A whitecap is the white, foamy portion formed upon the crest of a wave. They are commonly observed by ocean beach goers; as waves approach shore they begin to crest and break, forming whitecaps. Other, more dangerous oppertunities to observe whitecaps in the wild is during a powerful weather event.
When the wind whips up the tops of waves faster than the wave energy can travel through the water, they break at sea-- causing whitecaps-- and dangerous sailing!
When the height of a wave meets or exceeds the length of the wave divided by seven, the wave will form a whitecap and break. This can be expressed by the formula:
H = L/7; If H > 1, the wave will break.