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Hurricanes feed on warm ocean water, are ranked by wind on the Saffir-Simpson scale, and do most harm with storm surge and rain; landfall time is distance over speed.
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By the end of this lesson you will be able to rank a hurricane, predict its landfall, work out a storm surge's depth, and explain why category is not the whole danger.
You know that rising air makes low pressure, clouds and rain, and that wind blows toward a low, spiraling counterclockwise in the Northern Hemisphere. A hurricane is that pattern at its most extreme, powered by warm ocean water.
| Term | What it means |
|---|---|
| Hurricane | A tropical storm with sustained winds of at least 74 miles an hour. |
| Saffir-Simpson scale | A ranking of hurricanes from category 1 to 5 by sustained wind speed. |
| Storm surge | A rise in sea level pushed ashore by a storm's winds and low pressure. |
| Eye | The calm center of a hurricane, where air sinks. |
| Eyewall | The ring of towering clouds and strongest winds around the eye. |
| Landfall | The moment a hurricane's center crosses the coast. |
Hurricanes form where the sea is warm enough to power them.
Another way: picture
Picture a giant chimney over the ocean, drawing in warm, wet air at the bottom, sending it spiraling up, and releasing heat as the water vapor turns to rain. As long as the sea below stays warm, the chimney keeps drawing. Over land or cold water, it chokes and weakens.
Another way: steps
Floods, tropical storms and earthquakes require different hazard evidence. Flood likelihood concerns water reaching places; a storm brings wind, rainfall and surge with distinct footprints; an earthquake brings shaking modified by depth, distance and ground conditions. A wind category does not predict flood depth, and earthquake magnitude is not a count of deaths.
Exposure identifies the people and assets in the affected footprint. Vulnerability concerns how easily they are harmed, while capacity includes warnings, evacuation access and construction practices. An empty floodplain can have a high flood hazard and low present human exposure. A modest event in a poorly protected settlement can be disastrous. Compare similar periods and population denominators before ranking losses.
For an invented coast, two towns face the same wind forecast. One is on low ground with one evacuation road; the other has higher ground and several routes. Wind alone cannot explain their likely losses. Add a surge map, road capacities and information about residents who need assistance. Retain forecast uncertainty: the event footprint can shift, and preparedness should not rely on an exact landfall time from a constant-speed exercise.
Hurricanes are ranked by their sustained wind speed.
| Category | Wind (mph) | Typical damage |
|---|---|---|
| 1 | 74 to 95 | some roof and tree damage |
| 2 | 96 to 110 | major roof damage, widespread power loss |
| 3 | 111 to 129 | serious damage to homes |
| 4 | 130 to 156 | severe damage, many homes destroyed |
| 5 | 157 or more | catastrophic damage |
Categories 3 and above are called major hurricanes.
Atlantic hurricanes form mostly between June 1 and November 30, peaking from August to October, when the ocean is warmest. Many start as clusters of thunderstorms moving west off Africa, grow over the warm Atlantic, and curve toward the Caribbean, the Gulf of Mexico or the East Coast.
They do not form right at the equator, because the Coriolis effect there is too weak to start the spin.
At the center is the eye, often fifteen to forty miles across, where air sinks and skies can be clear and calm. Around it the eyewall holds the tallest clouds, the heaviest rain and the strongest winds. Spiral rain bands stretch hundreds of miles outward.
People caught in the eye sometimes think the storm is over, only for the eyewall's winds to return from the other direction.
A hurricane's winds pile sea water against the coast, and its low pressure lets the sea rise a little more. The result is a storm surge that can reach more than twenty feet on shallow coasts, pushing miles inland.
Surge adds to the tide: a surge at high tide reaches higher than the same surge at low tide. The depth of water over a street is the water level minus the street's height.
Hurricanes carry vast amounts of water. A slow-moving storm can drop several feet of rain, flooding rivers far from the coast. In 2017, Hurricane Harvey stalled over Texas and dropped more than sixty inches in places.
