Many tourists vacationing in the resorts of Thailand or Vietnam, faced with natural phenomena such as the tide of the sea. At a certain hour, the water suddenly recedes from the usual edge, exposing the bottom. This pleases the locals: women and children go ashore to collect those who have not had time to evacuate along with the tidal wave of crustaceans and crabs. And at another time, the sea begins to advance, and after about six hours, a deck chair standing at a distance is in the water. Why is this happening? What is the reason for this? Why, for example, in the Black or Azov Seas we do not observe tides, and near Murmansk the daily fluctuations in water level are significant? Let's look at these riddles of the ocean.
Physics of Natural Phenomenon
Paradoxical as it may seem, the cause of the ebb and flow of the ebb on planet Earth is its satellite. It would seem that what the bottomless depths of the seas have in common with a celestial body? The fact is that not only the Earth holds the Moon in orbit with its attraction. This process is mutual. The moon also has weight (and not small), and therefore the forces of gravity act on our planet. A month does not raise stones, but such light matter as water can. The World Ocean as if bends towards the moon. And since the Earth’s satellite moves in its orbit (for us - in the horizon), then high water moves behind it. Invisible in the open ocean, the wave manifests itself off the coast, in narrow bays and in shallow water, causing an ebb and flow. The sun also affects the force of attraction of huge masses of water. This luminary has a mass much larger than that of the Month, but it is also four hundred times farther from Earth than our satellite. Therefore, solar tides are twice as weak as lunar ones.
The frequency of the tides
Logically, the highest water level should be observed at that moment when the moon is at its zenith. When the month is in the nadir, we can expect a low, outgoing wave. But the strange thing: the ebb and flow are observed twice a day. And the second time, exactly when the moon is in a nadir (a point opposite the zenith). This is because the satellite still draws water, even across the entire thickness of the globe. Thus, the level of the World Ocean can be compared with an ellipse, the elongated ends of which lie on the same axis as the Moon, and the flattened ones are perpendicular to it. In addition, you should not discount such an important factor as the Earth’s own rotation around its axis. Huge masses of water under the action of centripetal force form two waves at mutually opposite points on the planet.
Why the power of this phenomenon is heterogeneous at different points on the Earth
In theory, on all coasts, we should observe the same tidal forces. Murmansk, however, can boast that its waterfront rises four meters, while in the Gulf of Finland off the coast of St. Petersburg this natural phenomenon is barely noticeable, and even then only in shallow water. The main factor enhancing the manifestation of the tide is the connection of the water area with the oceans. In the inland seas - Black, Baltic, Marmara, Mediterranean, and even more so Azov - this phenomenon is almost not felt. The water level can rise by 5-10 centimeters, no more.
Another factor that can increase the tide is the indented coast. In narrow bays with a shallow bottom, these phenomena are expressed more powerfully. If the mouth of the river has an eastern direction (opposite to the passage of the moon), then a tidal wave drives water upstream, sometimes several tens of kilometers from the sea. This is especially pronounced on the Amazon. Water rises to four meters. The wave moves deep into the mainland at a speed of 25 km / h.
What affects the intensity of the phenomenon
Staying for a long time on the same beach, we notice that on different days the tide has unequal strength. At one time, the sea approaches the coast very intensively, and also goes far from it. And after a week, the tide does not differ by such a force. The reason lies in the action of the sun. We have already noted that the sun also attracts the thickness of the water, although not as much as the moon. Therefore, in geography, two types of tides are distinguished - syzygy and quadrature. It all depends on the relative position of the moon and the sun in relation to the earth. If the luminary and the satellite of our planet are on the same axis (this is called syzygy), the tides intensify. When the Sun and the Moon are located at right angles (quadrature), their effect on the attraction of water decreases. Then the smallest tide occurs.
Record holders
Where do the greatest ebbs and flows occur? The first place was shared by two geographical points. Both are in Canada. This is the Ungawa Bay north of Quebec and the Bay of Fundy that lies between Nova Scotia and New Brunswick. Here the syzygy tide reaches the mark of eighteen meters! But even when the Sun and the Moon are in this area, the level of water rise is serious - fifteen and a half meters. In Europe, the largest tide is observed near the city of Saint-Malo, in the French province of Brittany. Due to the characteristics of the coastline and the English Channel, the natural phenomenon intensifies and the water height reaches 13.5 m.
The third place in terms of tidal height (almost thirteen meters) is occupied by the Penzhinskaya Bay in the Sea of Okhotsk. This place is also a champion on the entire Pacific coast. The estuaries and the prevailing winds also make adjustments to the tides. Arkhangelsk, located at the confluence of the Northern Dvina Sea, knows such a thing as a manich. It is nothing but a tide. He drives river waters upstream.
The ebbs and flows in Murmansk
The Mezensky Bay of the White Sea also boasts a serious arrival of water - as much as ten meters! However, in the Murmansk port itself, the difference between full and low water (the height of the end of the ebb and flow) is not so significant - only four meters. But since the coast is shallow, the entrance to the sea is gentle, a large area is exposed. Tourists specially go to watch the ebb. Where waves raged a few hours ago, birds roam, looking for mollusks and crustaceans in the pits. And so that the ships do not run aground when leaving the bay, the port authorities have a special table where it is calculated when the tide begins on a given day.
