Tides And Currents

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About the tides and currents of NY Harbor, our region's largest public open space! Created for the open-water rowers who use the Whitehall gigs, for a presentation at the BMCC Rowing Club, Nov. 11, 2009, Borough of Manhattan Community College.

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Tides And Currents

  1. 1. New York Harbor
  2. 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. 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. 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. 5. Diagrams Google Earth NYC view with clickable markers at http://tidesandcurrents.noa a.gov/ports/index.shtml?p ort=ny
  6. 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. 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. 8. More about Tides Photo credits: Bay of Fundy, Canada, http://en.wikipedia.org/wiki/Tide Haines, Alaska, photo by MN Hawk
  9. 9. New York Harbor Tides Times of High and Low Water / NOAA Tides at http://tidesandcurrents.noaa.gov/tides09/tpred2.html#NY
  10. 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. 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. 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. 13. High Water – Flood to Ebb
  14. 14. Low Water - Ebb
  15. 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. 16. Thank you for Watching this Show! Happy and Safe Boating on New York Harbor!

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