Friday, November 26, 2010

That’s the Way the Wind Blows

We were accustomed on the mainland to the prevailing wind coming from the West, and tailwinds making the travel from say Denver to New York generally much faster than the return trip.  When we moved to the east side of the Big Island I assumed that we would be on the leeward (“away from the wind”) side of the island.  But no, in the middle of the Pacific the prevailing winds turn out to be from the east, putting us on the windward side after all.
It so happens that solar radiation warms the air over the equator, causing it to rise. The rising air then proceeds south and north toward the poles. From approximately 20° to 30° North and South latitude, the air sinks. Then, the air flows along the surface of the earth back toward the equator.  This phenomenon creates several effects well known to pilots of ocean sailing vessels.

The Doldrums
Sailors noticed the stillness of the rising (and not blowing) air near the equator and gave the region the name "doldrums." The doldrums, usually located between 5° north and 5° south of the equator, are also known as the Intertropical Convergence Zone or ITCZ for short. The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world's heaviest precipitation regions.

The Horse Latitudes
Between about 30° to 35° north and 30° to 35° south of the equator lays the region known as the horse latitudes or the subtropical high. This region of subsiding dry air and high pressure results in weak winds. Tradition states that sailors gave the region of the subtropical high the name "horse latitudes" because ships relying on wind power stalled; fearful of running out of food and water, sailors threw their horses and cattle overboard to save on provisions.
The Trade Winds
In the central North Pacific, the trade winds represent the outflow of air from a great region of high pressure, known as the North Pacific High, typically located well north and east of the Hawaiian Islands. The North Pacific High is a semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter.  

Blowing from the subtropical highs (or horse latitudes) toward the low pressure of the ITCZ are the trade winds. Named from their ability to quickly propel trading ships across the ocean, the trade winds between about 30° latitude and the equator are steady and blow about 11 to 13 miles per hour. In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.  http:/
Hilo Hawaii is located roughly at 19.7 degrees north of the equator (see my earlier blog Changes in Latitudes).  As described above the trade winds at this latitude blow westward from the northeast thus putting the east side of the Big Island facing the prevailing wind known as the Northeast Trade Wind.  Whereas, in the mid-latitudes where most of the United States mainland is located the “westerlies” blow eastward. In fact in both the northern (30N to 60N) and southern (30S to 60S) latitudes, the prevailing winds are from the west. wikipedia

Confusion resolved!

1 comment:

  1. I love the name horse latitudes and think of horse's tails as the froth of the waves blown backwards like horse tails in the winds. Sad that it means something else, something so sad! Thanks for this wind expose. aloha!


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