How are waves formed? Surf reports and wave forecasts are compiled from scientific research and weather modeling. In order to find out what waves will form in the near future, it is important to understand how they are formed.

The main cause of wave formation is wind. The waves best suited for surfing are formed by the interaction of winds above the ocean surface, away from the shore. The action of wind is the first stage of wave formation.

Winds blowing offshore in a particular area can also cause waves, but they can also lead to deterioration in the quality of breaking waves.

It has been found that winds blowing from the sea tend to produce unstable and uneven waves as they affect the direction of wave travel. The winds blowing from the coast serve, in a certain sense, as a kind of balancing force. The wave travels many kilometers from the depths of the ocean to the shore, and the wind from land has a “braking” effect on the face of the wave, allowing it to avoid breaking longer.

Low pressure areas = good waves for surfing

In theory, areas of low pressure promote the formation of nice, powerful waves. In the depths of such areas, wind speeds are higher and wind gusts form more waves. The friction created by these winds helps create powerful waves that travel thousands of kilometers until they hit their final obstacles, the coastal areas where people live.

If winds generated in areas of low pressure continue to blow on the ocean surface for a long time, the waves become more intense as energy accumulates in all the resulting waves. In addition, if winds from areas of low pressure affect a very large area of ​​​​the ocean, then all the resulting waves concentrate even more energy and power, which leads to the formation of even larger waves.

From ocean waves to surf waves: the seabed and other obstacles

We have already analyzed how disturbances in the sea and the waves generated by them are formed, but after “birth” such waves still have to travel a huge distance to the shore. Waves originating in the ocean have a long journey to travel before they reach land.

During their journey, before surfers even get on them, these waves will have to overcome other obstacles. The height of the emerging wave does not match the height of the waves the surfers are riding.

As waves move through the ocean, they are exposed to irregularities in the seabed. As gigantic moving masses of water overcome high spots on the seafloor, the total amount of energy concentrated in the waves changes.

For example, continental shelves far from the coast offer resistance to moving waves due to the force of friction, and by the time the waves reach coastal waters, where the depth is shallow, they have already lost their energy, strength and power.

When waves move through deep waters without encountering obstacles on their way, they usually hit the coastline with enormous force. The depths of the ocean floor and their changes over time are studied through bathymetric studies.

Using the depth map, it is easy to find the deepest and shallowest waters of the oceans of our planet. Studying the topography of the seabed is of great importance for preventing shipwrecks and cruise liners.

In addition, studying the structure of the bottom can provide valuable information for predicting the surf at a particular surf spot. When waves reach shallow water, their speed usually decreases. Despite this, the wavelength shortens and the crest increases, resulting in an increase in wave height.

Sandbanks and wave crest increase

Sandbanks, for example, always change the nature of beach breaks. This is why the quality of waves changes over time, for better or worse. Sandy irregularities on the ocean floor allow the formation of distinct, concentrated wave crests from which surfers can begin their slide.

When a wave encounters a new sandbar, it will typically form a new crest, since such an obstacle causes the crest to rise, that is, the formation of a wave suitable for surfing. Other obstacles to waves include groins, sunken vessels, or simply natural or artificial reefs.

Waves are generated by the wind and as they travel are influenced by the topography of the seabed, precipitation, tides, rip currents off the coast, local winds and bottom irregularities. All these weather and geological factors contribute to the formation of waves suitable for surfing, kitesurfing, windsurfing and boogie surfing.

Wave forecasting: theoretical foundations

• Long-period waves tend to be larger and more powerful.
• Waves with a short period tend to be smaller and weaker.
• The wave period is the time between the formation of two clearly defined crests.
• Wave frequency is the number of waves passing through a certain point in a certain time.
• Big waves move fast.
• Small waves move slowly.
• Intense waves form in areas of low pressure.
• Low pressure areas are characterized by rainy and cloudy weather.
• Areas of high pressure are characterized by warm weather and clear skies.
• Larger waves form in deep coastal areas.
• Tsunamis are not suitable for surfing.

Wave is a form of periodic, continuously changing motion in which water particles oscillate around their equilibrium position.

If, for some reason, water particles are removed from the equilibrium position, then under the influence of gravity they will strive to restore the disturbed equilibrium. In this case, each water particle will perform an oscillatory motion relative to the equilibrium position, without moving along with the visible form of wave motion.

