Geoscience Reference
In-Depth Information
Moon, it is nevertheless important. When the Sun, Moon, and
Earth are aligned, the tide-raising force is maximized, result-
ing in the very highest tides called
spring tides
. In this case,
“spring” refers to upwelling, rather than the season. When the
Sun and Moon are at right angles to Earth, however, somewhat
lower high tides result; these tides are called
neap tides
.
In addition to the tidal variations that occur with respect to
the geometric relationship of the Earth, Moon, and Sun, the overall
tidal range can vary dramatically between places. For example, as
you can see in Figure 19.9, the average tidal range at San Francisco,
California, is about 2.5 m (8.2 ft). The largest tidal range on Earth
occurs in the Bay of Fundy, which lies between the Canadian prov-
inces of New Brunswick and Nova Scotia in eastern North America
(Figure 19.1). The shape of the coastline here has a funneling effect
that produces a tidal range varying from about 6 m (~20 ft) at the
mouth of the bay to 16 m (53 ft) at the head of the bay!
distance of unimpeded airflow across the ocean or large water
body. Although waves appear to be bodies of water that are
moving horizontally, they are, in fact, nothing more than ris-
ing and falling disturbances of water masses. Very little water
moves forward in a wave; instead, it is the wave energy that is
transmitted forward through the elastic medium of the water. If
you want to test this concept yourself, place a floatable object
in the water the next time you are at the ocean and watch as the
undulations pass beneath it. You will see that, although many
different waves pass by, the object moves only slightly.
Water waves are similar to the electromagnetic waves dis-
cussed in Chapter 4 because they consist of various measurable
components. The
wave crest
is the high spot of an individual
wave and forms when a vertical column of circularly rotating
water particles passes through the water and causes the water to
rise. Although the column of rotating water continues to some
depth in the water, the diameter of individual oscillations pro-
gressively becomes less until they disappear at the
wave base
.
After the wave crest passes, the water lowers into the
wave
trough
. The horizontal distance between successive wave crests
is the
wavelength
.
Wave amplitude
refers to the vertical distance
between the wave crest and the level of the water if it were still.
Wave height
is somewhat different from wave amplitude in that
it measures the vertical distance between the wave crest and
the base of the wave trough. In general, stronger winds result
in greater wave heights and amplitudes. When waves move out
of the area where the generating winds occur, they continue to
move forward and are then known as
swells
.
Waves
The most dynamic form of water movement along
coastlines is associated with
waves
, which are oscillations in a
body of water resulting mostly from the force of friction gen-
erated by wind blowing across the water surface. The specific
wind factors involved in the formation of waves are (1) wind
strength, (2) wind duration, and (3) wind
fetch
—that is, the
Waves
Oscillations in a body of water that form mostly due
to the frictional force generated by wind blowing across the
surface of the water.
High-water level
2
6
4
1
2
0
0
Low-water level
Noon
3/17/04
12:00 pm
3/18/04
Noon
3/18/04
12:00 pm
3/19/04
Noon
3/19/04
Figure 19.9 Tidal range at San Francisco, California, over a two-day period.
The point of refer-
ence used here is the Mean Lower Low Water (MLLW), which is the average of the lower low-water
height of each day and is represented by the 0 baseline in the graph. Relative to this level, the average
tidal range is about 2.0 m (about 6.5 ft). (
Source
: NOAA.)
