Roaring Forties

The "Roaring Forties" is a parallel of latitude between 40 and 50 degrees south around the world where the most intense storms in the Southern Hemisphere take place. Cold air sinks relative to warm air, creating a constant high pressure area over Antartica. As air is forced from the area it heats up and starts to rise. As the air (which has become wind) rises it starts spiralling in a clockwise fashion. The spiralling wind mixes with relatively moist warm air from warmer climates, continuing to rise as it warms, creating a low pressure cyclonic system and PRESTO! a storm is born. The strength of the storms depends on many factors including the relative humidity of the warmer air, the strength of high pressure systems neighboring the low pressure, whether the storm is over land or water and many elements of chaos and unpredictability.

Fetches, Sea and Swell

The many different terms associated with surfing can be confused if not defined properly. Continuing from the last section, as storms spin they don't always spin in perfect circular patterns. In reality the storms assume more oval shaped patterns as they are pressed from behind and in front by high pressure systems or land masses and obstructions. Because the shape is oval, the wind travels in one direction longer than it does in other directions. The length of area where the wind travels in one direction is termed a FETCH. Fetches are responsible for creating SWELL. As the wind passes over the surface of the ocean, the friction creates waves that travel in the direction of the wind. As long as the waves created by the wind are still in the area whereby the wind still affects the waves they are termed SEA.

In this isobaric map you can see the isobars are very tightly packed together and all headed in one direction. This FETCH created a large SWELL for south and south-west Australia. If the isobars and FETCH were angled more towards Indonesia, the Mentawai Islands would have received a large SWELL as well.

Mentawai Swell

The Mentawai Islands are located over 2400-3000 nautical miles away from the roaring forties. Continuing from the last section, as the SEA leaves the area of wind creating it, the waves start organizing into paths of least resistance and begin to form sets of SWELL. As the waves move farther and farther away from the storm that created them, they lose all the SEA element in the waves and keep only the SWELL. After traveling the entire length of the Indian Ocean, only the swell remains. Breaking on coral reefs, these waves have no bumps or sea and create perfect waves for surfers to ride.