Marine Glossary

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dangerous semicircle

Also called dangerous side.

That half of a cyclonic storm area in which the rotary and progressive motions of the storm reinforce each other and the winds are in such a direction as to tend to blow a vessel into the storm track. In the Northern Hemisphere this is to the right of the storm center and in the Southern Hemisphere it is to the left. The opposite is the navigable semicircle.

Often it is shifts in the wind direction of the approaching Low that tells us the relative course of the storm and which side would be the better choice if we have a choice. Here a key element is that on the dangerous side the building wind is in the direction toward pushing us back into the path of the storm, as shown in G171 of the Starpath Weather Trainer.

Finally there is the all important state of the sea. Winds on the dangerous side are stronger, so they make bigger waves. Waves are also bigger on this side because those made in the aft dangerous quarter are the least affected by the changing wind direction as they move through the front part of the storm–the waves generally move faster than the storm. Waves on the navigable side, though still very serious, are actually diminished by this effect.

The dangerous-side waves can be especially enhanced for fast storms (>15 kts) traveling in a straight line for half a day or more. This can lead to what is called a trapped fetch, meaning the waves from the dangerous stern quarter travel at about the speed of the storm, and riding along with it experience effectively very large fetch and duration. Waves built by tropical storms and hurricanes reach phenomenal heights, so any maneuver to avoid the worst of them by staying on the navigable side is crucial to the best handling of the situation.

ART-1 discusses the sides of a storm in detail. See also G103.