Read about Hurricane Categories in “Some hurricane facts” in your textbook. Hurricane Camille was a Category 5 hurricane when it hit Mississippi on August 17, 1969. Camille’s 190 mph sustained winds at landfall were the highest winds ever recorded for a hurricane hitting the US. On August 29, 2005, Hurricane Katrina hit the same region of the Mississippi coast, making landfall as a Category 3 hurricane with 130 mph sustained winds. Camille’s eye had an 11 mi diameter eye, and the radius of maximum sustained winds reached approximately 15 miles. Katrina, though, had a larger 37-mile diameter eye, and hurricane-force winds extended out 120 miles to the east of the center. Katrina’s radius of maximum winds was about 30 miles. Camille drove a record storm surge of 22.6 feet to Pass Christian, Mississippi, while Katrina’s storm surge exceeded Camille’s at all locations, topping out at 27.8 feet at Pass Christian (Fritz et al., 2008). With the given information and assuming the flow outside of the hurricane’s eye is approximated as a free vortex, perform a simple calculation indicating how a Category 3 hurricane’s storm surge could exceed that of a Category 5. Also discuss wind speeds, pressures, densities, and temperatures at the eye of hurricanes.
The Correct Answer and Explanation is:
Simplified Calculation: Free Vortex Model
In a free vortex, tangential wind speed VV is inversely proportional to the distance rr from the center: V∝1rV \propto \frac{1}{r}
Given:
- Camille’s radius of maximum winds: rC=15r_C = 15 mi, wind speed VC=190V_C = 190 mph
- Katrina’s radius of maximum winds: rK=30r_K = 30 mi
Using the free vortex model: VK=VC⋅rCrK=190⋅1530=95 mphV_K = V_C \cdot \frac{r_C}{r_K} = 190 \cdot \frac{15}{30} = 95 \text{ mph}
However, Katrina’s recorded wind speed was 130 mph, not 95 mph. This suggests Katrina had a more complex wind field not perfectly modeled by a free vortex, but the model does highlight that larger hurricanes can exert significant forces over a wider area.
Why Katrina’s Surge Was Higher
Despite Camille’s higher peak wind speed:
- Katrina had a larger eye (37 mi vs. 11 mi)
- Katrina had hurricane-force winds extending 120 mi from the center, much farther than Camille
- Katrina’s radius of maximum winds was twice as large, leading to greater water displacement over a larger area
Storm surge depends heavily on wind field size, storm size, duration, bathymetry, and coastal shape—not just peak wind speed. Katrina’s expansive wind field and longer duration over water enabled it to pile up more water over time.
Atmospheric Conditions at the Eye
- Wind Speed: Drops sharply to near zero at the eye’s center
- Pressure: Lowest in the entire storm system, typically 870–950 mb
- Temperature: Warmer in the eye due to sinking air and latent heat release
- Density: Lower due to high temperature and low pressure
In summary, storm size and wind field extent—not just category—are crucial in determining storm surge. Katrina’s broad, sustained winds caused more extensive ocean displacement, leading to a higher surge despite lower peak winds.
