Tides

 

The rise and fall of the tide is the result of the gravitational effects of the sun and the moon. In most places around the world you have two  high tides and two low tides in a 24 hour period. The time taken to fall from a high to a low tide (high water to low water) is approximately 6 hours. As can be seen from the graph the rate of fall is not constant but is  rather what is called the rule of twelfths ie one twelfth of the distance is changed in the first and last hour, two twelfths is changed in the second and fifth hours and three twelfths in the third and fourth hours.

 

The distance that the water rises and falls is known as the tidal range. This is not constant but there are generally two ranges:

• Spring Tides
  These are not as the names suggests tides that only occur in the spring but are tides that occur when the moon and the sun are aligned ie full moon and new moon. In this position they exert the greatest gravitational pull and hence we get higher tides.
• Neap Tides
  These occur when the sun and the moon are out of alignment the most ie first and last quarter moon. In this position they exert the least gravitational pull and so we get lower tides.

As the moon rotates around the earth every 28 days we get a cycle of spring, neap, spring and neap tides each month.

 

Estimates of what part of the tidal cycle we are in for each day can be calculated and the time of high and low water plus the expected heights can be deduced. Tide tables containing this information are published by many authorities. It should be noted that times stated in such tables may be local time or GMT and so adjustmenst may have to be made. Similarly tables are applicable only to the area stated on them ie a tide table for Portsmouth will be no good for Plymouth. Adjustment values may be available to apply the tables to your own area.

 

When the tide goes out or comes in we get what is known as the tidal stream ie the water flowing in or out (ebb tide) as applicable. The rate at which the water flows may be significant and should always be considered when going on the water. The greater the tidal range (distance between high and low water) the faster the tidal flow since more water has to move in the time period. Similarly since we already stated that the tide changes most during the third and fourth hours of the cycle we get the strongest tidal streams then. Stronger tidal streams will be found in mid channel or deep water and weaker ones along the edge of the channel. The way boats on moorings behave is a good indication of the tidal flow.