Waves

Q: How do waves form?

Krishna: Waves are created by energy passing through water, causing it to move in a circular motion (water's motion ). The rise and fall of water molecules creates a wave that moves in the direction of the wind. The wave transports energy, not water.

Waves in oceans and seas are caused by a number of factors, including wind, gravity, and underwater disturbances:

• Wind: The friction between wind and water creates waves, which are most commonly seen at the beach. As the wind blows, it pulls the water forward, which creates a wave crest. Waves are most commonly caused by wind. Wind-driven waves, or surface waves, are created by the friction between wind and surface water. As wind blows across the surface of the ocean or a lake, the continual disturbance creates a wave crest. These types of waves are found globally across the open ocean and along the coast.
• Gravity: The gravitational pull of the sun and moon on Earth causes tides, which are also a type of wave. The gravitational pull of the sun and moon on the earth also causes waves. These waves are tides or, in other words, tidal waves.
• Underwater disturbances: Earthquakes, landslides, and volcanic eruptions can displace large amounts of water quickly, creating tsunamis. Tsunamis are very long waves that roll into the shore like a massive rise in sea level.
  Hazardous waves can be caused by severe weather, like a hurricane. The strong winds and pressure from this type of severe storm causes storm surge, a series of long waves that are created far from shore in deeper water and intensify as they move closer to land. Storm surge and tsunamis are not the types of waves you imagine crashing down on the shore. These waves roll upon the shore like a massive sea level rise and can reach far distances inland.

As waves reach the shore , the shallow floor begins to run into their base, slowing their deeper parts. This causes the wave to stand up then pitch forward before eventually breaking.

The height of a wave is determined by the wind strength, the length of time it blows and its fetch – the distance it travels over the water. Oceanographers measure wave height using a term called “significant wave height”, which is the average height of the largest 33 per cent of waves in that region of ocean at the time.

Wind blowing for an hour at a speed of 55.6 kilometres per hour over a stretch of water 1000km wide would generate waves with a significant wave height of 0.7 metres. But if it blew for 48 hours, the wave’s crest would increase to 10.5 metres, about twice the height of a double-decker bus.

Occasionally waves merge to form one that is far higher than those surrounding it. These rogue waves were once thought to be fictional, or vanishingly rare. However, through better monitoring of the seas, we now know these freak waves are surprisingly common. The complex forces that give rise to them are now better understood, raising the possibility of rogue wave forecasts.

 Some research suggests that extreme waves are becoming more likely as a result of climate change, due to an increase in storms and melting of polar ice.

There are three main factors that affect wave formation: wind velocity, fetch, and duration. Wind velocity is the speed of the wind, fetch is the distance over the water that the wind can blow uninterrupted (which can be huge distances out at sea), and duration is the amount of time the wind blows over that patch of water. The greater the wind velocity, the longer the fetch, and the greater duration the wind blows, then the more energy is converted to waves and the bigger the waves. However, if wind speed is slow, the resulting waves will be small, regardless of the fetch or duration. It takes all three factors acting together to create big waves.

Waves often result from storms, which tend to move across the ocean with the prevailing winds. So although a storm might only have 500 nautical miles (nm) of fetch, the storm can travel greater distances, say 1,000 nm, creating a travelling fetch of more than 1,000 nm.

When wind moves across the water’s surface, it creates frictional drag. Drag is a force that acts against the relative motion of one fluid with respect to another fluid. A "fluid" is anything that flows easily, such as gases (air) and liquids (water). (1)

For our situation, the two fluids are air (atmosphere) and water (ocean). The texture of the ocean’s surface can influence the way the wind moves over the water and the amount of friction, or drag, produced. The sea is much smoother than your typical land features (e.g. mountains), so drag tends to be much less over the ocean than over land. The same drag that slows the wind tends to accelerate the ocean-surface water -- making ocean currents and waves.

The frictional drag felt by the air is limited to the bottom 300 m to 3 km of the atmosphere.  This portion of the troposphere is called the "boundary layer".  The thickness of the atmospheric boundary layer depends on the nature of the water’s surface, the wind, and the temperature. Above this boundary layer, the winds are not slowed by drag and will be much stronger than winds near the Earth’s surface.

Waves move energy, not water. 

Riding a wave ( Art work by Dr. Krishna Kumari Challa)

( http://www.kkartfromscience.com )

There are mainly seven types of waves (2): 

1 Beach Breaks

These waves are the most popular and break over a sandy bottom. The shape and quality of the waves can change due to the changing quality of sand and shifts in the seabed.

A sandbank may stay for months or disappear in a few days. The waves may be long and gentle or hollow and powerful, depending on factors such as the time.

2 Crumbly Waves

Also known as mushy waves, these are easy to characterize as they’re not too steep, fast or hollow, and break gently. Very gradual bottom contours cause these waves.

3 Point Breaks

A point break forms beside a headlock. It usually breaks over sand or rock. These waves peel for much longer than beach or reed breaks. These waves are said to be suitable for beginners and can be a lot of fun.

4 Reef Breaks

These waves break over a rocky bottom. The line-up changes depending on the size and direction of the waves but the seabed remains constant. You can paddle out through a clear channel.

5 Reform Waves

This unpredictable wave can die down when it hits deep water and then reform or break again due to the bottom’s varying depth. 

6 River Mouth Waves

They’re extremely rare. They are quite like point breaks as they are produced where the water deposits sand on to sandbars resulting in the waves peeling off in a neat and predictable manner. 

7 Double-up waves

This wave is the result of two waves meeting and coming together As a result, you get a very powerful and large wave.

These waves are very dangerous and ultra-hollow especially when they begin to break. 

Other than this, you should be able to identify where a wave is going. A left-hand wave, for example is a wave that peels or breaks to the left .

Footnotes:

1. https://www.eoas.ubc.ca/courses/atsc113/sailing/met_concepts/08-met...(e.g.%20tsunamis).

2. https://www.pacificsurf.com/7-types-of-waves-every-surfer-should-kn...