The main reason for the formation of waves is the wind blowing over the water. Therefore, the magnitude of the wave depends on the strength and time of its impact. Due to the wind, water particles rise up, sometimes breaking away from the surface, but after some time, under the influence of natural gravity, they inevitably fall down. From afar, it may seem that the wave is moving forward, but in fact, if this wave, of course, is not a tsunami, (tsunamis have a different nature of occurrence), it only descends and rises. So, for example, a sea bird that has landed on the surface of a rough sea will sway on the waves, but will not budge.
Only near the shore, where it is no longer deep, the water moves forward, rolling onto the shore. By the way, according to the scallop of spray from the detached drops forming a crest on the wave, experienced sailors determine the degree of sea disturbance, if the crest and foam on it have just begun to form, then the sea is 3 points.
What kind of sea wave is called a coast.
Waves on the sea can exist without wind, this is a tsunami caused by natural disasters like underwater volcanic eruptions, and a wave that sailors call a coast. It is formed at sea after a strong storm, when the wind died down, but due to the large mass of water that came into motion from the wind and a phenomenon called resonance, the waves continue to sway. It should be noted that such waves are not much safer than a storm and can easily capsize a ship or boat with inexperienced sailors.
The surface of the seas and oceans is rarely calm: it is usually covered with waves, and the surf continuously beats on the shores.
An amazing sight: a massive cargo ship, which is played by giant storm waves in the open ocean, seems no larger than a nutshell. Disaster movies are replete with such pictures - a wave as high as a ten-story building.
Wave oscillations of the sea surface occur during a storm, when a long gusty wind combined with drops atmospheric pressure forms a complex chaotic wave field.
Running waves, boiling foam of the surf
Moving away from the cyclone that caused the storm, one can observe how the wave pattern is transformed, how the waves become more even and slender rows move one after another in one direction. These waves are called swell. The height of such waves (that is, the difference in levels between the highest and lowest points of the wave) and their length (the distance between two adjacent peaks), as well as the speed of their propagation, are fairly constant. Two crests can be separated by a distance of up to 300 m, and such waves can reach a height of 25 m. Wave vibrations from such waves propagate to a depth of up to 150 m.
From the region of formation, swell waves propagate very far, even with complete calm. For example, cyclones passing off the coast of Newfoundland cause waves that reach the Bay of Biscay off the western coast of France in three days - almost 3000 km from the place of their formation.
When approaching the shore, as the depth decreases, these waves change their appearance. When wave oscillations reach the bottom, the movement of waves slows down, they begin to deform, which ends with the collapse of the ridges. Such waves are eagerly awaited by surfers. They are especially spectacular in areas where the seabed drops sharply near the coast, for example, in the Gulf of Guinea in western Africa. This place is very popular with surfers all over the world.
Tides: global waves
Tides are a completely different phenomenon. These are periodic fluctuations in sea level, clearly visible off the coast and repeating approximately every 12.5 hours. They are caused by the gravitational interaction of ocean waters mainly with the Moon. The period of the tides is determined by the ratio of the periods daily rotation Earth around its axis and the rotation of the Moon around the Earth. The sun is also involved in the formation of tides, but to a lesser extent than the moon. Despite the superiority in mass. The sun is too far from the earth.
The total value of the tides depends, therefore, on relative position Earth, Moon and Sun, which changes during the month. When they are on the same line (which happens on the full moon and new moon), the tides reach their maximum values. The highest tides are observed in the Bay of Fundy on the coast of Canada: the difference between the maximum and minimum sea level positions here is about 19.6 m.
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We have long been accustomed to many phenomena occurring on our planet, without thinking at all about the nature of their occurrence and the mechanics of their action. This is climate change, and the change of seasons, and the change of time of day, and the formation of waves on the sea and in the oceans.
And today we just want to pay attention to the last question, the question of why waves form on the sea.
Why do waves form in the sea
There are theories that waves in the seas and oceans arise due to pressure drops. However, these are often just the assumptions of people who quickly try to find an explanation for such a natural phenomenon. In reality, things are somewhat different.
Remember what makes the water "worry". This is a physical effect. Throwing something into the water, running a hand over it, hitting the water sharply, vibrations of different sizes and frequencies will certainly begin to go through it. Based on this, it can be understood that waves are the result of a physical impact on the surface of the water.
