1) Tides-
a. The vertical movement (rise and fall) of the water level.
b. Due to the gravitational attractions of the moon and sun. The moon
is the biggest factor- it’s closer.
c. Roughly 6 hour cycle in most locations, including the west coast. Resulting
in 2 highs and 2 lows each day.
d. “Spring” Tides- around the time of full and new moon. Earth,
moon and sun are aligned. Greater range, higher highs/lower lows.
e. “Neap” Tides- at moon quarter phases. Sun and moon counteract
each other. Less range of tide, lower highs and higher lows.
f. Level is in feet relative to the NOAA datum at mean lower low water
(MLLW). (same as most charts)
g. Given for a “primary station” like the Golden Gate, with
corrections (or “differences”) for other “sub-stations”
h. Differences in tides are given as time (+ or -) and height (+, - or
*(ratio)).
i. Other factors include: weather, moon perigee/apogee, declination (monthly),
sun perihelion/aphelion (annually).
j. Tidal range in SF Bay is roughly 6 feet. (5.8 feet between MLLW and
MHHW)
k. Tides are said to “stand” at high and low, before moving
in the opposite direction.
2) Currents-
a. The horizontal movement (ebb (out) and flood (in)) of the water.
b. Because of tidal effects.
c. Same (roughly) 6 hour cycle in most areas.
d. Given as times and velocities (in knots) for maximum ebb or flood and
times (only) for slack water (no current).
e. Directions can be determined by current charts found in most booklets.
These are usually provided by hours relative to max current. Some current
tables give compass directions.
f. During “Spring” tides, currents are stronger.
g. During “Neap” tides, currents are weaker.
h. Given for a “primary station” like the Golden Gate, with
corrections (or “differences”) for other “sub-stations”
i. Differences for currents are given as time (+ or -) for low slack,
flood, high slack and ebb, and speed ratios for flood and ebb.
j. Other factors include weather as well as tidal factors mentioned.