“Clarifying the ‘dirty side’ of a hurricane”

Winds and Waves with Thunderstorm

When there’s an Atlantic hurricane in the forecast, you see a lot of these diagrams. These graphics show what’s called the Cone of Uncertainty: the range of potential paths the center of a tropical storm could take. It’s often shortened to just the cone, or forecast cone.

It fans out like this because meteorologists can’t know for sure which way the center will go. But these familiar diagrams leave out something that’s super important for mapping how far-reaching a hurricane’s impact area may be.

You don’t wanna be on this side of a hurricane as it starts to make landfall. Atlantic hurricanes, like Milton, Helen, Katrina, ones in the northern hemisphere, rotate counterclockwise. The center of a hurricane is the eye, a relatively calm column of air which scientists still don’t fully understand. Around that is the eye wall.

Moving out from that are the rain bands-swirling winds in moisture that are very strong near the eye wall and weaken as they move to the outer edge of the hurricane.

From above, a hurricane looks pretty symmetrical, but the energy isn’t distributed equally. Meteorologists divide a tropical cyclone into quadrants like this: the right front, left front, left rear, and right rear.

It’s the right side of the storm-and particularly the right front quadrant-that’s often considered the most dangerous. We call this the dirty side. But what actually makes the dirty side so dangerous. That’s because the dirty side of the storm is where the greatest winds, the greatest tornado risk, and crucially, the greatest storm surge and flooding usually occur.

That’s because the winds here are moving in the same direction as the storm, combining their speeds, whereas the winds on the left side are moving in the opposite direction of the storm, which slows them down. You can see the dirty side more clearly in radar images, like this one of Hurricane Katrina.

The track of the storm is key are you on the clean or the dirty side of the storm. Some of the worst damage and threat to human life during hurricanes is from storm surge. When the forward-moving winds of an approaching hurricane push seawater toward the shore, this causes major flooding even before the storm hits.

These forecast storm surge maps from previous hurricanes show time and again that the most significant threat of storm surge is at the center of the storm, in the eyewall, and to its right-the dirty side. Storm surge in coastal areas isn’t the only way the dirty side can bring on huge amounts of rainfall and deadly flooding.

Thankfully, many meteorologists do provide warnings about the dirty side, and some are beginning to add context to Storm tracking graphics include possible impact zones like this one from Hurricane Helen, where you can see that the Impact Zone reaches further to the right than to the left.

As climate change continues to warm our oceans, we’ll continue to see tropical storms build incredible amounts of power in a very short period of time, and the resulting catastrophic flooding and property damage and threats to human life, both along coasts and inland. Knowing which side of a future hurricane you are will become super important pretty fast.