply exactly to Japanese coasts. Even with the same swell, the wave face can become smooth or messy depending on the wind that day.
6. How Offshore and Onshore Wind Change the Shape of a Wave
Surfers pay attention to wind direction. Wind blowing from land to sea is called "offshore," and wind blowing from sea to land is called "onshore."
Surfing magazines explain that offshore wind smooths the wave face, supporting the wave and making it less likely to break early. Onshore wind flattens the wave and causes it to break early, in deeper water [5]. Even with the same swell, the wave’s appearance can look very different depending on wind direction. Note that these are explanations from surfing media, not public agencies, so treat them as "said to be" rather than absolute facts. Also, very strong offshore winds can make waves difficult to ride. Light offshore wind is considered best [5]. So, how do surfers read today’s waves using swells and wind?
7. Reading Surf Forecasts: Height, Direction, and Period, Then Checking at the Sea
In surf wave forecasts, the three main keys are swell height, direction, and period [6]. Since this is from an encyclopedia, read it as a supplementary guide. Swells with longer periods are said to wrap around the shape of the coast more easily. Swells created by distant storms are said to be good for surfing [6].
Even so, the explanation states that you must ultimately decide by looking at the sea at the location with your own eyes [6]. Distant storms create swells, swells rise at the shore, and wind adjusts the face. Today’s waves arrive from the vast expanse of the weather map.
8. Two Ways to Investigate Waves Yourself
Here are two things you can check yourself. At a safe water’s edge at a sea or lake, measure the interval between wave crests reaching the shore using a clock’s second hand. Record the arrival times 6 times, then divide the difference between the first and last time by 5 to get an estimate of the period. Since the period does not change even in shallow water [4], you can measure it at the shore. Do not enter the water; observe from a distance with an adult.
Check the weather map on the Japan Meteorological Agency’s homepage. On days when a typhoon or low-pressure system is far from Japan, compare the waves at a nearby sea with news or local conditions over several days and keep a note. If you want to read more, the Japan Meteorological Agency’s explanations on wave phenomena [1] and high waves from typhoons [3] are available in Japanese.