However, why do large waves appear on the sea, coming to the shore from afar? Everything else is to blame a natural phenomenon- wind.
The fact is that gusts of wind pass over the water along a tangent line, exerting a physical effect on the sea surface. It is this action that pumps water and makes it move in waves.
Someone, of course, will ask another question about why the waves on the sea and in the ocean go oscillatory movements. However, the answer to this question is even simpler than the very nature of waves. The fact is that the wind has a non-permanent physical effect on the surface of the water, because it is directed towards it in gusts. different strength and power. This affects the fact that the waves have a different size and frequency of oscillation. Of course, strong waves, a real storm, occur when the wind exceeds the norm.
Why are there waves on the sea without wind
A very reasonable nuance is the question of why there are waves on the sea even if there is absolute calm, if the wind is completely absent.
And here the answer to the question will be the fact that water waves are an ideal source of renewable energy. The fact is that waves are able to store their potential for a very long time. That is, the wind that brought the water into action, creating a certain number of oscillations (waves), can be enough for the wave to continue its oscillation for a very long time, and the wave potential itself has not exhausted itself even after tens of kilometers from the point of origin of the wave.
That's all the answers to questions about why there are waves on the sea.
Waves are created by the wind. Storms create winds that affect the surface of the water, causing ripples. Just like the ripples in your cup of coffee after surfing when you blow on it. The wind itself can be seen on weather forecast maps: these are low pressure zones. The greater their concentration, the stronger the wind will be. Small (capillary) waves initially move in the direction the wind is blowing. The stronger and longer the wind blows, the greater its effect on the surface of the water. Over time, the waves begin to increase in size. As the wind continues to blow and the waves generated by it continue to be affected by it, small waves begin to grow. The wind has a greater effect on them than on a calm surface of the water. The size of a wave depends on the speed of the wind that forms it. Wind blowing at some constant speed will be able to generate a wave of a certain size. And as soon as the wave reaches its maximum possible size with a given wind, it becomes “fully formed”. The generated waves have different wave speeds and periods. (See wave terminology for more details) Waves with a longer period move faster and overcome long distances than their slower counterparts. As they move away from the source of the wind (spread), the waves form lines of surfs (swells), which inevitably roll onto the shore. You are probably already familiar with the concept of "wave set" (wave set)! Waves that are no longer affected by the wind that generated them are called bottom waves (groundswell). This is exactly what surfers are looking for! What affects the size of the surf (swell)? There are three main factors that affect the size of waves on the high seas: Wind speed - the higher it is, the larger the wave will be. The duration of the wind is similar to the previous one. Fetch (fetch, "coverage area") - again, the larger the coverage area, the larger the wave is formed. As soon as the influence of the wind on them stops, the waves begin to lose their energy. They will move until the moment when the protrusions of the seabed, or other obstacles in their path (a large island for example) absorb all the energy. There are several factors that affect the size of a wave at a particular location in the surf. Among them: The direction of the surf (swell) - will it allow us to get the swell to the place we need? The ocean floor is a swell moving from the depths of the ocean to the reef, forming large waves with barrels inside. A shallow long ledge stretching towards the shore will slow down the waves and they will lose their energy. Tides - some sports are completely dependent on it. Find out more in the section on how the best waves appear.
The waves that we are used to seeing on the surface of the sea are formed mainly by the action of the wind. However, waves can also occur for other reasons, then they are called;
Tidal, formed under the action of the tide-forming forces of the Moon and the Sun;
Baric, arising from abrupt changes atmospheric pressure;
Seismic (tsunami), resulting from an earthquake or volcanic eruptions;
Shipborne, arising from the movement of the vessel.
Wind waves are predominant on the surface of the seas and oceans. Tidal, seismic, baric and ship waves do not have a significant effect on the navigation of ships in the open ocean, so we will not dwell on their description. Wind waves are one of the main hydrometeorological factors that determine the safety and economic efficiency of navigation, since a wave, running into a ship, falls on it, sways, hits the side, floods the decks and superstructures, and reduces the speed. The pitching creates dangerous rolls, makes it difficult to determine the position of the vessel and greatly exhausts the crew. In addition to the loss of speed, the wave causes the ship to yaw and evade from a given course, and constant rudder shifting is required to keep it.
