結了冰的浪
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海浪是發生在海洋表面上的表面波，亦即沿著水與空氣界面間傳行的一種波動，屬於重力波的一種類型。當風吹起時，風所帶來的壓力及摩擦力對海洋表面的平衡態產生擾動，一些能量自風轉移到水上。水能夠自風得到能量是因為兩者間的摩擦力，使得表面粒子以橢圓式運動移動著，這種橢圓式運動是縱波(往覆運動)與橫波(上下運動)所合成。在後面這個網頁有此種波動的優良展示：
* Prof. Robert Dalrymple Java applet.
As the depth into the ocean increases, the radius of the elliptical motion decreases. At a depth equal to half the wavelength λ, the orbital movement is zero. The speed of the surface wave is also called celerity or phase velocity because it corresponds the speed of the shape of the wave, but is different from the speed of the water particles. This celerity is well approximated by
当一个物体上下颠簸于a ripple, it experiences an elliptical trajectory because ripples are not simple transverse sinusoidal waves.
當一個物體上下顛簸於a ripple, it experiences an elliptical trajectory because ripples are not simple transverse sinusoidal waves.
c=\sqrt{\frac{g \lambda}{2\pi} \tanh \left(\frac{2\pi d}{\lambda}\right)}
where
c = phase speed in m/s;
λ = wavelength in m;
d = water depth in m;
g = gee in m/s2.
In deep water, where d \ge \frac{1}{2}\lambda, s approximates 1.25 \sqrt\lambda. This expression tells us that waves of different wavelengths travel at different speeds: waves disperse. The fastest waves in a storm are the ones with the longest wavelength. As a result, when waves a arrive on the coast from a storm the first ones to arrive are the long swells.
When several wave trains are present, which is always the case in the ocean, the waves form groups. In deep water the groups travel at a group velocity which is half of the phase velocity. Following a single wave in a group one can seen the wave appearing at the back of the group, growing and finally disappearing at the front of the group.
As the water depth d decreases towards the coast, this will have an effect on the speed of the crest and the trough of the wave; the crest moves faster than the trough. This causes surf, a breaking of the waves.
Individual "freak waves" (also "rogue waves", "monster waves" and "king waves") sometimes occur in the ocean, often as high as 30 metres. Such waves are distinct from tides, caused by the moon and sun's pull, and tsunamis that are caused by underwater earthquakes or landslides.
The movement of ocean waves can be captured by wave energy devices. The energy contained in 1 m2 of sea depends on the water density γ and the wave height h (= 2·amplitude (y):
E=\frac{1}{8}\gamma{h}^2=\frac{1}{2}\gamma y^2.
[編輯] 種類
When waves "break", they have different characteristics depending on factors including the structure of the ocean floor. There are three main types that are identified by surfers or surf lifesavers: plunging waves (also known as "dumpers"), spilling waves and surging waves. Their varying characteristics make them more or less suitable for surfing and present different dangers.
