Tides
JetStream,
the National Weather Service Online Weather School
Tides are another kind of wave motion in the ocean.
Tides are a change in the ocean water level, typically
reaching a high and low level twice a day usually occurring about six hours
apart.
The term for the change from low to high tide is called the
"flood tide".
The change from high tide to low tide is called the
"ebb tide".
Tides result from the pull of gravity; on the earth alone,
between the earth and moon and between the earth and the sun.
The gravitational pull of the sun on the earth is about 178
times stronger than the gravitational pull on the earth from the moon.
However, because of the close proximity of the moon, when
compared to the sun, the tidal pull by the moon is over twice that of the sun.
The result of this tidal pull is a bulge in the ocean water
almost in line with the position of the moon; one bulge toward the moon and one
on the opposite side of the earth, away from the moon.
When we observe the tides what we are actually seeing is
the result of the earth rotating under this bulge.
It is easy to understand why there should be a bulge of
water, producing a high tide, on the side of the earth facing of the moon. But
why is there a bulge on the opposite side as well?
It is obviously not gravity that is doing it but rather, it
is the difference in gravitational force across the earth that causes the
bulge. This difference in gravitational force comes from the moon's pull at
various points on the earth.
Because the pull of gravity becomes stronger as the
distance decreases between to object, the moon pulls a little harder at point
"C" (closest point to the moon) than it does at point "O"
(in the center of the earth), and the pull is weaker still at point "F"
(farthest point from the moon).
 |
| Figure 2 |
If it were not for the earth's gravity, the planet would be pulled apart (figure 2).
Yet also because of the earth's gravity which pulls us
toward the center of the planet we can, mathematically subtract the moon's pull
at the center of the earth from the moon's pull at both point "C" and
"F".
 |
| Figure 3 |
When this vector-based subtraction occurs, we are left with
two smaller forces; one toward the moon and one on the opposite side point away
from the moon (figure 3) producing two bulges.
As the earth makes one rotation in 24 hours, we pass under
these areas where the tidal force pulls water away from the earth's surface and
experience high tides.
Also, since the difference in gravitation force is constant
across the earth, the bulge on both side of the earth is essentially the same.
Which explains why consecutive high tides are nearly the
same height each time regardless whether the moon is overhead or on the
opposite side of the earth.
The change in the water level with the daily tides from
location to location results from many factors.
The oceans and shorelines have complex shapes and the
depth, and configuration, of the sea floor varies considerably.
As a result, some locations only experience one high and
low tide each day, called a diurnal tide.
Other locations experience two high and low tides daily,
called a semi-diurnal tide.
Still, other sites have mixed tides, where the difference in successive high-water and low-water marks differ appreciably.
Another factor in the variation of tides is based on the
orbit of the moon around the earth and the earth around the sun. Both orbits
are not circles but ellipses.
The distance between the earth and moon can vary by up to
13,000 miles (31,000 km).
Since the tidal force increase with decreasing distance
then tides will be higher than normal when the moon is at its closest point
(called perigee) to the earth, approximately every 28 days.
Likewise, the earth's elliptical orbit also causes
variations in the sun's pull on the tides as we move from the closest point to
the farthest point (called apogee) over the course of a year.
And just to complicate things even more, the moon's orbit
is inclined 5° to the earth's rotation.
So, the north/south orientations of the bulge also varies
between the northern and southern hemisphere over this same 28-day orbital
period.
 |
| Earth-Moon-Sun configuration for Spring tide |
As the moon completes one orbit around the earth (about
every 28 days), there are two times in each orbit when the earth, moon and sun
are in line with each other and two times when the earth, moon and sun are at
right angles.
When all three are in line (around full and new moons), the
combined effect of the moon's and sun's pull on the earth's water is at its
greatest resulting in the greatest ranges between high and low tide.
This called a "spring" tide (from the water
springing or rising up).
Seven days after either full or new moon, the earth, moon
and sun are at right angles to each other.
 |
| Earth-Moon-Sun configuration for Neap tide |
At this time the pull of the moon and the pull of the sun partially cancel each other out. The resulting tide, called a "neap" tide, has the smallest range between high and low tide.
This graph indicates how the ocean level changes in height
daily for May 2019 in Santa Barbara, CA. For this location there are two high
and low tides daily with one ebb and flow greater than the other.
 |
| Example of the daily tide and differences between Spring and Neap tides at Santa Barbara, CA from May 2019. |
The difference in sea-level height between each high and
low tide changes daily depending upon the position of the Moon.
The greatest difference in height occurs around new and
full moons; 6.27 ft. (1.91 m) and 7.18 ft. (2.19 m) respectively.
The least difference in height occurs at
both first- and last-quarter moon phases; 4.72 ft. (1.44 m) and 3.16 ft.
(0.96 m) respectively.
Local influences effect the actual timing of
spring and neap tides and therefore they may not necessarily align precisely
with the phases of the moon.
Also, this graph represents this location for
this month and year only. High and low tides, and their timing, for every
location world-wide are different day-by-day, month-by-month, and year-by-year.
Welcome
to JetStream, the National Weather Service Online Weather School. This site is
designed to help educators, emergency managers, or anyone interested in
learning about weather and weather safety.
https://www.weather.gov/jetstream/tides
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