How the Moon Affects Ocean Tides? (Part 2)

Moon, ebb

Gradual Ebb and Flow

The change from high tide to low is referred to as ebb tide, while the change from low tide to high is known as flood tide. A tidal range is a technical term that describes the difference in sea level between the high tide and the succeeding low tide.

It is not accurate to conclude how long a high or low tide will last – let’s say around 6 hours and 12 minutes (¼ of a lunar day) – because the flow and ebb are gradual. During this period, the speed of the flow varies, and it also varies from place to place.

The Rule of 12th

People, such as fishermen, sailors, and surfers consider tides because of its importance when it comes to wave riding and surfing. The height of tides influences water volume, water depth and breaking of waves; thus, people usually use the rule of 12ths in calculating the expected level of water.

According to the rule, in the first hours of a tidal shift, the water level will increase by 1/12 of the predicted tidal range in any given area. The water level will rise by 2/12 in the second hour, 3/12 in the third hour, it will also increase by 3/12 in the fourth hour, 2/12 in the fifth hour and it will increase by 1/12 in the sixth hour. The sequence to this rule is 1-2-3-3-2-1.

So, for instance, if the predicted tidal range in an area is 12 feet, the tide will rise by 1 foot in the first hour, 2 feet in the second hour, 3 feet in the third and fourth hour. And then it will rise by 2 feet and 1 foot in the fifth and sixth hour respectively.

Storm Tides and Surge

It is very easy to predict the astronomical forces driving tides; you will find these predictions in local tidal tables. However, the water level can be affected by different weather conditions; these conditions could change the expected tides. A storm usually increases the level of sea water, this is known as a storm tide which occurs as a result of combining storm surge – the rise in seawater level caused mainly by a storm – and normal tidal movement.

Low tides can also occur when strong offshore winds move water away from coastlines. Also, onshore winds may cause water to pile up onto the shoreline, resulting to a low tide higher than the usual.

Tides can also be influenced by high-pressure weather systems and low-pressure systems. Low-pressure weather systems can lead to much higher tides than expected, while high-pressure systems can lead to days with unusual low tides.