Thursday, March 09, 2006

Southern cyclones and fear

In case you cant tell, this is a map of the southern tip of South Africa and Madagascar. Pressure bars and wind patterns are superimposed. The big orange circular thing on the right is a massive cyclone spinning just SE of Madagascar. A cyclone is essentially the same thing as a hurricane, a huge mass of coulds spinning around a low pressure area. But here in the southern hemisphere, cyclones spin in the opposite direction (clockwise) just like the toilets do. Its called the Coriolis effect

This cyclone is in perfect position to be spraying Durban with lots of swell. The system has two important aspects to wave generation: its very powerful with a low central pressure and there is a long distance between the storm and the shore. Its just starting to be the end of summer and the beginning of winter here in Durban, and the wind and waves are beginning to pick up.

This particular storm is causing pretty big waves in Durban. Nothing like the waves we saw last November at the Goodwave competition, but still powerful and tall. I paddled out yesterday afternoon and was a little frightened. Most sets were medium sized waves, around 4-6 feet. But then a big 8 foot set would come through and, to be honest, it scared me a little bit. I still have a strong respect and pretty healthy fear of the ocean. I looked around me at the handful of other surfers around, and decided I was in over my head. I fought the current for a while, and finally caught a smallish wave all the way to the beach.

Its always hard to tell if you are playing it smart, or just being a big pansy. Its such a fine line in surfing and in life. The balance of caution and hazard is something we deal with everyday. In this case, my perhaps exagerated fear of the powerful sea was too overwhelming and I decided to play it safe. Whether it was the right decision or not, I'll never know. Such is life.


At 3/09/2006 10:33:00 PM, Anonymous Anonymous said...

Cully -

As a (former) oceanographer by training and trade, I feel it is my duty to do everything in my power to dispel a common myth/misconception ...

It is true that the Coriolis Force causes wind (and ocean) currents to deflect (ie Coriolis Effect), however in the northern hemisphere they deflect to the right (clockwise) and to the left (counterclockwise) in the southern hemisphere.

Why then do hurricanes spin in the opposite direction from the moving air mass (counterclockwise vs clockwise in the northern hemisphere)? In the northern hemisphere, as the high-pressure air mass curves to the right, it drags the air flowing around a low pressure system in the center of the hurricane with it ... causing it to spin counterclockwise.

It also happens to "pile" water masses in the middle of the circle thus making the ocean surface not flat, but angled with the Western edge higher than the eastern edge (caused by the equatorial current) and pock-marked with water domes (caused by the Coriolis Force).

THE CORIOLIS EFFECT IS TOO WEAK TO AFFECT TOILET BOWLS, TUBS, AND SINKS. You can find water swirling down the drain in either direction in either hemisphere. The direction water spins in these basins is determined by micro-vortices within the water and imperfections in the surface of the basins themselves.

A quick web search, much to my dismay, had a huge amount of hits perpetuating this myth (thus proving that any person can write anything on the web, even when they are ignorant of the facts ... if only I could convince my patients of this truth). However, if you go to a reputable oceanography text or site and search the myth, it should be dispelled.

Here are some interesting links ...

Hope I can bring enlightenment to even a few ...


At 3/09/2006 10:40:00 PM, Anonymous Anonymous said...

One other thing ... those tourist traps at the equator that "prove" the effect by showing water spinning in opposite directions on either side of the line are a huge hoax ... the Coriolis Effect is essentially ZERO at the equator.



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