The Earth, the Moon, the Sun and other celestial bodies have a gravitational pull between them. This gravitational pull is balanced by the centrifugal force cause by two celestial bodies circling a common centre of gravity.
The gravitational pull depends on the mass of the bodies and the distance between them. This is why the gravitational pull on the Earth is dominated by the Moon and Sun and why the pull of the Sun is about half (0.46) of that of the Moon.
The gravitational pull affects all mass but since water is fluid it pulled faster towards the Moon or the Sun. If the whole Earth was covered by a uniformly deep sea over a homogenous interior, and the only gravitational force was from the Moon, then high tide would occur at the position closest to the Moon and also on the opposite side of the Earth - since the gravitational force here is less than the centrifugal force.
The Moon orbits the Earth in 24 hours and 50 minutes. Lunar tides therefore have a period of 12 hours and 25 minutes.
Many factors influence the tides
In reality the tides are not only caused by the Moon and the Sun but also by the Earth’s rotation, the angle of the Earth’s axis, variations in the distance to the Moon and the Sun, and the angle of their paths. The continents hinder the tide and the water (tidal) waves cannot move as quickly as the Sun and Moon move relative to the Earth.
All these factors mean that the tides vary both from place to place and at with time in a complicated but regular characteristic pattern, which is also regionally-specific.
Tides in Swedish waters
The Skagerrak and Kattegat have semi-diurnal tides because of the opening into the North Sea and the Atlantic. The amplitude of the tide is normally around 10 cm in the Skagerrak and 5 cm in the Kattegat, but when the various forces combine (spring tides) the amplitude can be as high as 40 cm and 20 cm respectively.
The dominant tidal period is 12 h 25 min which is half of the time interval between the moon’s minimum distance from a specific location. This can be clearly seen on the sea level curves for the measurement stations along the West Coast.
The Baltic Sea is itself too small to have its own significant tides, and it has too narrow an opening to the North Sea to be influenced by the North Atlantic tides. The total tidal effect is only a few centimetres. Tidal waves from Kattegat through the Sound contribute a few centimetres of tide in the Southern Baltic.
Why are there hardly any tides in Sweden?
The size of the tide depends on the topography and size of the water basin. In Swedish waters the locally generated tides have little significance.
The tides we experience in Sweden depend on waves coming from the North Sea, Skagerrak and Kattegat. Since the waters are shallow the waves are significantly dampened due to friction.
As the waves continue to move into the Baltic they are further limited due to the narrow openings via the Sound and the Belt, leaving only a few centimetres of variation.
Tides in other countries
In parts of Asia you can experience low water in the morning and high water in the afternoon, while other days can be the opposite. Here there are daily or mixed diurnal and semi-diurnal tides.
Diurnal tides have one high water and one low water per day, and semi-diurnal two of each. The two most significant daily components have periods of 23.93 and 25.82 hours.
When there are strong diurnal tides, the phase of high water and low water can switch during the month. It can also be linked to local effects such as friction or the shape of the sea bottom.