The following article provided by the marine weather blog Ocean Weather Services blog and written by Fred Pickhardt, a professional marine meteorologist and owner of Ocean Weather Services. Ocean Weather Services provides professional marine meteorological research reports to admiralty law firms and insurance underwriters, Ocean weather forecasts and ship routing services.
Wind-generated waves are the most common waves found on the ocean and are the result from stress on the water surface caused by the wind. The smallest of these are capillary waves which can be quickly brought back to equilibrium solely by the cohesion of the individual water molecules. Most wind-generated waves, however, are referred to as gravity waves since it is gravity that acts to restore them to equilibrium. Wind driven waves are the waves that have the greatest impact on ships.
Tides are the rise and fall of sea level caused by the gravitational attractions of the moon and sun and by the centrifugal force of the spinning earth.
When the solar and lunar gravitational forces are in line they combine to create the highest of the high tides and lowest of the low tides which are referred to as “spring tides.” When the forces are perpendicular to each other, the forces are pulling the water in different directions so the difference between high and low tides are minimized and is referred to as a “neap tide”.
A seiche is the sloshing of water back and forth in lakes and other large bodies of waters. Seiches can be caused by a disturbance such as an earthquake or landslide, changes in air pressure, or changes in the wind. The most common cause of seiches are persistent strong winds blowing along the long axis of large water body causing a rise in the water level at the down-wind side and a lowering of the water level at the up-wind end.
When the wind abates, the water is released as a seiche wave. Flooding and erosion can occur at one end of the lake, while at the other end the decreased water depth can cause hazards to ship navigation.
Recent events in Japan have focused our attention on tsunamis. Tsunamis are long-period waves generated by undersea earthquakes, volcanic eruptions and landslides. In the open deep oceans a tsunami will have extremely long wavelengths with small amplitudes and might go unnoticed by ships. Tsunami waves travel at very high speeds, often at hundreds of miles per hour through deep water but as the tsunami waves reach shallow water near the coast, they begin to slow down while gradually growing steeper, due to the decreasing water depth and can grow to tens of meters or more as they reach the shoreline. The effects can be further amplified where a bay, harbor, or lagoon funnels the waves as they move inland and well document during Japan’s recent event. Another potential cause of a tsunami is an asteroid impact in the deep ocean which could produce a tsunami waves of over 100 meters (more than 330 feet)!
5. Pressure Induced
The 5th but less significant type of wave develops as air pressure perturbations move over the water surface. The sea surface height rises or falls slightly as the atmospheric pressure changes. Low air pressure within a strong storm can elevate the ocean’s surface up to 0.5m (1.6ft), creating an atmospherically forced pressure wave beneath the storm.
Wave definitions: Image credit NOAA
A wave crest is the highest point in the wave and a wave trough is the lowest point in the wave. Wave height (H) is the vertical distance between the wave crest and the wave trough.
Wavelength (L) is the distance from one crest to the next crest or from one trough to the next trough.
Wave period (T) is the time it takes successive wave crests or successive wave troughs to pass a fixed point. In the real world, the wave period is actually a spectrum of periods scattered about a mean wave period.
Wave steepness (S) is defined as wave height divided by wavelength (S = H/L). Therefore, the same wave height will result in high steepness if the wavelength becomes smaller. A small height divided by a large length will produce a low steepness. When the wave steepness exceeds about 1/7 the wave will begin to break or “white cap.”
Wave speed (C) is the speed an individual wave moves through water. If the wave period (T) and wave length (L) are known, then the wave speed (C) can be determined by C=L/T
Other Wave Facts:
Deep Water or Shallow Water
A wave is considered to be a deep water wave as long as water depth exceeds 1/2 the wavelength. A wave is considered to be a shallow water wave as long as water depth is less than 1/20 the wavelength. The area between deep and shallow water is transition zone.
Wave energy increases by a factor of 4 as the wave height doubles so a 10ft wave is four times more powerful than a 5 ft wave.
The significant wave height (Hs) is the mean height of the highest one third of the waves passing a point. This is of interest as this wave height correlates best with the wave height a trained observer reports after examining a group of wave heights from a ship or platform. The averaged periods of the waves used to compute significant wave height is known as the significant wave period.
Useful wave height relationships:
Hm (Mean wave height) = 0.64 times Hs
Hs or H1/3 = Significant wave height
H1/10 (Highest 10% wave height) = 1.27Hs
H1/100 (Highest 1% wave height) = 1.67Hs
Hmax (Max probable wave height for a large sample) = about 2.0Hs
Ocean Swell is defined as any wave that has moved out of its wind generation source region. Swells characteristically exhibit smoother, more regular and uniform crests and a longer period than wind waves.
Combined Seas describes the combination or interaction of wind waves and swells in which the separate components are not distinguished. Combined Seas (CS) is the square root of the square of swell plus the square of wind waves: The National Weather Service considers the combined seas as being the same as significant wave height.
Rogue waves (sometimes called freak waves) are simply unusually large waves appearing in a set of smaller waves. A rogue wave will have a height of at least twice the size of surrounding waves, often come from a direction different than the prevailing waves, and they are unpredictable. Most reports of extreme storm waves say they look like “walls of water,” and are seen as steep-sided with unusually deep troughs. The USS Ramapo reported one such wave with a height of 112 feet in the Pacific in 1933. Another report of a freak wave occurred when one struck the Queen Mary amidships, south of Newfoundland, at the end of World War II, rolling her to within a degree or two of capsizing.
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