High tides & low tides are caused by the moon. The moon’s gravitational pull generates something called tidal force. The tidal force causes Earth & its water to bulge-out on the side closest to the moon and side farthest from the moon. These bulges of water are of high tides.
As Earth rotates, your region of the Earth passes through both of those bulges every day. When you are in one among the bulges, you experience a high tide. When you are not in one among the bulges, you experience a low tide. This cycle of two high tides & two low tides occurs most days on most of the coastlines of the world.
More About Tides
Tides are all about gravity and when we are talking about the daily tides, it is the moon’s gravity that’s causing them.
As Earth rotates, moon’s gravity pulls on different parts of our planet. Even though, moon only has about 1/100th the mass of Earth, since it is so close to us, it’s enough gravity to move things around. The moon’s gravity even pulls on the land, but not enough for anyone to tell (unless they use special & really precise instruments).
When moon’s gravity pulls on the water in the oceans, however, someone’s sure to notice. Water has a much easier time-traveling and the water wants to bulge in the direction of the moon. This is called tidal force.
Because of tidal force, water on the side of the moon always wants to bulge-out towards the moon. This bulge is, what we call a high tide. As your part of world rotates into this bulge of water, you would possibly experience a high tide.
One thing to note, however, this is just an explanation of the tidal force, not the actual tides. In real life, the Earth is not a global ocean, covered in an even layer of water. There are 7 continents, in which land gets in the way. The continents prevent the water from perfectly following moon’s pull. That’s why in some places, difference between high & low water is not big, and in other places, the difference is drastic.
That explains the first high tide each day, but what about the second high tide?
The ocean also bulges-out on the side of Earth opposite the moon.
If moon’s gravity is pulling the oceans towards it, how can ocean also bulge on the side of Earth away from the moon? It does seem a little weird. It is all because tidal force is a differential force, meaning that it comes from differences in gravity over surface. Here’s how it works:
On the side of Earth that’s directly facing the moon, moon’s gravitational pull is the strongest. The water on that side is pulled strongly in the direction of moon.
On the side of Earth farthest from the moon, moon’s gravitational pull is at its weakest. At the middle of Earth is approximately the average of the moon’s gravitational pull on the whole planet.
To get tidal force, the force that causes the tides, we subtract this average gravitational pull on the Earth from the gravitational pull at each location on Earth.
Tidal force = moon’s gravitational pull during a specific location on Earth — moon’s average gravitational steer the entire Earth
The results of the tidal force stretching & squashing of Earth. This is what causes the 2 tidal bulges.
These two bulges explain why in one day, there are two high tides & two low tides, as the surface rotates through each of the bulges once a day.
Does anything else affect tides?
The Sun causes tides, just like the moon does, although they’re somewhat smaller. When the Earth, moon & Sun line-up, which happens at the time of full moon or new moon, the lunar & solar tides reinforce one another, resulting in more extreme tides, called spring tides. When lunar & solar tides act against one another, the result is unusually small tides, called neap tides. There’s a new moon or a full moon about every 2 weeks, so that’s how often we see large spring tides.
Wind & weather patterns can also affect water level. Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much-less noticeable.
High-pressure weather systems can push-down sea levels, resulting in lower tides. Low-pressure systems brought on by strong storms & hurricanes, can cause tides than are much higher than predicted, so watch out!