2. Diurnal tide type: A lunar day has only one high tide and one low tide. Such as Shantou in the South China Sea and Qinhuangdao in the Bohai Sea. Beibu Gulf in the South China Sea is a typical diurnal tide area in the world.
3. Mixed tidal pattern: there are two high tides and two low tides on some days of 1 month, but the tidal range between the two high tides and low tides is quite different, and the time of high tides and low tides is also different; On other days, there are high tides and low tides.
Most parts of the South China Sea belong to mixed tidal type. For example, in Yulin Port, there is a diurnal tide on 15 days, and the other days are irregular semidiurnal tides with large tidal range. No matter what kind of tide, within two or three days after the first and fifteenth day of each month in the lunar calendar.
There will be a spring tide with the largest tidal range, with the highest high tide and the lowest low tide. Two or three days after August 23rd of the lunar calendar, there is a small tide with the smallest tidal range. At this time, the tide does not rise too high, and it is not too low at the end.
Extended data
Because the gravity distribution of the moon and the sun on the earth is different, the earth is constantly rotating. Take the moon as an example. The earth revolves around the moon for about 24 hours and 48 minutes.
In this process, most parts of the earth except the north and south poles are always facing the moon. In the area where the earth is opposite to the moon, the gravity of the moon is greater than the centrifugal force, and gravity plays a leading role, which makes the seawater form a climax.
As a result, within 24 hours and 48 minutes, there will always be two high tides and two low tides every day due to the influence of the water on the earth. This is what we often call a semidiurnal tide.
At the same time, although the tidal force of the sun on the fluctuation of the earth's seawater is not as great as that of the moon, if their gravitational forces overlap or weaken each other, their tidal force will produce complex tidal phenomena. The monthly two tides and two tides are formed because the tidal force of the sun and the moon is superimposed and weakened.
As we all know, the earth revolves around the sun while rotating, and the moon revolves around the earth. The positions of the three have been changing. Whenever the first day of the lunar calendar (new moon period) or the fifteenth day of the lunar calendar (full moon period), the positions of the earth, the moon and the sun are basically on the same axis, and their tidal force adds up to form a strong gravitational force, which makes the sea water on the earth rise higher and fall even more.
Commonly known as the spring tide period. However, whenever the seventh and eighth lunar month (the first quarter moon) and the second and third lunar month (the second quarter moon), the gravitational forces of the moon and the sun are just opposite, and the tidal force of the sun greatly weakens the tidal force of the moon, so the fluctuation of seawater will be much smaller, and there will be a small tidal period (commonly known as "death").
At the same time, due to the complexity of marine meteorology, the fluctuation of Shanghai water on the earth is deeply influenced by the gravity of the moon and the sun, and the geographical environment and wind power of the sea area are also different. Sometimes there will be a diurnal tide, that is, there will only be one high tide and one low tide in the sea for 24 hours and 48 minutes, but it is not very common every year. Most sea areas in China are basically regular semidiurnal tides.
Baidu encyclopedia-Chao