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post #142 of Old 02-03-2012
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Originally Posted by Southcoasting View Post
I just learned about this type of cloud at my Coast Guard Auxiliary Boating Safety Class...We were taught these are basically a sign that a thunderstorm or cold front is coming with a squall line that can turn a 5 knot wind to 60 knots in no time...

So basically, a huge risk to take if you decide to sail if you see one of these...Or if you see one while sailing, the idea is to sail away from it and run for cover...
Yeah man, in the States we call that a Roll cloud. It marks the gust front of a severe thunderstorm - the leading edge of the forward-flank-downdraft of cold, dry outflow air in the front of a thunderstorm. This is that first cool dry gust you feel when a normal thunderstorm approaches. In a big'n, it's powerful. Cold dry air being more dense, it "shovels" warm moist air up into the storm. That air, in addition to feeding the storm, condenses when it gets lifted and turns into that ominous line. In a microburst under there, straight line winds can exceed 100mph. I've been chasing storms on the Plains for 15+ years and I've never seen one that perfect. That is AWESOME. Unless you're stuck under it in a boat I guess.

These pics are all absolutely incredible. Thanks everyone for posting! I already plan to make it as far north as the San Juans this summer... looks like next summer I may have to shoot for Desolation!

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