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Secrets of the St. Lawrence

Sailors take warning - The sky above us


- "Hey, Mom, look at that enormous black cloud headed our way! What should we do?

- Quick, help me tie everything down and shorten the sail. It's sure to hit us in a couple of minutes. Don't be afraid!"


A huge, black, anvil cloud is approaching. At its base, a roll cloud is moving very quickly above a choppy sea. Spray is blowing. The wind picks up speed quickly and shifts. Get ready for a good blow.

A squall is a sudden gust of wind that literally sweeps the sea. The wind rages and often thunder, lightning and heavy showers make for a dramatic scene. Squalls are so violent that they can tear the rigging and keel a sailboat over on its side.

If the squall is produced by an isolated thunderstorm moving out from the shore, it should disperse as it moves offshore. It is often possible to avoid these storms by altering course.

This phenomenon, which normally lasts 10 to 20 minutes, may go on for several hours if it accompanies a cold front, but won't necessarily continue with the same intensity as it hit with.

You can often see a cold front approaching - there will be storm cells along much of the horizon. As the cold front passes, the gust of wind will create a cross sea that will make for difficult sailing.

Cross section of an anvil-shaped cumulonimbus cloud. Air ahead of the cloud rises upward creating a roll cloud. Air from behind and directly below the cloud sink into the water. An anvil is starting to form, at the top, in front of the cumulonimbus cloud.

An anvil-shaped cumulonimbus cloud approaching generally means that you are in for a squall.

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