Tides

Tides are the regular rise and fall of the ocean due to the gravitational attraction of the moon and the sun on the Earth. As storms approach a coastline, the tide level is very important, as a high tide will likely result in a greater chance of coastal flooding.

Tide gauges around the world measure the tides, and accurate tide predictions can be made for most parts of the world today. The diagram below shows a typical tidal sequence for Newport, RI. Both the predicted and measured values are shown.

Tides at Newport, RI at the beginning of March 2009. Red crosses are measured values, the blue line is the predicted value and the green is the difference between predicted and measured. Graph from NOAA tides site - http://www.co-ops.nos.noaa.gov/

The difference between the measured and predicted tidal levels may be due to winds, water build-up from a storm, or low pressure changing the water level locally.

The tides are a bulge of water from the “pull” of the moon and sun. However, there are actually two tidal bulges on the earth. One tidal bulge is caused by the pull of gravity and the other tidal bulge is caused by centrifugal forces or this can be described as an inertial bulge. For a detailed explanation of how the tides work, visit the educational pages at the NOAA Tides and Currents website.

Tidal bulges on the Earth. One tidal bulge is caused by the pull of gravity and the other tidal bulge is caused by centrifugal forces or this can be described as an inertial bulge. Image adapted from NOAA

There are generally three types of tides: diurnal – one high and low tide each day, semi-diurnal – two high and low tides each day, and mixed – two high and low tides each day of different heights. The different types of tides occur due to the arrangement of the land on earth, the shape of the local coastline, and the depth of the water near the coastline.

Types of tides: diurnal, semidiurnal, and mixed. Figure from NOAA Co-OPS Education.

The tidal type only refers to the pattern of high and low tides each day, not to the height of the water or tidal range (change in water height between high and low tide). Different locations have different tidal ranges depending on the coastline, location in the world and other factors.

This map shows the geographic distribution of different tidal cycles along the Earth's coastlines. Regions experiencing diurnal tides are marked in yellow, regions experiencing semidiurnal tides are drawn in red and regions with mixed semidiurnal tides are outlined in blue. Image from NOAA Ocean Service Education.

Higher tidal ranges are called spring tides and lower tidal ranges are called neap tides. Variation in tidal range as illustrated in the graph of the Newport tide occurs because of the orientation of the sun and the moon to the Earth. When the moon and sun are lined up straight with the earth, the gravity from both acts together and produces spring tides. When the sun and moon are at right angles to each other, the gravity from both works at right angles, producing neap tides.

Relative orientations of the Earth, Sun, and Moon that produce spring and neap tides. Modified from NOAA.