Tides are one of the most reliable phenomena in the world, and we know that they move in and out around twice a day, but not exactly. So, why is that?
A day on Earth is the time it takes our planet to spin once around its own axis and return to the same point under the Sun. This is known as a solar day, and it lasts around 24 hours.
However, the time it takes Earth to reach the same position in relation to the Moon, takes, on average, 24 hours and 50 minutes, known as a lunar day. The reason the lunar day is longer than a solar day is that the Moon revolves around Earth in the same direction as Earth rotates around its axis, so it takes Earth, on average, an additional 50 minutes to “catch up” to the Moon.
Because the tidal force of the Moon is more than twice as strong as the Sun’s, the tides follow the lunar day, not the solar day. It takes half a lunar day, on average 12 hours and 25 minutes, from one high tide to the next, so we have high and low tides nearly twice a day.
The change from low to high tide is known as flood tide, while the change from high to low tide is called ebb tide. The technical term for the difference in water level between high tide and low tide is tidal range.
The flow and ebb are gradual, so it is not accurate to say that a high or low tide lasts around 6 hours and 12 minutes, i.e. a quarter of a lunar day. The speed of the water flow varies during this period, and it also varies from place to place.
The astronomical forces which drive the tides can be predicted very accurately, and these predictions are published in local tidal tables. However, different weather conditions also affect the sea level and may cause both lower and higher tides than expected. If there is a storm, the seawater level often increases. This is called a storm tide and is caused by a combination of storm surge and normal tidal movement.
Strong offshore winds can move water away from coastlines, exaggerating the low tide, while onshore winds may cause the water to pile up onto the shoreline, making the low tide higher than normal.
High pressure weather systems can lead to days with exceptionally low tides, while low pressure systems may contribute to causing much higher tides than predicted.
The depth and shape of the ocean and the distance between continents are also important in determining the water level along the shores. In the northern parts of North America, Europe, and Asia, the continents are close together, which creates a bigger difference between high and low tides than in areas farther south, where the continents are farther apart.