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--- un:tide [2025/03/28 01:09] – created asad
+++ un:tide [2025/03/31 02:43] (current) – [1. Causes of Tides] asad
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-====== Tide ======
+====== Tides ======
+Due to the moon's tidal forces, the Earth's surface water experiences high tide twice a day and low tide twice a day. Additionally, due to the combined effect of the sun and moon, spring tides occur twice a month and neap tides occur twice a month. These phenomena are caused by gravitational forces.
+===== - Causes of Tides =====
+{{:bn:un:tidal-force.webp?750&nolink|}}
+The Earth and the moon rotate around their common center of mass, the barycenter of the Earth-moon system (located 70% of the Earth's radius away from the Earth's center). This rotation creates an imaginary force in our rotating frame of reference, known as inertial force or centrifugal force ($F_r$). This force is uniform across the Earth's surface and always points away from the moon. In contrast, the moon's gravitational force ($F_g$) acts towards the moon. Subtracting the inertial force from the gravitational force yields the tidal force, depicted as red arrows in the image; the length of the arrow represents the force's magnitude, and its direction indicates the force's direction. In the image above, the tidal forces at points $A$ and $C$ are equal and opposite, and those at points $B$ and $D$ are also equal and opposite, though smaller in magnitude.
+However, this inertial force is merely an imaginary force, not a real one. We perceive its existence because we are in a non-inertial frame of reference. The effect of the tidal force at the four points above can be explained without considering this imaginary force. If we subtract the moon's gravitational force at the Earth's center from the moon's gravitational force at every point on the Earth's surface, we obtain the same result. In this case, we are calculating the gravitational force at every point on the surface relative to the center, hence it's differential gravity. The two methods yield the same result because the inertial force is equal and opposite to the moon's gravitational force at the Earth's center.
+{{https://science.nasa.gov/wp-content/uploads/2023/08/tide-generating-force.gif?700&nolink}}
+The tidal force affects ocean water, causing it to rise higher where the force is greater and remain normal where it is smaller. By observing the magnitude and direction of the tidal force at points $A, B, C, D$ in the first image, it becomes clear that the ocean water takes on an egg-like shape, bulging towards the moon and remaining normal in the vertical direction. This bulge of water is the cause of tides. To understand why this oval shape leads to tides, we need to consider the Earth's rotation around its axis.
+{{https://science.nasa.gov/wp-content/uploads/2023/08/high-and-low-tides.gif?nolink&700}}
+The Earth rotates once around its axis approximately every twenty-four hours. However, the oval shape of the ocean does not rotate with it; the ocean bulges along the line towards the moon. It's like the Earth is rotating inside the ocean. Therefore, an observer standing on the Earth's surface will see the ocean rise twice and fall twice in a day. This is why there are two high tides and two low tides each day.
+{{https://science.nasa.gov/wp-content/uploads/2023/08/moon-earth-lag-updated.gif?nolink&700}}
+However, high and low tides do not occur at the same time every day because the moon is also orbiting the Earth, making tide time calculations a bit complex. If the moon did not orbit, the moon would be in the same position at midnight today and midnight tomorrow. But since the moon moves during these twenty-four hours, it will appear in the same position about fifty minutes later the next day, around 1 AM. This is because it takes fifty minutes more than twenty-four hours for the Earth to rotate to the same position relative to the moved moon. Consequently, the direction of the ocean bulge also shifts by the same amount each day, as the major axis of the oval must always point towards the moon. This is why the timing of high or low tide shifts by about fifty minutes each day. Therefore, the tidal period, the interval between two high tides, is 12 hours and 25 minutes.
+{{https://science.nasa.gov/wp-content/uploads/2023/08/lunar-and-solar-tides.gif?nolink&700}}
+Twice a month, the high tides are very high, called spring tides, and twice a month the high tides are at their lowest, called neap tides. This is due to the sun. The Earth-moon system orbits the sun. When the Earth, moon, and sun are aligned, i.e., during new moon and full moon, the tidal forces of the moon (blue line) and the sun (orange line) add up. And when the Earth-moon line is at a 90-degree angle to the Earth-sun line, the sun's tidal force reduces the moon's force. This is why spring tides and neap tides occur twice a month. During neap tides, the moon appears half full. Neap tide occurs one week after the spring tide of the new moon, the spring tide of the full moon occurs one week later, neap tide again one week later, and the spring tide of the new moon again one week later.
+===== - Tide Maps =====
+{{:bn:un:tide-gauge.webp?nolink|}}
+Due to the terrain, seawater cannot flow everywhere, so tidal water accumulates more on the coast and not in the open sea. At various locations along the coast, tide gauges are used to measure the height of the sea level at different times. The image above shows approximately fifteen days of data, from one spring tide to another. There are special names for several heights relative to mean sea level. The highest level to which water can rise under the influence of the moon is called the Highest Astronomical Tide, HAT. The average height of all high tides in a month is called Mean High Water. The average height of the two high tides during spring tide is called Mean High Water Springs. And the average height of the two high tides during neap tide is Mean High Water Neaps. In contrast to all of these, there are almost symmetrical depths of low tide, such as the deepest possible low tide, called the Lowest Astronomical Tide, LAT.
+{{:bn:un:lat-atlas.webp?nolink|}}
+The map of LAT for all the seas of the world is shown above in meters; 0 means mean sea level. It can be seen that LAT can be as low as 6 meters below mean sea level. LAT is very low, close to zero, in the open ocean, where neither high nor low tides are significant. The value of LAT is highest near the coast, so HAT will also be much higher in these places. On the coast of Bangladesh, the value of LAT or HAT is close to the maximum, about 4 meters.
+===== - Tidal Coast of Bangladesh =====
+The coast of Bangladesh is divided into three regions: the Sundarbans and the Ganges Tidal Zone in the west, the Meghna Delta in the center, and the Chittagong Coast in the east. More than 20 million people live in the coastal region, and about 60 percent of this region is less than 3 meters above mean sea level. The southernmost part has an elevation of less than 1 meter. In the map below, the coast is shown in red-yellow color. The areas that may be submerged if the sea level rises by 1 meter and 3 meters are shown by blue and black lines, respectively. The color indicates the amount of future rainfall (in millimeters).
+{{:bn:un:bd-coast.webp?nolink&750|}}
+Tides coming from the Indian Ocean enter the Bay of Bengal and reach Hiron Point in the west and Cox's Bazar in the east at almost the same time. These two places are located at the edges of two submarine canyons of the Bay of Bengal, Hiron Point at the edge of the Swatch of No Ground, and Cox's Bazar near the Burma Trench. In the coast between these two regions, there are two high tides and two low tides a day, with differences in the height of the two high tides. Tidal range (distance from high tide height to low tide depth) is higher in relatively shallow areas in the east. At Hiron Point and Cox's Bazar, the tidal range is about 10 feet (3 meters) during spring tides.
+{{https://www.fao.org/4/AC352E/AC352E09.gif?nolink&850}}
+Tides enter rivers and create tidal channels. Channels increase the salinity of river water. The graph above shows the salinity (below) and water temperature (above) data obtained from a station in Satkhira. This plot shows the relationship between tides and another factor in Bangladesh: the monsoon season. Salinity starts to decrease in June-July due to the influence of the monsoon and reaches its minimum in September. Water temperature reaches its minimum in January, in line with the seasons. But tidal range increases in winter and decreases in monsoon.
+Due to tides, the change in the direction of the river is visible up to about Louhajong in the Padma River. And the backwater effect (increase in water level upstream due to tides towards the estuary) can be found even further north.