Part D: Comparing North and South
Up to this point, you've been dealing with hurricanes that occurred in the Atlantic Ocean north of the equator. Now, you'll examine some visualizations of cyclones that occurred south of the equator. You'll be asked to draw conclusions about how these storms differ from those that North Americans are familiar with.
Step 1: Severe Tropical Cyclone George
Severe Tropical Cyclone George came ashore near Port Hedland on the northwest coast of Australia on March 8, 2007. The storm had average winds of about 195 km/h, gusting to 275 km/h.
In the South Pacific basin, they use the Australian Cyclone Scale to indicate storm intensities. Developed by the Australian Bureau of Meteorology, this scale places primary emphasis on the strength of individual wind gusts rather than the 10-minute sustained winds that the Saffir-Simpson scale uses. On the Australian scale, George was considered a borderline Category 4/Category 5 storm.
Click the image to see the animation. Check the color code to interpret what's happening in the storm.
Checking InAnswer the following questions to check your understanding of the information provided above.
- Given George's Australian Cyclone rating, how would this storm rank on the Saffir-Simpson Scale?
George would be a Category 3 hurricane on the Saffir-Simpson Scale.
- Based on the way the rain bands move across the view in the animation, which direction are winds spiraling into the center of the hurricane - clockwise or counterclockwise?
The winds are spiraling clockwise in towards the center.
The term "storm surge" refers to water that is pushed up to higher-than-normal levels at the shore by the winds of a storm. The spiraling nature of winds in cyclones result in winds on one side of the storm blowing on-shore (towards the land) in the same direction as the storm's motion. In the Northern Hemisphere, the storm surge occurs on the right side of the eye (using the direction of motion as facing forward) because that is where the winds and storm motion reinforce each other. On the other side, where winds are blowing offshore (away from the land) there is no storm surge and in fact most times the water level is lower than normal. If the storm surge occurs at high tide, then the coast experiences a worst case situation.
Stop and Think
1: Which side of a Southern Hemisphere cyclone will experience the higher storm surge? Draw a diagram of a hypothetical storm to illustrate how you came to your conclusion. Label the relevant parts of the storm as well as the wind and motion directions.
2: Based on your answer to #1, was the storm surge from Cyclone George to the East of the eye or to the West when it came ashore? Remember that George was moving generally south.
Step 2: Side-by-Side Comparison
These two pictures represent examples of hurricanes in the Northern and Southern Hemispheres respectively. On the left, you see Hurricane Katrina on August 29, 2005 just before it made landfall on the Gulf Coast of the US. On the right is Hurricane Catarina on March 28, 2004 as it was making landfall on the coast of Brazil. Note that the scale is not the same for each image.
Both of these storms were record breakers in their own ways.
- Catarina was the first ever hurricane known to form in the South Atlantic basin. Wind shear in the upper atmosphere of this part of the world usually prevents the formation of hurricanes and cyclones. Scientists are still studying the data that was gathered to see what factors led to this unprecedented event.
- Hurricane Katrina became the costliest hurricane ever to strike the United States. It was also one of the most powerful. The destruction caused by this storm along the Gulf Coast was staggering. It also ranks third on the list of the deadliest hurricanes to hit the US, having taken more than 1800 lives.
Stop and Think
3: Based on the images and text, compare and contrast these two storms.
4: Catarina was on the borderline between Category 1 and 2. Despite its relatively weak status, the storm damaged or destroyed about 26% of all the buildings in the region of Brazil where it came ashore. The storm caused about $25 million in damages. Based on what you've learned, formulate an explanation for how a relatively weak storm could cause so much damage.