TIDES
- periodic, short-term changes in the height of the ocean surface at a particular place caused
by a combination of the gravitational force of the moon & sun and the motion of the Earth
Causes of the Tides
- the moon’s gravitational attraction is greatest on the side of the Earth nearest the
moon & least on the opposite side. The centrifugal force (inertia) is equal all over. On the
side near the moon, attraction of the moon is greater than the centrifugal force so water is
attracted to the moon; on the other side, the opposite is true, so water tends to be flung
away. This causes water to bulge directly under the moon & at a point on the opposite side
of the Earth. The bulges are the high tides with the low tides being halfway between. As
the Earth rotates beneath the bulges, the bulges appear to move across the ocean surface
giving highs & lows
Examination of tidal curves from many ports shows that each has a different tide. Tides are
grouped into 3 types based on the number of highs & lows per day, the relation-ship
between the heights of successive highs & lows, and the time between highs & lows:
1) diurnal - 1 high & 1 low per day (ex: Gulf of Mexico)
2) semidiurnal - 2 highs & 2 lows per day (ex: N. Atlantic)
3) mixed - 2 highs & 2 lows per day but both highs & both lows differ (higher high water,
lower high water, lower low water, higher low water) (ex: Pacific)
Tidal Range - difference between high & low tide
With a tidal record of only a few days length, the type of tide characteristic can be
classified. However, with a record of several weeks, a pattern of changes emerges.
Spring Tide - occurs near the times of a full & new moon and the Spring tidal range is
greater than the mean tidal range
Neap Tide - occurs during the 1st & 3rd quarter moons and the tidal range is the least
Other variations occur over periods of years.