Waves can arise under the influence of various reasons (forces). Depending on the origin, i.e., on the causes that caused them, the following types of sea waves are distinguished.

1. Friction waves (or friction waves). These waves primarily include wind waves, which arise when the wind acts on the surface of the sea. These also include the so-called internal, or deep, waves, which arise at depths when a layer of water of one density moves over a layer of water of another density.

Research has established that if another liquid of a different density moves over a liquid of one density, then waves are formed on the surface separating both liquids. The size of these waves depends on the difference in the speed of movement of liquids in relation to each other and the difference in density of the two media. This also applies to the case of air movement over water. This is why waves arise both in the depths of the ocean and in the high layers of the atmosphere, if there is a similar movement of two water or air masses of different densities.

1. Baric waves occur when atmospheric pressure fluctuates. Fluctuations in atmospheric pressure cause rises and falls of water masses, in which water particles strive to occupy new equilibrium positions, but, having reached them, perform oscillatory movements by inertia.


2. Tidal waves arise under the influence of the phenomenon of ebb and flow of tides.


3. Seismic waves are formed during earthquakes and volcanic eruptions. If the source of an earthquake is located under water or close to the shore, then the vibrations are transmitted to the water masses, causing seismic waves in them, which are also called tsunamis.


4. Seiches. In seas, lakes, and reservoirs, in addition to vibrations of water particles in the form of translational waves, periodic vibrations of water particles only in the vertical direction are often observed. Such waves are called seiches. During seiches, vibrations occur, similar in nature to vibrations, in a periodically rocking vessel. The simplest type of seiche occurs when the water level rises at one edge of the reservoir and simultaneously falls at the other. In this case, in the middle of the reservoir there is a line along which water particles do not have vertical movements, but move horizontally. This line is called the seiche node. More complex seiches are two-node, three-node, etc.

Seiches can occur as a result of various reasons. A wind blowing over the sea for some time in the same direction produces a surge of water at the leeward coast. With the cessation of wind, seiche-like level fluctuations immediately begin. The same phenomenon can occur under the influence of differences in atmospheric pressure in different places in the water basin. Senche fluctuations in sea level are created by seismic vibrations in very small basins (in a harbor, in a bucket, etc.) Seiches can occur during the passage of ships.

There are numerous theories of wave formation. None of the theories describes the phenomenon completely, but since it is the effects, not the causes, that are of practical interest, this state of science should not particularly worry us. At the initial stage, the waves are apparently caused by friction between the moving air flow and the stationary surface of the water. Water slows down the air flow, causing vortices in it, while the surface of the water becomes uneven and ripples form on it. A vicious circle arises: due to the fact that the surface of the water has become uneven, friction increases and the vortices above the surface of the water intensify.

Immediately after the formation of waves, the so-called Jeffreys screening mechanism begins to work, according to which the air flow over large waves is significantly distorted. This affects the sail of small yachts such as single dinghies. According to Jeffreys' theory, the air flow presses on the windward slope of the wave, rises more or less smoothly along the slope and is directed slightly upward at the crest, and then, falling, presses on the slope of the next wave; the gap under the smooth air flow on the windward slope of the wave is filled with a turbulent vortex in such a way that this part of the wave is shielded from the action of the wind. Figure 27 helps to understand this mechanism*.

Jeffreys' theory is not entirely correct, since it does not take into account the speed of the fastest waves, which can move at the speed of the wind or even faster, whereas usually when the wind is steady for a significant time, the waves move at a speed of about 3/4 of the speed wind. However, the shielding mechanism plays an important role in the formation of slower waves. Updrafts of air over the crests are used by soaring seabirds.

Theoretically, ripples appear when the wind is about 2 knots, but at the same time real waves are not formed and the existence of already formed waves is not maintained, moving to areas with lower wind speed. If the wind that formed the ripples subsides, then the surface of the water again becomes smooth as a mirror.

Therefore, dark streaks of ripples on a calm day are a good indicator of wind speed at the surface of the water, although this does not necessarily mean that where there are no windbreaks, there are several above the water surface there is no wind. For a yachtsman, observing wind stripes in calm conditions is very important, but these stripes are not always unmistakable signs of the best sailing wind, since the yacht is driven by the wind not at the very surface, but somewhat higher.