Wind waves are the process of formation, development and propagation of wind-induced waves on the sea surface. Wind waves have two main features. The first feature is irregularity: the disorder of the sizes and shapes of the waves. One wave does not repeat another, a large one can be followed by a small one, and maybe even a larger one; each individual wave continuously changes its shape. Wave crests move not only in the direction of the wind, but also in other directions. Such a complex structure of the disturbed sea surface is explained by the eddy, turbulent nature of the wind that forms the waves. The second feature of the wave is the rapid variability of its elements in time and space and is also associated with the wind. However, the size of the waves depends not only on the speed of the wind, the duration of its action, the area and configuration of the water surface are essential. From the point of view of practice, it is not necessary to know the elements of each individual wave or each wave oscillation. Therefore, the study of waves is ultimately reduced to the identification of statistical patterns, which are numerically expressed by the dependencies between the elements of the waves and the factors that determine them.
3.1.1. Wave elements
Each wave is characterized by certain elements,Common elements for waves are (Fig. 25):
Top - highest point wave crest;
Sole - the lowest point of the hollow of the wave;
Height (h) - excess of the top of the wave;
Length (L) is the horizontal distance between the tops of two adjacent crests on a wave profile drawn in the general direction of wave propagation;
Period (t) - the time interval between the passage of two adjacent wave tops through a fixed vertical; in other words, it is the time interval during which the wave travels a distance equal to its length;
Steepness (e) - the ratio of the height of a given wave to its length. The steepness of the wave at different points of the wave profile is different. The average steepness of the wave is determined by the ratio:
Rice. 25. Basic elements of waves.
For practice importance has the largest slope, which is approximately is equal to the ratio wave height h to its half-length λ/2
- wave speed c - the speed of the wave crest in the direction of its propagation, determined for a short time interval of the order of the wave period;
Wave front - a line on the plan of a rough surface, passing along the tops of the crest of a given wave, which are determined by a set of wave profiles drawn parallel to the general direction of wave propagation.
For sailing highest value have such wave elements as height, period, length, steepness and general direction of wave movement. All of them depend on the parameters of the wind flow (wind speed and direction), its length (acceleration) over the sea and the duration of its action.
Depending on the conditions of formation and propagation, wind waves can be divided into four types.
Wind - a system of waves, which is at the time of observation under the influence of the wind with which it is caused. The directions of propagation of wind waves and wind in deep water usually coincide or differ by no more than four points (45°).
Wind waves are characterized by the fact that their lee slope is steeper than the windward one, so the tops of the ridges usually collapse, forming foam, or even break off by a strong wind. When waves enter shallow water and approach the shore, the directions of wave and wind propagation can differ by more than 45°.
Swell - wind-induced waves propagating in a wave formation area after the wind weakens and / or changes its direction, or wind-induced waves that come from a wave formation region to another area where the wind blows with a different speed and / or direction. special case swell that propagates in the absence of wind is called dead swell.
Mixed - excitement resulting from the interaction of wind waves and swell.
Transformation of wind waves - a change in the structure of wind waves with a change in depth. In this case, the shape of the waves is distorted, they become steeper and shorter, and at a shallow depth not exceeding the height of the wave, the crests of the latter overturn, and the waves are destroyed.
In my own way appearance wind waves are characterized by different forms.
Ripples - initial form development of wind waves, arising under the influence of a weak wind; the crests of the waves with ripples resemble scales.
Three-dimensional excitement - a set of waves, the average length of the crest of which is several times greater than the average wavelength.
Regular wave - wave in which the form and elements of all waves are the same.
Crowd - chaotic excitement arising from the interaction of waves running in different directions.
Waves breaking over banks, reefs or rocks are called breakers. Waves breaking in the coastal zone are called surf. At steep coasts and at port facilities, the surf has the form of a reverse fault.
Waves on the surface of the sea are divided into free, when the force that caused them stops acting and the waves move freely, and forced, when the action of the force that caused the formation of waves does not stop.
