Hurricanes may seem like “polite” disasters. After all, there is usually advance warning that the hurricane is coming, unlike tornadoes that sometimes erupt suddenly in the middle of the night. For those readers who do not live in coastal areas, believe me when I say this is an illusion. As we watch Isaac hit Louisiana on the anniversary of Katrina, allow me to share my version of “Hurricanes 101.”
Hurricane and typhoon are actually regional names for what is properly termed a tropical cyclone. Cyclone comes from the Greek word for “to rotate.” Because cyclones rotate counterclockwise in the northern hemisphere, cyclonic is sometimes used to describe other entities that rotate counterclockwise. Without getting too technical, when the pressure in one area of the atmosphere drops, Mother Nature tries to add air to bring it back up to average – this causes winds. The earth’s rotation impacts how the winds attempt to fill that low pressure area causing the low pressure area to rotate. Sitting out over the ocean, if the water is warm, this adds energy, which can result in a hurricane. From above the spiral of the hurricane is readily visible with the center of the storm – the “eye” being relatively calm. In the northern hemisphere the front of the hurricane blows more or less east to west followed by the calm eye and the back of the storm is closer to west to east winds.
We have advance warnings about hurricanes because they can be tracked by radar, weather buoys and special aircraft that fly into the hurricane and take pressure, speed and other readings. When a hurricane is being tracked, the process is complicated and not as precise as we would like. The path is impacted by such external factors as wind, high and low pressure systems, and water temperature. A change in direction is not easy to accurately identify. When a hurricane’s course looks as if it will change, meteorologists debate whether it is a real direction change or just a “wobble” in the existing track. They try to err on the side of caution and project whichever path encourages people to prepare for the worst.
If you don’t live in a hurricane area, it might seem that there is always plenty of time to evacuate ahead of the storm. For all intents and purposes, evacuees need to be on the road about 3 days ahead of the hurricane’s eye. Even though they reverse highway lanes so that all traffic is headed away from the coast, traffic tends to be bumper to bumper, stop and go. Since hurricanes tend to occur in warmer weather, people keep their cars running even when stopped to keep the air conditioning running. Inevitably some will run out of gas, further complicating traffic. Those who try to evacuate with less than 3 days head start run a significant risk of getting stuck and trying to ride out the storm inside an automobile. This is one reason why so many people tend to stay at home and “ride out” the storm.
The actual weather associated with a hurricane is experienced well ahead of the hurricane’s eye. The spiral rainbands can extend up to 100 miles ahead of the eye and deliver both significant rain and high winds. Sometimes we’ll see a hurricane “stall” and just keep raining on one area. If part of the storm is over open water, the hurricane can continue to pick up water vapor and immediately dump it on land. While the wind is the most visually impressive, it is actually the tremendous amounts of heavy rain that are dumped on the land referred to as “storm surge”. Add to this the fact that the hurricane’s right side in relation to its direction of travel often spawns tornadoes.
Since hurricanes affect coastal areas, there are additive issues that make them especially deadly. Start with the wind and rain ahead of the eye, add the tornadoes and then factor in tides. The leading edge of an east coast hurricane rotates so that the wind is blowing from the Atlantic toward land. This can keep a high tide from emptying into the ocean such that the next high tide is added to the previous one. Now add the rain from the hurricane itself and the inevitable flooding is the sum of all of these factors. The debris from structures damaged by both wind and water now can be carried by the flood waters and strike other structures with the force of raging flood waters creating even more damage.
Legend has it that early meteorologists used the first names of politicians to name hurricanes alluding to both having lot of hot air and creating a lot of damage. To hide this practice, they switched to the first names of politicians’ wives, which is why for many years hurricanes had female names. This may not be true, but it’s a great story. Today, male and female names are alternated and the names of particularly deadly hurricanes are retired from the list.
For more information, NOAA – the National Oceanic and Atmospheric Administration, the agency that includes the National Weather Service has a great site on hurricanes including a free downloadable booklet called “Hurricane Basics.”