Tides And Currents

Text-only Preview

New York HarborWhat 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/What causes TidesGravity 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 earthhttp://oceanservice.noaa.gov/education/kits/tides/tides02_cause.htmlTides are the rise and fall of sea levels caused by the combinedeffects 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/TideToday’s Tides - 11/11/2009At the Battery, NYC:High Tide3:12 am3:26 pmLow Tide9:34 am10:06 pm Tomorrow’s Low10:32 am10:55 pmSource: Eldridge 2009 Tide and Pilot Book, p 127.DiagramsGoogle Earth NYC view with clickable markers athttp://tidesandcurrents.noa a.gov/ports/index.shtml?p ort=nyHigh tide at different timesUnlike 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.More about TidesIn 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!More about TidesPhoto credits: Bay of Fundy, Canada, http://en.wikipedia.org/wiki/TideHaines, Alaska, photo by MN HawkNew York Harbor TidesTimes of High and Low Water / NOAA Tides at http://tidesandcurrents.noaa.gov/tides09/tpred2.html#NYWhat 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 acurrent in the oceans, near the shore, and in bays and estuariesalong 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 currentsare 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.Document Outline
  • Tides and Currents
    • ÿ
    • ÿ
    • ÿ
    • ÿ
    • ÿ
    • ÿ
    • ÿ
    • ÿ