According to the variability of wave elements in time, they are divided into steady, i.e., wind waves, in which the statistical characteristics of the waves do not change in time, and developing or damping - changing their elements in time.
According to the waveform, they are divided into two-dimensional - a set of waves, the average length of the crest of which is many times greater than the average wavelength, three-dimensional - a set of waves, the average length of the crest of which is several times greater than the wavelength, and solitary, having only a dome-shaped crest without a sole.
Depending on the ratio of the wavelength to the depth of the sea, waves are divided into short ones, the length of which is much less than the depth of the sea, and long waves, the length of which is greater than the depth of the sea.
By the nature of the movement of the waveform, they are translational, in which there is a visible movement of the waveform, and standing - not having movement. According to how the waves are located, they are divided into surface and internal. Internal waves are formed at one or another depth on the interface between water layers of different density.
3.1.2. Methods for calculating wave elements
When studying sea waves, some theoretical provisions are used to explain certain aspects of this phenomenon. The general laws of the structure of waves and the nature of the motion of their individual particles are considered by the trochoidal theory of waves. According to this theory, individual water particles in surface waves move along closed ellipsoidal orbits, making a complete revolution in a time equal to the wave period t.The rotational motion of successively located water particles shifted by a phase angle at the initial moment of motion creates the appearance forward movement: individual particles move in closed orbits while the wave profile moves forward in the direction of the wind. The trochoidal theory of waves made it possible to mathematically substantiate the structure of individual waves and to interconnect their elements. Formulas were obtained that make it possible to calculate the individual elements of the waves
where g is the free fall acceleration, Wave length K, its propagation velocity C and period t are interconnected by the dependence K=Cx.
It should be noted that the trochoidal theory of waves is valid only for regular two-dimensional waves, which are observed in the case of free wind waves - swells. With three-dimensional wind waves, the orbital paths of the particles are not closed circular orbits, since under the influence of the wind there is a horizontal transfer of water on the sea surface in the direction of wave propagation.
The trochoidal theory of sea waves does not reveal the process of their development and attenuation, as well as the mechanism of energy transfer from wind to wave. Meanwhile, the solution of precisely these issues is necessary in order to obtain reliable dependencies for calculating the elements of wind waves.
Therefore, the development of the theory of sea waves followed the path of developing theoretical and empirical relationships between wind and waves, taking into account the diversity of real sea wind waves and the non-stationarity of the phenomenon, i.e., taking into account their development and attenuation.
AT general view formulas for calculating wind wave elements can be expressed as a function of several variables
H, t, L, C \u003d f (W, D t, H),
Where W - wind speed; D - acceleration, t - duration of the wind action; H is the depth of the sea.
For shallow water areas of the seas, to calculate the height and wavelength, you can use the dependencies
Coefficients a and z are variable and depend on the depth of the sea
A \u003d 0.0151H 0.342; z = 0.104H 0.573 .
For open areas of the seas, the elements of waves, the height coverage of which is 5%, and the average values of the wavelengths are calculated according to the dependencies:
H = 0.45 W 0.56 D 0.54 A,
L \u003d 0.3lW 0.66 D 0.64 A.
Coefficient A is calculated by the formula
For open areas of the ocean, wave elements are calculated using the following formulas:
where e is the steepness of the wave at small accelerations, D PR is the maximum acceleration, km. The maximum height of storm waves can be calculated using the formula
where hmax - maximum wave height, m, D - acceleration length, miles.
At the State Oceanographic Institute, on the basis of the spectral statistical theory of waves, graphic relationships were obtained between wave elements and wind speed, the duration of its action and the length of the acceleration. These dependences should be considered the most reliable, giving acceptable results, on the basis of which nomograms were constructed at the Hydrometeorological Center of the USSR (V.S. Krasyuk) to calculate the height of the waves. The nomogram (Fig. 26) is divided into four quadrants (I-IV) and consists of a series of graphs arranged in a certain sequence.