It must be emphasized that the ripples are caused relative movement water and air directly above the surface. Therefore, in the presence of a current, ripples can form when there is calm. Thus, dark stripes on the water do not always indicate wind; they can equally be a consequence of the current. It should also be noted that the wind, which usually causes ripples, does not form them if the water is moving in approximately the same direction and at the same speed as the air. Under such conditions, smooth areas of the water surface may indicate the presence of a passing current. Likewise, if a light wind blows in the direction of the current, and the calm current has already formed a ripple, then the wind can destroy it. Therefore, when using ripples as an indicator of the presence of wind or current, it is necessary to remember all of the above circumstances. (See also pp. 71-76.)

The size of the waves is affected by the distance at which the wind waves developed. This distance is called wind (or wave) acceleration. To understand the effect of acceleration on waves, let us consider how a breakwater acts on them. The same wind can be both before and after the breakwater, but the waves will be significantly different. Behind the breakwater, it is formed by the wind with limited acceleration and is therefore relatively small and can be safe.

Viscosity also affects the formation of waves: under natural conditions, large waves rarely form at wind speeds of less than 8 knots. Continuous exposure to increasing winds produces large waves, but then dissipation and turbulence limit the size of the waves, and further energy supplied by the wind is used only to increase their length and speed. For example, a strong squall is known to produce steep, erratic small waves that, unlike relatively tall waves, have not had enough time to reach significant length or speed.

Swell

Absolutely regular big waves are relatively rare even in the open ocean, and they are even rarer in coastal waters. Waves generated by strong winds decay slowly and therefore travel long distances; Such waves that move without the help of wind are called swell. Very often, two or three swell systems can be observed simultaneously in the same area. Often, with local wind, waves of smaller sizes and a different direction are formed on the crests of the swell. All this can happen in the open sea, hundreds of miles from land, so it is easy to imagine a complex pattern of interference in shallow waters off the lee shore and in the presence of currents.

Strangely enough, and probably contrary to popular belief, is my opinion that the excitement in dinghy and other small yacht competitions is often more regular than in the mid-Atlantic; the reason is that acceleration is limited in the competition areas, so the waves here are young and therefore coincide with the observed wind and are not “confused” with waves originating in other areas.

It is well known that swells created by strong winds can persist long after the wind has died down. It is not surprising that the speed of the swell very often significantly exceeds the speed of the local wind. What is less known (we have already mentioned this) is that with sufficiently high acceleration and stable wind, the speed of waves can be significantly greater than the speed of the wind that generated them. There is a recording of waves at 60 knots; 30 knots is quite normal speed.

The world's oceans are in constant motion. In addition to waves, the calm of waters is disturbed by currents, ebbs and flows. All these are different types of water movement in.

Wind waves

It is difficult to imagine an absolutely calm surface of the ocean. Calm - complete calm and absence of waves on its surface - is very rare. Even in calm and clear weather, ripples can be seen on the surface of the water.

Both these ripples and the raging waves of foam are generated by the force of the wind. The stronger the wind blows, the higher the waves and the greater the speed of their movement. Waves can travel thousands of kilometers from the place where they originated. Waves contribute to the mixing of sea waters, enriching them with oxygen.

The highest waves are observed between 40° and 50° S. sh., where the strongest winds blow. Sailors call these latitudes stormy or roaring latitudes. Areas where high waves occur are also located off the American coast near San Francisco and Tierra del Fuego. Storm waves destroy coastal buildings.

The highest and most destructive waves. The reason for their occurrence is underwater earthquakes. In the open ocean, tsunamis are invisible. Along the coast, the wave length decreases, and the height increases and can exceed 30 meters. These waves bring disaster to residents of coastal areas.

Ocean currents

Powerful water flows - currents - are formed in the oceans. Constant winds cause surface wind currents. Some currents (compensatory) compensate for the loss of water, moving from areas of its relative abundance.

A current whose water temperature is higher than the temperature of the surrounding waters is called warm; if it is lower, it is called cold. Warm currents carry warmer waters from the equator to the poles, cold currents carry colder waters in the opposite direction. Thus, currents redistribute heat between latitudes in the ocean and have a significant impact on the climate of the coastal areas along which they carry their waters.

One of the most powerful ocean currents is. The speed of this current reaches 10 kilometers per hour, and it moves 25 million cubic meters of water every second.

Ebbs and flows

The rhythmic rise and fall of water levels in the oceans are called tides. The reason for their occurrence is the effect of the gravitational force of the Moon on the earth's surface. Twice a day the pod rises, covering part of the land, and retreats twice, exposing the coastal bottom. People have learned to use the energy of tidal waves to generate electricity at tidal power plants.

