Climatologically, this is the coldest time of the year in the Northern Hemisphere. So, it's the right time of year to think about being some place warm, and a sunny beach wouldn't hurt. If you're into surfing, a place that has nice sand, a palm tree or two, a sultry breeze, and reliable ocean swells sounds especially inviting.

In general, the windier regions and consequently the heaviest seas, are found in the zone of the trade winds and the westerlies. Of course, the biggest waves result form the most powerful storms, but also, the greater the fetch (the distance of open water over which the wind is blowing) the higher the waves are likely to be. Southern California, South Africa, and the southeastern coast of Australia all have some wonderful beaches and outstanding surf. However, just like many mountaineers look forward to some day climbing the highest peaks in the Himalayas, most surfers set their sights on Hawaii's big waves.

If you're a surfer dude, Hawaii has about everything you need, and in particular, big waves you can usually count on. The three main factors that determine the size of waves are wind speed, fetch and the duration of the wind. Since Hawaii sits pretty much in the middle of the Pacific Ocean, it has a tremendous fetch, regardless from which direction the wind is blowing.

Hawaii lies in the zone of the trade winds. Winds here typically blow from the southeast. While hurricanes (not typhoons) may on rare occasion visit the Hawaiian Islands, extratropical storms may be even more rare. However, you don't need to be in a storm to feel its affects. Because of the long fetch, a storm may miss Hawaii by thousands of miles and still generate potent waves on the island's distant shores. That's what happened in late January of 1998. A powerful storm that formed near southern Japan and moved east across the Pacific with the prevailing westerly winds, missed the Hawaiian Islands by about 3,000 km (1,900 miles). This was regarded as one of the two strongest storms (based on wave height) to effect the island of Oahu in the past 100 years.

The beaches on the north shore of Oahu have the reputation as the world's best for big wave surfing. Not only are the waves reliable, but the configuration and geometry of the shore guarantees that the waves that come ashore there will be bigger in comparison to most sandy beaches. The size and shape of the waves are partially determined by the shape of the bottom of the shore. However, for any wave that enters shallow water, its speed and wavelength will decrease, and its height will increases. All waves, whether they're ocean swells or tsunamis, will break when the ratio of wave height to wavelength (wave steepness) exceeds 1/7. The submerged shoals off the north shore of Oahu are sufficiently high and far enough from the shore so that waves approaching the beach rise up steeply, allowing surfers to enjoy a long, fast ride.

During the winter of 1998, El Nino conditions existed in the Pacific Ocean, and frequent storms moved across the northern Pacific, eventually pounding the US and Canadian coasts. One of these was the storm that produced monster waves on the north shore of Oahu on the 28th of January. This storm generated swells (long period waves) that were 9 m (28 feet) high in the open water between Japan and Hawaii. When these waves reached Waimea Bay on Oahu's north shore, they rose to gigantic 13 m (40 foot) behemoths! The wave faces were in some cases 26 m (80 feet) - the slope distance between the crown and the wave trough. The "tubes" or tunnels created by such huge plunging breakers could have swallowed a bus. Interestingly, when the waves from this storm finally reached Hawaii, the skies were clear and the winds were nearly calm.

January is the time of year when the biggest waves are most often seen on Oahu. Big waves can come in at any time of the year, but since extratropical storms are most prevalent during the winter, this is when the biggest waves are most often observed, and it's also the time when a lot of the big surfing competitions are held. On 28 January 1998, the waves were so big that only a handful of the world's best surfers would attempt to ride them. Just off shore, the swells were nearly the width of a football field.

When you're waiting for a wave, the idea is to match its speed by paddling at the same speed. Big waves move faster than slower ones, and these giants were moving so fast that the surfers couldn't possibly paddle fast enough to get past the point where the big waves were breaking. The few who managed to do so had to be towed out past the breakers with jet skis. To say the least, surfing in these conditions is extremely dangerous. Since a cubic meter of water weights about 1,000 kg (2,205 lbs), the tons of weight and terrific force of each wave could have crushed the surfers had they wiped out. Surfers also had to be wary of unmanned surf boards, which in those waves, were like out of control missiles. Fortunately, no one was seriously hurt. An IMAX film crew was on-hand for the surfing competition, and with the aid of a helicopter hovering just above the sea, they were able to capture some incredible footage. At times, the helicopter was actually filming in the trough of the huge waves.

For more about this see the Public Television Show "Nature" http://www.pbs.org/wnet/nature/conditionblack/
and the Earth Science Picture of the Day for Friday January 25
(http://epod.usra.edu/)