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The question was inspired by an answer to this question on History SE, which referred to the "at times enormous waves" at the Cape of Good Hope in South Africa, and the even more "treacherous" passage around Cape Horn.

Why is the sea so hard to navigate at the southern extremities of the land masses.?Cape Horn, with a latitude in the high 50's, is easy to understand, but the Cape of Good Hope is in the 30s. Yet from what I understand, "navigability" does not extend far south of the Cape of Good Hope.

Is southern Australia and Tasmania (in the 40s latitudes) similarly hard to navigate? Would it be fore the same reasons?

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  • $\begingroup$ The examples you mention are in the southern hemisphere. I'm wondering how the comment applies to southern Asian - from the Arabian peninsular to southern Vietnam, which is in the northern hemisphere & relatively close to the equator? Then there's the southern coast of the USA between Florida & the Rio Grande (border with Mexico). Southern Europe can be ignored because the Mediterranean Sea is effectively a large lake between southern Europe, north Africa & western Asia. $\endgroup$ – Fred Apr 6 at 13:42
  • $\begingroup$ @Fred: I would say that the Gulf of Mexico is effectively a lake between North and South America. I believe that part of the issues is that South America and Africa "juts" into the southern seas, whereas South Asia is relatively "smooth" In latitude) for a long distance. Also, South Asia is near the equator, while South America is closer to the South Pole than the equator. I was wondering if South Africa is an "intermediate" case. $\endgroup$ – Tom Au Apr 6 at 22:29
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Fetch. The longer the wind has to act on the water the larger and more developed the waves become, see this site from the NOAA. If you look at the southern portion of our planet you will note that there are few land masses to affect the wind or the wave development. In the Wikipedia entry about prevailing winds you can see that the Southern Westerly winds are only affected by the Cape of Good Hope, Cape Horn, and to a lesser extent Australia. Contrast this to the Northern hemisphere where North America, Europe and Asia divide the seas into smaller isolated sections.

There are additional factors involved particularly around the capes. If you look at the prevailing ocean currents, see global map on this site you will see there are strong currents, driven by the wind, around the southern pole region. At the capes, these currents become more chaotic and lead to eddies. Sometimes the eddies make the current flow against the wind. This results in even larger and less predictable sea states.

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  • $\begingroup$ Further to my discussion above, I searched for a reference to the fetch needed to develop full sea state and couldn't find anything. I did come across this site, woodshole.er.usgs.gov/project-pages/coastal_model/Tools/…, from the US Geological Survey which allows you to enter wind speed and fetch to calculate wave height. For 30 knot winds here are the wave heights: 1nm / 0.452m, 10nm / 1.4m, 100nm / 4.3m, 1000nm / 10.058m, 2500nm/11.6m, 10000nm / 12.2. After 2500nm it doesn't increase as rapidly. $\endgroup$ – Friddy Apr 5 at 22:01
  • $\begingroup$ To add to this, the prevailing winds in the southern hemisphere have the names: Roaring Forties (latitudes 40-50), Furious Fifties (lat 50-60) & Shrieking or Screaming Sixties (lat 60-70) $\endgroup$ – Fred Apr 7 at 4:04

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