The document summarizes information about tides and currents in New York Harbor. It explains that tides are caused by the gravitational pull of the moon and sun, and that they cause water levels to rise and fall approximately every 12 hours. It also discusses how tidal changes cause tidal currents to flow in and out with the rising and falling tides. Maps and diagrams show the typical tidal currents in New York Harbor on both high and low tides.

1. New York Harbor

2. What are Tides?
The water in all oceans and estuaries rises to a high level
approximately every 12 hours, and to a low level 6 hours
earlier and later.
An estuary is where salt and fresh water mix, in harbors
and rivers near where they meet the ocean. The Hudson
River is part of an estuary that extends 50 miles north of
NYC. The entire estuary feels the up and down effect of
tides.
http://oceanservice.noaa.gov/

3. What causes Tides
Gravity is one major force that creates tides. In 1687, Sir Isaac
Newton explained that ocean tides result from the gravitational
attraction of the sun and moon on the oceans of the earth
http://oceanservice.noaa.gov/education/kits/tides/tides02_cause.html
Tides are the rise and fall of sea levels caused by the combined
effects of the rotation of the Earth and the gravitational forces
exerted by the Moon and the Sun. The tides occur with a period
of approximately 12 and a half hours. Most coastal areas
experience two daily high (and two low) tides.
http://en.wikipedia.org/wiki/Tide

4. Today’s Tides - 11/11/2009
At the Battery, NYC:
High Tide 3:12 am 3:26 pm
Low Tide 9:34 am 10:06 pm
Tomorrow’s Low 10:32 am 10:55 pm
Source: Eldridge 2009 Tide and Pilot Book, p 127.

5. Diagrams
Google Earth NYC view
with clickable markers at
http://tidesandcurrents.noa
a.gov/ports/index.shtml?p
ort=ny

6. High tide at different times
Unlike a solar day, however, a lunar day is 24 hours and 50
minutes. The lunar day is 50 minutes longer than a solar day
because the moon revolves around the Earth in the same
direction that the Earth rotates around its axis. So, it takes the
Earth an extra 50 minutes to “catch up” to the moon (Sumich,
J.L., 1996; Thurman, H.V., 1994).
Because the Earth rotates through two tidal “bulges” every
lunar day, coastal areas experience two high and two low tides
every 24 hours and 50 minutes. High tides occur 12 hours and
25 minutes apart. It takes six hours and 12.5 minutes for the
water at the shore to go from high to low, or from low to high.

7. More about Tides
In New York Harbor, the difference between high and low
tide is about 5½ feet.
Closer to the Equator, the difference between high and low
tide is less – as small as two feet.
Closer to the North and South poles, the difference
between high and low tide is greatest, as much as 35-feet
of difference every 12 hours!

8. More about Tides
Photo credits: Bay of Fundy, Canada, http://en.wikipedia.org/wiki/Tide
Haines, Alaska, photo by MN Hawk

9. New York Harbor Tides
Times of High and Low Water / NOAA Tides at
http://tidesandcurrents.noaa.gov/tides09/tpred2.html#NY

10. What are Currents?
Oceanic currents are driven by several factors. One is the rise
and fall of the tides, which is driven by the gravitational
attraction of the sun and moon on Earth's oceans. Tides create a
current in the oceans, near the shore, and in bays and estuaries
along the coast. These are called "tidal currents." Tidal currents
are the only type of currents that change in a very regular
pattern and can be predicted for future dates.
A second factor that drives ocean currents is wind. Winds drive
currents that are at or near the ocean's surface. These currents
are generally measured in meters per second or in knots (1 knot
= 1.15 miles per hour or 1.85 kilometers per hour). Winds drive
currents near coastal areas on a localized scale, and in the open
ocean on a global scale.

11. Currents II
Tidal currents occur in conjunction with the rise and fall of the
tide. The vertical motion of the tides near the shore causes the
water to move horizontally, creating currents. When a tidal
current moves toward the land and away from the sea, it
“floods.” When it moves toward the sea away from the land, it
“ebbs.” These tidal currents that ebb and flood in opposite
directions are called “rectilinear” or “reversing” currents.
Check out the animation at
http://oceanservice.noaa.gov/education/kits/currents/02tidal1.html

12. New York Harbor Currents
NY Harbor current charts scanned from EldridgeTide and
Pilot Guide at
http://www.weerow.org/content/tides+currents/
Next 2 slides show currents at High and Low tide

13. High Water – Flood to Ebb

14. Low Water - Ebb

15. Conclusion
The tides' influence on current flow is much more difficult to
analyze, and data is much more difficult to collect. A tidal
height is a simple number which applies to a wide region
simultaneously. A flow has both a speed and a direction, both
of which can vary substantially with depth and over short
distances.
Animated hourly currents with stop action available at
http://tidesandcurrents.noaa.gov/ofs/nyofs/now_cg_cur.shtmlb

16. Thank you for
Watching this Show!
Happy and Safe Boating on New York Harbor!