Education

Winds, Weather, and Currents of the Pacific

Quotations are from William G. Van Dorn's Oceanography and Seamanship , 2nd Edition (Centreville, Maryland: Cornell Maritime Press, 1993). Click Here for Map.

NE Trade Wind Belt (25 degrees N to 9 degrees N)

The ENE trade winds are produced by air circulating clockwise around an area of high pressure centered northeast of Hawai'i. During the summer (Leg 1), the trade winds prevail about 90% of the time; during the winter (Leg 5), they blow 40-60% of the time and are more easterly and lighter, though episodes of strong, gusty trade winds are somewhat more frequent than during the summer months. Squalls in the trade wind flow may carry brief bursts of winds up to 40 knots. In winter, Kona storms bring southerly winds and rain. Winter and spring cold fronts from storms in the North Pacific bring southwesterly winds and rain, followed by cool, dry northerly winds.

Wind Conditions: NE Trades, generally from 'Aina Ko'olau (ENE); 10 - 20 knots

Current: North Equatorial Current; west flowing; 0.5 knots; 12 miles per day (0.5 knots x 24 hours)

Canoe Performance: 5 knots; 120 miles per day (5 knots x 24 hours)

Intertropical Convergence Zone

(Varies Between 10 degrees N to 0 degrees)

Intertropical Convergence Zone (ITCZ) shifts between 10 degrees N and the Equator, on average between 9 degrees N and 3 degrees N on the route between Hawai'i and Tahiti. Converging winds from the Northeast and Southeast Trade Wind Belts and warm air rising from equatorial waters produce doldrum conditions: variable winds, calms, thunderstorm activity, and dense cloud cover. The zone is charcterized by an "impressive wall of clouds"; "confused state of the swell, flukey winds that blow intensely and then subside, and intermittent showers of rain that come from nowhere in a solid, opaque overcast." The light, shifting winds and the confused swells and cloud cover make sailing slow and navigation difficult. North. Sometimes, however, cloudless skies and easterly winds prevail across the zone. In the 1995, the Hokule'a and Hawai'iloa sailed down to Tahiti without encountering doldrum conditions.

From June to October, hurricanes develop between 5 degrees-20 degrees N, from the waters off Mexico to about 140C W, and generally drift westward before curving north.

Wind Conditions: Variable; generally out of the East; 0 to 10 knots

Current: Equatorial Countercurrent; east flowing, but unpredictable

Canoe Performance: 2.5 knots; 60 miles per day (2.5 x 24 hours)

SE Trade Wind Belt (0 degrees to 25 degrees S)

Southeast trade winds: "generally stronger, steadier, and cover a much wider zone of latitudes" than the Northeast trade winds. The ESE trade winds are produced by air circulating counterclockwise around an area of high pressure centered around 30C S and stretching westward off the coast of South America. During the southern hemisphere summer and fall (December-April /Leg 5), hurricanes, though infrequent, may form around Tahiti.

Wind Conditions: Southeast trades, generally from Hikina (E), La Malanai (E by S) or 'Aina Malanai (ESE); 10 to 20 knots.

Current: South Equatorial Current; west-flowing; 0.5 knots; 12 miles per day (0.5 knots x 24 hours)

Canoe Performance: 5 knots; 120 miles per day (5 knots x 24 hours)

Southern Subtropical Divergence Zone

(25 degrees S to 30 degrees S)

This area of high pressure, which produces the Southeast Trade winds, has light, variable winds and mainly clear skies. In September and October, around Rapa Nui, the wind are easterly (from NE to SE) 60% of the time, averaging 12 knots. "...balmy weather and light winds; although February there is a 5 percent probability of gale winds, usually from the northwest."

Wind Data for Sept./Zone 20 (E of Heragi/Pitcairn Island)

Wind Data for Sept./Zone 21 (around Rapa Nui)

Wind Data for Oct./Zone 20 (E of Heragi/Pitcairn Island)

Wind Data for Oct./Zone 21 (around Rapa Nui)

Cold fronts generated by low pressure systems to the south during the Southern Hemisphere Winter (May through September) bring westerly winds.

Currents in this zone are weak and variable, generally east-flowing at 0.1 to 0.5 knots.

Southern Hemisphere Westerlies (35 degrees S to 50 degrees S)

"...the [westerly] winds blow unhindered around the world [except at Cape Horn at the tip of South America], with remarkably little variation in average weather throughout the year"; "light breezes to fresh gales, from days of bright sun to days of lowering, squally weather, with intermittent rain, with air temperature generally in the 50s and 60s."

Hurricanes and Cold Fronts

Hurricanes: Hurricanes (tropical cyclones whose wind speed exceed 64 knots) form in the warm waters of the equator and are steered away from it by surface winds. Chances of surviving a hurricane on a voyaging canoe are minimal, so they must be avoided. The hurricane season in the Northern Hemisphere is May-November with the peak months August-September. The leg from Hawai'i to the Marquesas (June-July) is scheduled to take place before the peak months arrive. The hurricane season in the Southern Hemisphere is November-May with the peak months January-March. The leg from Tahiti to Hawai'i in December is scheduled to take place before the peak months arrive. Should a hurricane threaten any part of the route, the voyage will be postponed. Once the canoe reaches the area of the equator, hurricanes are no longer a threat, as the Coriolis effect doesn't exists there. (Due to the Coriolis effect, rising air around low pressure systems circulates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. When the air circulates fast enough (over 64 knots), the gyre is called a hurricane.

Cold Fronts: Cold fronts accompany winter and early spring storms, usually at latitudes above 30 degrees. But these fronts may extend below 30 degrees into the trade wind zone (below 25 degrees latitude) in both hemispheres and break down the trades, bringing westerly winds. While the weather associated with these westerly winds is stormy, Hokule'a can use these westerly winds to speed it on its way to Rapa Nui.