2004-2005
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Ocean Planet Vendée Globe Update:
Tuesday, November 23, 2004
Position 12 51S, 26 21N, @ 10:00 UT
Hurricane Formation
The infamous Doldrums are a couple of days behind us now, but the experience is still fresh in my mind. The large, angry thunderstorms, torrential downpours, and the humidity are easy to remember.
But what also lingers is the somewhat ominous atmosphere that I felt not only from the local storms, but from the knowledge that this area is also the primary breeding ground for the north Atlantic hurricanes. It is indeed a boiling cauldron of rising, evaporating water that is waiting for the right conditions for a larger storm to begin taking shape.
I have never been in a hurricane, and I don’t intend to. Having experienced winds of a max of about 60 kts on the water and perhaps a little more on land, I can only imagine the fury and waves formed at sea once full hurricane force (greater than 74kts) is reached. I have sailed through the “eye” of tropical storms which is an eerie experience, but just reading about passing through the eye of a hurricane is enough for me!
Several factors need to be in place for a tropical storm to form and then develop into a hurricane. From what little I know, plenty of warm, wet air at the surface is the starting ingredient. Air with a lot of evaporated moisture in it is actually lighter than dryer air, so this is one reason you see clouds and thunderstorms billowing up. As the air moves up, it can create lower pressure on the surface which pulls in yet more warm wet air.
What usually happens in a big thunderstorm is that the rising air sort of “tops out” and then much of the water in the air condenses (it gets cooler as it rises so the air can no longer “hold” all the water and droplets form) and begins falling. If the rising air can not escape out the top, then the lower pressure at the surface begins to “fill” and the process slows down. Eventually the thunderstorm will “rain itself out”, dissipating its energy with powerful downdrafts and torrential rain. Often this energy will reform in another nearby thunderstorm and the process will repeat itself, pummelling the poor sailors below but sparing them a true tropical storm. This is what I and the other Vendee racers had to deal with in the doldrums a few days ago. We could see the towering clouds and the darkness underneath, spelling potential trouble if you couldn’t get away.
But, while all this is going on, if in the upper atmosphere something called “divergence” happens to occur, then a new process starts up. Divergence is when more air is moving away from an area then is coming in, creating lower pressure in the area from whence the air is departing. So, if our boiling area of rising air is able to escape….then the low pressure at the surface formed by the rising air “deepens” and more wet air rushes in to fill the void. So, if there is enough divergence in the upper atmosphere then a kind of huge air/water vacuum cleaner begins to form and suck in more and more wet air below. In a hurricane this occurs on a massive scale.
How does it stop? Since the fuel for the hurricane is the warm wet air, once they move over land they start to die out. Or, if the low pressure above the storm caused by divergence above begins to fill, then effectively that will slow it down too. But what often will be left is gobs and gobs of rain from all the moisture sucked into the whole thing so even after the winds die there can be floods and associated damage.
In any case, I’m sailing away from the doldrums as fast as I can! Only it is off to the deep south where a whole other kind of huge weather pattern will govern our fates…..
Bruce
To hear more of Bruce’s first hand experience in the Vendee click here for streaming audio interviews from Ocean Planet.
Bruce Schwab, Skipper
USA 05/Ocean Planet