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Tide Prediction

The content and activities in this topic will work towards building an understanding of how the earth, the sun, and the moon interact to influence the time patterns of tides.

Predicting Tides

Navigating ships through shallow-water ports, boat channels, man-made canals, and estuaries requires knowledge of the time and height of the tides, as well as the speed and direction of the water as the tides move in and out. This information is critical to sailing ships because they have to take advantage of the tides and currents to maneuver correctly. The captains of large transport ships also need to be aware of the tides because navigating a large ship through shallow, narrow channels leaves little room for error.

 

Because the movements of the earth, moon, and sun are predictable, tide characteristics such as tide height, zero point, and mean lower low-water level are predictable. Tide height is usually measured from the low-water mark, which is considered the zero point. This procedure is simple in areas where the tide is relatively constant. However, in areas where tides are continually changing, scientists use an average (mean) of the low tide levels. This average is called the mean lower low-water level (MLLW). All tide heights at a location are measured from the zero point. When the low-water level is lower than the zero point, tide height is measured with a negative number and is called a minus tide.

 

Tide tables are based on average tidal data obtained at a given location for many years. An example is shown in Table 6.2. Tide table data can be plotted to make tide graphs.

 

Table 6.2. Tide table for Hilo Bay, Hawai‘i, July 2010
Moon Phase Day Date High Tide Low Tide High Tide Low Tide
      Time Height (ft) Time Height (ft) Time Height (ft) Time Height (ft)
  Thu 8     5:58 a.m. -0.3 1:39 p.m. 2.7 8:25 p.m. 0.8
  Fri 9 12:23 a.m. 1.2 6:47 a.m. -0.5 2:19 p.m. 2.9 9:03 p.m. 0.6
  Sat 10 1:20 a.m. 1.3 7:34 a.m. -0.6 2:59 p.m. 3.1 9:40 p.m. 0.5
new moon Sun 11 2:13 a.m. 1.3 8:21 a.m. -0.7 3:38 p.m. 3.2 10:17 p.m. 0.4
  Mon 12 3:04 a.m. 1.4 9:07 a.m. -0.6 4:17 p.m. 3.1 10:55 p.m. 0.3
  Tue 13 3:57 a.m. 1.5 9:53 a.m. -0.4 4:55 p.m. 3.0 11:34 p.m. 0.2
  Wed 14 4:53 a.m. 1.6 10:41 a.m. -0.2 5:32 p.m. 2.8    

 

Activity

Activity: Tide Prediction

Plot a tide graph from a tide table.

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.