Rain can do more harm than wind: a weak but slow storm may flood more homes than a strong, fast one.
The National Hurricane Center in Miami tracks every storm with satellites, aircraft that fly into the eye, and ocean buoys. It forecasts the storm's path, shown as a cone, and its strength.
Dividing the distance to the coast by the storm's forward speed gives the hours to landfall. But storms speed up, slow down and turn, so forecasts are updated every few hours.
Officials order evacuations early, because moving hundreds of thousands of people takes many hours, roads can jam, and the storm's outer winds and rising water arrive before its center. If an evacuation takes eighteen hours and landfall is thirty hours away, there are twelve hours to spare, but less in practice.
Coastal areas are divided into evacuation zones by surge risk, so those in the most danger leave first.
The hurricane that struck Galveston, Texas, in 1900 remains the deadliest natural disaster in American history: its storm surge swept over the low island city, killing somewhere between 6,000 and 12,000 people. There was little warning and no way to evacuate.
Hurricane Katrina in 2005 killed more than 1,300 people, mostly when the storm surge broke the levees protecting New Orleans and flooded much of the city.
Checking an answer. A wind speed must sit inside its category's band. Hours to landfall should shrink if the storm speeds up.
Dividing distance by speed for time is allowed while the storm keeps a steady speed and direction, which is why forecasts are updated as it changes. Adding surge and tide is allowed because both raise the same sea surface.
Subtracting land height from the water level is allowed because both are measured from the same reference, normal sea level.
Warmer oceans give hurricanes more fuel. Scientists find that the strongest storms are becoming more intense and wetter, and that some storms strengthen very quickly just before landfall, leaving little time to prepare.
Rising sea level also means each storm surge starts from a higher sea, reaching farther inland than the same surge would have decades ago.
The most common slip is judging a hurricane's danger by its category alone. Another is thinking the calm eye means the storm has passed.
A third is forgetting to add the tide to the surge. A fourth is multiplying distance by speed instead of dividing when finding the hours to landfall.
On September 8, 1900, a powerful hurricane struck Galveston, Texas, then a booming port city built on a low barrier island whose highest point was less than nine feet above sea level. The storm surge, around fifteen feet, swept across the entire island.
Between 6,000 and 12,000 people died, the most of any natural disaster in American history. Weather forecasting was primitive, warnings were ignored or came too late, and there was no bridge wide enough for a mass evacuation.
Galveston responded by building a seawall about seventeen feet high and raising the level of much of the city by pumping sand beneath it, lifting buildings as the ground rose. The seawall still protects the city, and the disaster helped lead to better hurricane warnings across the country.
The National Hurricane Center in Miami, Florida, watches the Atlantic and eastern Pacific around the clock during hurricane season. Its forecasters use satellites, radar, ocean buoys, and Air Force and NOAA aircraft known as Hurricane Hunters, which fly directly into storms to measure their winds and pressure.
The center publishes a forecast cone showing where a storm's center will probably go over the next five days, along with its expected strength, storm surge and rainfall. Local officials use those forecasts, and the hours to landfall, to decide when to order evacuations.
Forecasts of a hurricane's track have improved greatly: the average error in a three-day forecast today is far smaller than it was a few decades ago. That extra accuracy gives coastal communities more time, which saves lives.
The Saffir-Simpson category is the number people remember, so it is natural to judge a storm by it alone. But the category measures only wind. Most hurricane deaths come from water: the storm surge along the coast and floods from heavy rain inland.
A large, slow category 1 storm can push a higher surge or drop far more rain than a small, fast category 4. Listen to the surge and rainfall forecasts, not only the category.
A hurricane's winds are $120$ mph. Recall where category 3 starts.
$111\ \text{mph}$
The band's floor.
Recall where category 4 starts.
$130\ \text{mph}$
The next floor.
Place the storm.
$111 \le 120 < 130$
Category 3.
Say what that makes it.
$\text{a major hurricane}$
Category 3 or above.
A storm is $300$ miles offshore at $12$ mph. Divide.
