Why tropical cyclones are often more dangerous on the ‘right’ side

If you watch a storm like Hurricane Barry as it moves over land, you might notice notice something odd: the right side of the system is producing heavier rainfall and higher winds:

This is very common among tropical systems, and can be explained by their structure and the way that they move.

Let’s start with defining the sides. The right side of a tropical cyclone is relative to the storm’s forward motion. So if we look at a tropical cyclone moving northward – like Hurricane Barry – the right side of the storm is the eastern side; if we look at a storm moving west – like Hurricane Florence when it made landfall in North Carolina – the right side is the northern side.

The right side of a storm produces higher wind speeds because the forward movement of the storm contributes to the right side’s winds. The counterclockwise flow around a tropical cyclone’s center orients the winds so that the entire storm is moving in the same direction as the winds on the right side. Meanwhile, the winds on the left side of the storm’s center are moving in the opposite direction as the cyclone as a whole, which means they’ll often be weaker than the winds on the right side.

As for rainfall, this often depends on how a storm moves onshore or over a coastline. The region of the storm with the best access to ocean moisture will often produce the heaviest rain. That’s usually the right side, since the right side winds are moving from the ocean onto land, while the left side winds are often moving from land out to sea. Thus, the right side winds help pull more moisture into the storm from the ocean. This can create ‘lopsided’ tropical cyclones, like what we saw on radar during Hurricane Barry’s landfall:

It’s important to note, that all of this only applies to storms in the Northern Hemisphere. In the Southern Hemisphere the winds flow around a cyclone in a clockwise direction, which reverses the entire process. So often times the highest winds are found on the left side of a tropical cyclone in the Southern Hemisphere!

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