Date       Day Time        Height
01/15/2005 Sat 02:30AM LST 10.9 H

This excerpt includes the date of the predictions (Saturday, January 15, 2005), and the time and height of the first high tide on that day. This tide will occur at 2:30am local standard time, and will have a height of 10.9 feet above mean lower low water, the "zero" depth on the chart. The "H" indicates a high tide.

Visit the NOS tide predictions web site and explore the information there. You'll find that predictions are offered for a large number of coastal locations, and that nearby locations usually have very similar tide times and heights.

The NOS predictions tell us the time and height of high and low tide, but what happens in between? Knowing the approximate water level at a given time could be very important while paddling, for example to determine whether you will be able to pass over a shallow bar that uncovers at low tide.

In order to calculate water level at any time, you need to know the approximate heights of the flanking tides (for example, the previous high tide and the coming low tide). Let's say the previous high tide height was 9.0 feet, and the coming low tide height is 0.0 feet. The water level change follows a regular, sinusoidal pattern as shown by the blue curve in the graph at right.

In the graph, the curve has been divided into time increments of one hour in length by the colored triangles. Note that each triangle shows the height change that occurs during that hour of the tide cycle.

We now divide the tide range into 12 equal blocks, which in this example are 0.75 feet, or 9 inches (9 feet divided by 12) in height. Notice that during the first hour after high tide, the water level drops by one block; during the second, two; and during the third hour, three. Then the pattern reverses until low tide, giving an overall configuration of 1-2-3-3-2-1.

To estimate the water level at a given time, say 4 hours after high tide, we count the number of blocks (1+2+3+3 = 9), multiply by the block size (9x0.75 = 6.75 feet), and subtract this height from the high tide height (9.0-6.75 = 2.25 feet) to obtain a water level of 2.25 feet.

This pattern also applies to the rising tide, though in this case the coming high tide height is only 7.5 feet. This means that one block will be 0.6 feet, or 7.5 inches. Be sure, however, to take into consideration the low tide height, which is not always zero.