In quadrant I (counted from the lower right corner) of the nomogram, a degree grid is given, each division of which (horizontally) corresponds to 1 ° meridian at a given latitude (from 70 to 20 ° N) for maps of a scale of 1:15 000000 polar stereographic projections. A degree grid is needed to convert the distance between isobars n and the radius of curvature of isobars R, measured on maps of a different scale, to a scale of 1:15 000000. In this case, we determine the distance between isobars n and the radius of curvature of isobars R in meridian degrees at a given latitude. The radius of curvature of the isobars R is the radius of the Circle with which the section of the isobar passing through the point for which the calculation is being carried out or near it has the greatest contact. It is determined with the help of a meter by selection in such a way that the arc drawn from the found center coincides with the given section of the isobar. Then, on the degree grid, we plot the measured values \u200b\u200bat a given latitude, expressed in degrees of the meridian, and with a compass solution we determine the radius of curvature of the isobars and the distance between the isobars, corresponding to a scale of 1: 15,000,000.
In quadrant II of the nomogram, curves are shown that express the dependence of wind speed on the pressure gradient and the geographical latitude of the place (each curve corresponds to a certain latitude - from 70 to 20 ° N). For the transition from the calculated gradient wind to the wind blowing near the sea surface (at a height of 10 m), a correction was derived that takes into account the stratification of the atmospheric surface layer. When calculating for the cold part of the year (stable stratification t w 2 ° C), the coefficient is 0.6.
Rice. Fig. 26. Nomogram for calculating the elements of waves and wind speed from maps of the surface pressure field, where isobars are drawn at intervals of 5 mbar (a) and 8 mbar (b). 1 - winter, 2 - summer.
Quadrant III takes into account the effect of isobar curvature on the geostrophic wind velocity. Curves corresponding different meanings curvature radii (1, 2, 5, etc.) are given by solid (winter) and dashed (summer) lines. The sign oo means that the isobars are rectilinear. Usually, when the radius of curvature exceeds 15°, no consideration of curvature is required in the calculations. The abscissa axis separating the yadrants III and IV determines the wind speed W for a given point.
In quadrant IV there are curves that make it possible to determine the height of the so-called significant waves (h 3H) with a probability of 12.5% by wind speed, acceleration or duration of the wind.
If it is possible to use not only data on wind speed, but also on the acceleration and duration of the wind when determining the height of the waves, the calculation is performed on the basis of the acceleration and duration of the wind (in hours). To do this, from quadrant III of the nomogram, we lower the perpendicular not to the acceleration curve, but to the curve of the duration of the wind action (6 or 12 hours). From the results obtained (acceleration and duration), the smaller value of the wave height is taken.
Calculation using the proposed nomogram can be made only for areas of the "deep sea", that is, for areas where the depth of the sea is not less than half the wavelength. For acceleration exceeding 500 km or wind duration greater than 12 h, the dependence of wave heights on wind corresponding to ocean conditions is used (thickened curve in quadrant IV).
Thus, to determine the height of the waves at a given point, it is necessary to perform the following operations:
A) find the radius of curvature of the isobar R passing through a given point or near it (using a compass by selection). The radius of curvature of isobars is determined only in the case of cyclonic curvature (in cyclones and troughs) and is expressed in meridian degrees;
B) determine the pressure difference n by measuring the distance between adjacent isobars in the area of the selected point;
C) according to the found values of R and n, depending on the season, we find the wind speed W;
D) knowing the wind speed W and acceleration D or the duration of the wind (6 or 12 hours), we find the height of significant waves (h 3H).
Acceleration is as follows. From each point for which the wave height is calculated, a streamline is drawn in the direction against the wind until its direction changes with respect to the initial one by an angle of 45 ° or reaches the coast or the ice edge. Approximately, this will be the acceleration or path of the wind, during which should be formed (waves arriving at a given point.
The duration of the action of the wind is defined as the time during which the direction of the wind is unchanged or deviates from the original by no more than ± 22.5 °.
According to the nomogram in Fig. 26a, one can determine the wave height from the map of the surface pressure field, on which the isobars are drawn through 5 mbar. If the isobars are drawn through 8 mbar, then the nomogram shown in fig. 26 b.
The period and wavelength can be calculated from the data on wind speed and wave height. An approximate calculation of the wave period can be made according to the graph (Fig. 27), which shows the relationship between the periods and the height of wind waves at different wind speeds (W). The wavelength is determined by its period and the depth of the sea at a given point according to the graph (Fig. 28).
