|The 48-foot fishing vessel Sea Hunter is hidden in the trough of a 15 - 18 foot|
wave in the Gulf of Alaska. Official Coast Guard photo courtesy of Coast
Guard Cutter Hickory.
Guest blog by Vincent Pica
Chief of Staff, First District, Southern Region (D1SR)
United States Coast Guard Auxiliary
When we first put together the column on waves (Wave Theory – and Practice), I noted that we’d get back to waves and talk about different kinds of waves - tsunamis, deep, shallow, non-wind, etc – and the effect they have on mariners. This column completes that promise.
Wind Creates Waves
The frequency of the tidal wave (not to be confused with a tsunami, often called a tidal wave) created by the rotation of the Moon around the Earth is about 12 ½ hours. The wavelength is half of the circumference of the Earth itself or roughly 12,451 miles. Looking at exhibit #1, you can easily the representation of the wavelength.
Wind DOES Create Waves
Jumping ahead since we’ve laid the ground work with the prior column, take it as a given that the wind does create waves. And, on a windy but pleasant day, you can see the white caps marching in lock-step down the bays and in the offing to the ocean’s horizon. But many mariners have experienced “confused seas” where the waves seem to coming from everywhere and typically these are foul weather seas. What it going on? The wind seems to be coming from one direction – but the waves aren’t. Why? This is due to the geometry of storms. Most storms are low pressure systems, with cyclonic winds orbiting the storm center. The cyclonic wind field creates waves moving in every direction. These newly formed waves tend to be steeper and more severe and have shorter periods than waves outside of a generation area. After wind waves form, their growth is limited by the duration of energy input. Energy input is determined by the velocity, duration, and fetch of the wind field. Waves reach an equilibrium state (the "fully developed sea") if the all three of these variables remain constant for sufficient duration. For higher velocities, these conditions are rarely realized, because the waves tend to run out of the area with high winds. Even in a hurricane 300 miles across, a typical large wave will move out of the source area within half a day, far too little time to allow development of a “full sea.”
So, there you have it. Wind creates waves – except for the longest wave, the tides, and the most destructive, the tsunami…
BTW, if you are interested in being part of USCG Forces, email me at JoinUSCGAux@aol.com or go direct to the D1SR Human Resources department, who are in charge of new members matters, at DSO-HR and we will help you "get in this thing…"