$\dfrac{300}{12} = 25\ \text{hours}$
Distance over speed.
Write its distance after $t$ hours.
$300 - 12t$
Miles offshore.
Find its distance in $10$ hours.
$300 - 120 = 180\ \text{miles}$
Substitute.
Say what happens if it speeds up to $15$ mph.
$\dfrac{300}{15} = 20\ \text{hours}$
Five hours less.
Say why forecasts update.
$\text{speed and path change}$
Every few hours.
A surge of $12$ ft arrives at a high tide $2$ ft above normal. Add them.
$12 + 2 = 14\ \text{ft}$
Water level.
A street is $5$ ft above normal sea level. Find the depth.
$14 - 5 = 9\ \text{ft}$
Water over the street.
Compare with a person's height.
$9 > 6$
Deeper than a tall adult.
Find the depth if it arrived at low tide, $1$ ft below normal.
$12 - 1 - 5 = 6\ \text{ft}$
Still dangerous.
Say what the difference shows.
$\text{timing with the tide matters}$
Three feet between them.
Say what officials do.
$\text{evacuate the lowest zones first}$
Where the water is deepest.
Write distance over speed.
$\dfrac{240}{15}$
Time.
Evaluate the expression.
$16\ \text{hours}$
Until landfall.
Say what arrives sooner.
A hurricane's sustained winds are $105$ miles an hour. What category is it on the Saffir-Simpson scale?
Complete the worked solution: a hurricane pushes a storm surge of $12$ feet onto the coast at high tide, which is $2$ feet above normal sea level. A street stands $5$ feet above normal sea level. Find the water level above normal sea level, and the depth of water over the street.
Find the water level.
$\text{surge} + \text{tide} =$ w
Feet above normal sea level.
Find the depth over the street.
$\text{water level} - \text{street height} =$ d
Feet of water.
Say why this is deadly.
$\text{deeper than a person is tall}$
Surge causes most hurricane deaths.
Say what waves add.
$\text{more height and force}$
On top of the surge.
Match each part of a hurricane to what it does.
| a dome of sea water pushed onto the coast | rivers flooding many miles inland | a ring of the storm's strongest winds | a calm, often clear center before the winds return | |
|---|---|---|---|---|
| the storm surge | ||||
| the heavy rain | ||||
| the eyewall | ||||
| the eye |
Sort four hurricanes by their sustained winds on the Saffir-Simpson scale: category 1 from 74 mph, 2 from 96, 3 from 111, 4 from 130, and 5 from 157.
| Category | |
|---|---|
| winds of 85 mph | |
| winds of 115 mph | |
| winds of 135 mph | |
| winds of 165 mph |
A hurricane is $360$ miles offshore and moving straight toward the coast at $20$ miles an hour. Write its distance from the coast, in miles, as a function of the hours $t$ from now.
Answer:
A hurricane is $180$ miles offshore, moving toward the coast at $10$ miles an hour. In how many hours will it make landfall, if its speed holds?
Answer: hours
Suppose a hurricane is $450$ miles from New Orleans, Louisiana, moving toward it at $10$ miles an hour, and officials expect an evacuation to take $30$ hours. If the evacuation starts now, how many hours will it have to spare before landfall?
Answer: hours
A report compares a flooded district, hurricane coast and earthquake zone, each with 7 thousand residents. It ranks expected deaths using only flood depth, wind category and magnitude. Mark every supported sentence that diagnoses the comparison's limits and identifies relevant missing evidence.
This task has no paper form; do it on a device.
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A hurricane is $300$ miles offshore and moving straight toward the coast at $12$ miles an hour. Write its distance from the coast, in miles, as a function of the hours $t$ from now.
Answer:
You can reason about hurricanes. Explain why a category 1 hurricane can be deadlier than a category 4.
24. Your turn: a storm is $240$ miles away, moving at $15$ mph. How many hours until landfall?, step 3
$\text{outer rain bands}$
Hours before the eye.