Tides

• What are the tides?

  The periodic rise and fall in the level of the sea.
• What are tides caused by?

  The gravitational pull of the sun and moon (although the moon has a much greater influence as it is closer).
• How does the moon create tides?

  It pulls water towards it, creating a high tide, and there is a compensatory bulge on the opposite side of the Earth (created by the Earth itself pulling away from the water on the other side). In areas of the world between the two bulges, the tide is at its lowest.
• 𝙏𝙃𝙀 𝙈𝙊𝙊𝙉:

  As the moon orbits the Earth, the high tides follow it. Twice in a lunar month, when the moon, sun and Earth are in a straight line, the tide-raising force is strongest. This produces the highest monthly tidal range - the 𝘴𝘱𝘳𝘪𝘯𝘨 𝘵𝘪𝘥𝘦.

  Also twice a month, the moon and sun are positioned at 90° to each other in relation to the Earth. This alignment gives the lowest monthly tidal range - the 𝘯𝘦𝘢𝘱 𝘵𝘪𝘥𝘦.
• 𝙏𝙄𝘿𝘼𝙇 𝙍𝘼𝙉𝙂𝙀:

  Tidal range is the difference in height between the high tide and the low tide, and can be a significant factor in the development of a coastline.

  Along the coasts of the Mediterranean Sea, tidal ranges are low. This restricts wave action to a narrow width in the coastal zone.

  In parts of the British Isles, however, tidal ranges are high. This gives a wide zone of wave attack, resulting in the formation of wide wave-cut platforms in many places.
• Tidal range is greatest where an ocean is funnelled into a constricted channel and the water is forced to rise as it is contained.
• Neap tides occur when the Sun, Moon and Earth are at right angles to one another. Their gravitational forces cancel out some of the effects of each other so that the high tides are not as high and the low tides are not as low as usual.
• Spring tides occur when the Sun, Moon and Earth are aligned with one another. The combined effect of their gravity causes higher than average high tides and lower than average low tides.
• 𝙎𝙄𝙂𝙉𝙄𝙁𝙄𝘾𝘼𝙉𝘾𝙀 𝙊𝙁 𝙏𝙃𝙀 𝙏𝙄𝘿𝙀𝙎:

  • the movement of the tides can generate ocean currents that influence the direction and scale of coastal sediment movement
  • when high tides are combined with a low-pressure weather system, an extremely high storm surge can be generated that leads to flooding along coastal areas
  • the regular movement of the tides can be used to generate renewable energy - Swansea Bay, Wales, is an area that utilises this energy to generate power
• What are ocean currents?

  The movement of water from one location to another.
• What are ocean currents measured in?

  Metres per second or knots (1 knot = 1.15mph).
• What are ocean currents driven by?

  • wind
  • water density differences (thermohaline circulation)
  • the rise and fall of the tides
• 𝙏𝙄𝘿𝙀𝙎 𝘿𝙍𝙄𝙑𝙄𝙉𝙂 𝙊𝘾𝙀𝘼𝙉𝙄𝘾 𝘾𝙐𝙍𝙍𝙀𝙉𝙏𝙎:

  Tides create a current in the oceans, which are strongest near the shore and in bays and estuaries along the coast - these are called "tidal currents" and change in a very regular pattern, allowing them to be predicted.

  In some locations, strong tidal currents can travel at speeds of 8 knots or more.
• 𝙒𝙄𝙉𝘿 𝘿𝙍𝙄𝙑𝙄𝙉𝙂 𝙊𝘾��𝘼𝙉𝙄𝘾 𝘾𝙐𝙍𝙍𝙀𝙉𝙏𝙎:

  Winds drive currents that are at or near the ocean's surface. Near coastal areas, winds tend to drive currents on a localised scale and can result in phenomena like coastal upwelling (the upward movement of deep, colder water). On a more global scale, in the open ocean, winds drive currents that circulate water for thousands of miles throughout the ocean basins.
• 𝙒𝘼𝙏𝙀𝙍 𝘿𝙀𝙉𝙎𝙄𝙏𝙔 𝘿𝙍𝙄𝙑𝙄𝙉𝙂 𝙊𝘾𝙀𝘼𝙉𝙄𝘾 𝘾𝙐𝙍𝙍𝙀𝙉𝙏𝙎:

  Thermohaline circulation is a process driven by density differences in water due to temperature (thermo) and salinity (haline) variations in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents.
• Currents affect the Earth's climate by driving warm water from the Equator and cold water from the poles around the planet.
• How do currents affect us?

  • real-time and forecasted current measurements are important to shipping, commercial fishing, recreational boating, and safe docking/navigation
  • search-and-rescue personnel can use predicted current patterns to determine where the water may carry a missing person or floating object(s)
  • GIS systems are used in search-and-rescue efforts to estimate the location of missing person(s) or item(s) using real-time current and weather data and drift patterns
• What is a storm surge?

  The pushing of water against a coastline to abnormally high levels (extreme low pressure and high tides).
• Storm surges are caused when atmospheric air pressure drops rapidly over an area, causing sea level to rise dramatically. This happens because the lower atmospheric pressure allows the atmosphere to pull seawater towards it, increasing its height above mean sea level.
• What are ocean currents?

  Large-scale movement of water in the oceans.
• What is the tidal range?

  The vertical difference in height of sea level between high and low tide.
• What is spring tide?

  Tides that occur twice a month when the sun, moon and Earth align in a straight line.
• What is neap tide?

  Tides that occur twice a month when the sun and moon are at right angles to the Earth.
• What are rip currents?

  Strong localised underwater currents caused when plunging waves cause a build-up of water at the top of the beach.
• 𝙍𝙄𝙋 𝘾𝙐𝙍𝙍𝙀𝙉𝙏𝙎:

  • caused by the topography of the beach
  • commonly formed when a series of plunging waves cause a temporary build-up of water at the top of a beach
  • met with resistance from breaking waves, water returning down the beach (backwash) is forced below the surface following undulations in the beach profile
  • this fast-flowing offshore surge can drag people into deep water