Explain why is the sea calm by the wind offshore and agitated near the coast?

In short (click here for detailed version)

Offshore, the wind can blow over large expanses of water, allowing waves to form and disperse, resulting in a calmer sea. Near the coast, the effect of topography and land obstacles limits the dispersion of waves, leading to rough seas.

Explain why is the sea calm by the wind offshore and agitated near the coast?
In detail, for those interested!

First factor: Water depth

When the wind blows over the sea, the first factor that influences its agitation is the depth of the water. In the open sea, far from the coasts, the water is generally deeper, allowing for a more effective dispersion of the wind's energy. This dispersion reduces the height of the waves and decreases the level of sea agitation. On the other hand, near the coasts where the water is shallower, the waves tend to be higher and closer together, creating a more agitated sea. The decrease in depth near the coasts causes more friction with the bottom, which also contributes to the agitation of the surface.

Second factor: Friction effect

When the wind blows on the surface of the water, it exerts a force that propagates and creates waves. These waves then move in the direction of the wind, but the speed of the waves decreases with the depth of the water. Indeed, near the surface, the water is slowed down by the friction with the air, creating a drag. This drag leads to the formation of shorter and closer waves. On the other hand, in deeper water, the water is less affected by this friction, allowing the waves to stretch out and move more quickly. Thus, offshore, where the water is deeper, the waves are longer and lower, making the sea calmer. Conversely, near the coasts, where the depth is shallower, the friction is more pronounced, creating shorter and higher waves, stirring up the sea more.

Third factor: Influence of the topography

The shape of the coast can greatly influence the behavior of the sea. Waves can be reflected, diffracted or amplified depending on the topography of the area. For example, a concave coast can concentrate wave energy at a point, creating more turbulent conditions. Conversely, a convex coast can disperse this energy, favoring a calmer sea. Islands and reefs can also play an important role in modifying wave behavior, creating shadow zones where the sea is calmer.

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Frequently Asked Questions (FAQ)

1

Why is the sea generally calmer offshore than near the coast?

The depth of the water offshore helps to mitigate the direct impact of the wind, which reduces the agitation of the sea surface.

2

What is the effect of coastal topography on sea agitation near the coast?

The shape of the coasts can create reflections and wave interferences, thereby increasing the agitation of the sea near the coasts.

3

How does the wind influence the formation of waves at sea?

The wind creates ripples on the surface of the water, which can transform into waves due to its strength and duration of action.

4

What is the role of water temperature in the behavior of the sea in response to the wind?

The water temperature can influence the density and viscosity of water, which can modify the transmission of wind energy to the sea surface.

5

How do tides impact sea agitation near the coast?

Tides can change the depth of the water near the coasts, which can amplify or decrease the sea agitation by modifying the initial conditions.

Natural Sciences

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