High tides and low tides are stages of the tidal cycle. They happen in periods of around 6 hours between each other and represent both the highest and lowest level of water of a specific location along the coastline.
Those who have spent a day at the beach probably noticed the constant variations on the level of the sea in relation to the land. At some point, the water begins to retreat and the beach widens; while in other moments the water advances over the sand and the beach narrows. Such phenomenon is explained by the interaction between the gravitational pull of the sun, the moon, and the Earth (as well as an array of environmental factors), creating what we know as tides.
Tides are events that happen in cycles – much like any process in the natural world. Despite having diverse times and amplitudes – which are strongly affected by aspects like the shape of coastlines or the nearshore bathymetry of a given place – all tidal cycles follow the same stages: the sea level rises over the course of a few hours (flood tide), reaches its highest point (high tide), falls over several hours (ebb tide), and hits its lowest level (low tide).
These tidal cycles can be semi-diurnal (two high tides and two low tides per day) or diurnal (a single cycle per day). In a semi-diurnal cycle, both the high waters and the low waters of each day usually differentiate in height, consequently creating the concepts of higher high water and lower high water for high tides, and higher low water and lower low water in low tides.
Among the stages of a tidal cycle, the most commonly discussed are the high tides and low tides. In any given location along the coastline, high tides occur when the moon is directly above – and thus closest to – that very region, its gravitational force pulling all bodies towards its core. This causes the waters to bulge from the surface of the Earth, and the sea level of that particular nearshore area to rise. But since the entire mass of the Earth is attracted by the force of the moon’s gravity, the waters in the opposite end of the surface also bulges and – as the planet is drawn towards the moon – simultaneously experiences a high tide in the opposite direction.
Conversely, low tides happen when the rotation of the Earth places the moon at its furthest from a given location, on a 90° angle in relation to the core of the planet. This transition between high tide and low tide takes around 6 hours, making it so that there are [usually] two high tides and two low tides in every lunar day (24 hours and 50 minutes), before the moon returns to the same location in the sky.