<P> Because these effects are related to a spatial variation in the phase speed, and because the phase speed also changes with the ambient current--due to the Doppler shift--the same effects of refraction and altering wave height also occur due to current variations . In the case of meeting an adverse current the wave steepens, i.e. its wave height increases while the wave length decreases, similar to the shoaling when the water depth decreases . </P> <P> Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse . A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces . When the slope, or steepness ratio, of a wave is too great, breaking is inevitable . </P> <P> Individual waves in deep water break when the wave steepness--the ratio of the wave height H to the wavelength λ--exceeds about 0.17, so for H> 0.17 λ . In shallow water, with the water depth small compared to the wavelength, the individual waves break when their wave height H is larger than 0.8 times the water depth h, that is H> 0.8 h . Waves can also break if the wind grows strong enough to blow the crest off the base of the wave . </P> <P> In shallow water the base of the wave is decelerated by drag on the seabed . As a result, the upper parts will propagate at a higher velocity than the base and the leading face of the crest will become steeper and the trailing face flatter . This may be exaggerated to the extent that the leading face forms a barrel profile, with the crest falling forward and down as it extends over the air ahead of the wave . </P>

The ratio of the wave height to its wave length is called