Introduction

There is no sea without waves; its surface always fluctuates. Sometimes these are just light ripples on the water, sometimes rows of ridges with cheerful white caps, sometimes menacing waves carrying clouds of spray. Even the calmest sea “breathes”. Its surface seems completely smooth and shines like a mirror, but the shore is licked by quiet, barely noticeable waves. This is the ocean swell, the harbinger of distant storms. What are the main reasons for the formation of waves, and how do sea waves affect a person and his activities?

The relevance of this issue is constantly increasing, in proportion to the development of human civilization and the development of sea spaces.

This work is intended to help in resolving issues related to wave processes and to describe in as much detail as possible everything connected with them, their nature and activity.

Types of sea disturbances

Sea disturbances can be divided into several types. These types are distinguished according to the characteristics of each of them.

Wave classification

There are many classifications of waves that differ in their physical nature, specific propagation mechanism, propagation medium, etc.

According to the nature of the wave-forming forces, waves are divided into 2 types: free and forced.

Free waves are not directly influenced by the forces that cause them, but are excited by initial or boundary disturbances. Depending on the nature of the disturbing force, the following subtypes of free waves are distinguished:

wind waves caused by initial disturbance - the action of wind tension;

seismic waves caused by underwater earthquakes and volcanic eruptions (tsunamis);

waves caused by the dynamic instability of large-scale currents.

Forced waves are under the direct influence of the forces that cause them. They are divided into 3 subtypes:

wind waves excited by the action of wind on the water surface;

baric waves excited by an atmospheric pressure gradient (see Anemobaric 1 waves);

tidal waves excited by the tidal forces of the Moon and the Sun.

Depending on the stratification of waters, all waves are divided into 2 types: surface and internal.

Surface waves have a maximum amplitude on the free surface, and their characteristics do not depend on the stratification of water by density. With increasing depth, the amplitude of such waves decreases according to a law close to exponential. Buoyancy forces play a significant role in the formation of internal waves; the characteristics of these waves significantly depend on the stratification and vertical stability of the waters. The amplitude of internal waves is inversely proportional to the vertical gradient of water density.

Anemobaric waves - Forced long gravitational or inertial-gravitational waves arising under the influence of wind and atmospheric pressure. They can be progressive or standing. The periods of anemobaric waves range from several minutes to a day, the height in the open sea does not exceed 1 m. In the coastal zone, long waves of anemobaric origin make a significant contribution to storm surges, sometimes leading to catastrophic floods.

Depending on the degree of participation in the formation of surface and internal waves of gravity and forces caused by the rotation and sphericity of the Earth, classes of waves are distinguished. The division into classes is based on the ratio of the period of waves T to the period of inertial oscillations Тр = р/ ьsinт, where у is the angular velocity of the Earth's rotation; c -- geogr. latitude of the place. The following classes of waves are distinguished:

gravitational, in the formation of which gravity forces play a dominant role (T<

inertial-gravitational, for the formation of which both gravity and the deflecting force of the Earth's rotation are essential (T<Тp);

inertial, or gyroscopic, in the formation of which the dominant force is the Coriolis force (T = Tp);

planetary (so-called Rossby waves), caused by the combined effect of rotation and sphericity of the Earth (T>>Tr).

The class of inertial waves in internal waves is not distinguished, since they propagate mainly in the horizontal plane and do not depend on the stratification of waters. In the classes of surface and internal gravity waves, types are distinguished: short waves, the length of which is significantly less than the depth of the sea, and long waves or waves of shallow water, the length of which is much greater than the depth of the sea.

In the class of planetary waves, short and long waves are divided depending on the ratio of wavelength to pool length. When this ratio is small, the waves are short, when it is high, the waves are long.

In the classes of inertial-gravity and inertial waves, types are not distinguished.

Finally, according to the nature of their propagation, waves are divided into progressive (progressive or, as they are also called, traveling) and standing. Translational waves (eg wind waves) have a visible movement of shape. Standing people do not have such movement. In real ocean conditions, the observed waves are a complex combination of free and forced, standing and forward wave systems of various origins. The nature of wave processes is especially complicated in coastal areas due to the influence of the topography of the bottom and shores, reflection, diffraction and refraction of sea waves.