As anyone knows, it's easy to make waves. But a lot more than sloshing in the tub goes into the making of the sort of 100-plus-foot (30.5 meter) wave that surfer Garrett McNamara rode on Jan. 28, in Nazaré, Portugal.

For one thing, bathtub waves are essentially micro-tsunamis – very different beasts than the wind waves surfers ride. Tsunamis are created by a pulse of energy, like a sudden rupture in the Earth's crust, a landslide, glacier calving or even a large meteor impact. Their size depends on the size of the pulse. The sky is literally the limit.

Wind waves, on the other hand, are grown. They can be seen forming on any pond when there is a breeze. Add more wind, time and a longer expanse of water, called fetch, and you get larger waves, explained Hendrik Tolman chief of Marine Modeling and Analysis for the National Atmospheric and Oceanic Administration (NOAA). Tolman is also the man who leads the development of the WAVEWATCH III™ forecasting model, which is the same model used by commercial surf forecasters like Surfline.

“Theoretically, with hurricane force winds you could get waves much, much higher” than 100 feet, said Tolman. The largest ocean wind waves recorded are between 120 and 130 feet, and have have been detected by automated buoys inside monster storms. But in that environment, waves are all mixed up and a mess – impossible to ride on a surfboard. Instead, the big surfing waves are found hundreds of miles from the storm where waves have had a chance to get sorted out into neat swells, Hendrik said.

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Surprisingly, the best storms for generating these large waves are not necessarily hurricanes. Despite their powerful winds, hurricanes tend to be compact, and so they blow across smaller patches of ocean. It's the broader winter storms, like that which generated the waves in Portugal, that are the giant makers, said Hendrik. The reason is that they blow over larger fetches and blow longer.

“For any constant wind speed, wave growth is limited by the time that this constant wind blows over a given area,” said Scott Stripling a wave and wind forecaster for NOAA in Miami. “Longer fetch lengths yield larger waves.”

That said, wave forecasters have discovered that even some lower-power hurricanes, like Sandy, can make very large swells if they develop something called “trapped fetch waves” that last for more than 18 hours.

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“When Atlantic hurricanes make the turn and go up the East coast of the US, they travel in a straight line,” explained Stripling. On the right side of such storms the winds are blowing north, in the same direction the storm is moving. If the storm's speed syncs with the waves it's generating and they are all traveling in the same direction, the winds just keep growing the same waves bigger and bigger as the storm moves up the coast.

“It was the Canadians who noticed it first, when they were being pounded by incredibly big waves that were way larger than expected,” said Stripling.

The final part of the recipe for giant surfing waves is the place where the waves break. The geometry of the sea bottom and coast can focus the waves, making them taller as well as shaping how they break. This is why there are certain places famous for their giant waves – when the swells are there. Places like Nazaré, Mavericks and Cortes banks in California, Jaws in Maui, and many other big wave sites worldwide are watched religiously by surfers and Surfline, so that the world's big wave surfers, like Garret McNamara, can hop on planes and arrive in time to catch the largest waves, which may only last a few hours.