Rogue waves are open-water phenomena, in which winds, currents, nonlinear phenomena such as solitons, and other circumstances cause a wave to briefly form that is far larger than the "average" large wave (the significant wave height or "SWH") of that time and place. The basic underlying physics that makes phenomena such as rogue waves possible is that different waves can travel at different speeds, so they can "pile up" in certain circumstances, known as " constructive interference ". (In deep ocean, the speed of a gravity wave is proportional to the square root of its wavelength, the peak-to-peak distance between adjacent waves.) However, other situations can also give rise to rogue waves, particularly situations where nonlinear effects or instability effects can cause energy to move between waves and be concentrated in one or very few extremely large waves before returning to "normal" conditions.