Supercomputer simulations show there will be more typhoons with winds of 179 miles (288 kilometers) per hour—considered an F3 on the five-level Fujita

Scale—by 2074.

By definition, supertyphoons carry winds of at least 150 miles (241 kilometers) per hour.

Such storms would be more destructive than Hurricane Katrina, which slammed into U.S. states along the Gulf of Mexico in August 2005.

“The most important factor in the creation of these typhoons is the warming of sea-surface temperatures in the western Pacific,” said researcher Kazuhisa

Tsuboki of Nagoya University.

Small But Severe

If global warming continues at its present pace, by 2080 the western Pacific Ocean will be 3.6 degrees Fahrenheit (2 degrees Celsius) warmer, according to

Tsuboki, who worked with a team from Japan’s Meteorological Research Institute.

“That sounds like a small difference, but it will have a very big impact on a typhoon,” Tsuboki said.

That’s because even a relatively minor increase in seawater temperature adds an exponentially larger amount of energy to a storm, he said.

A rise in air temperature will also increase the amount of water vapor in the lower atmosphere, adding yet more fuel to the system.

Typhoons generally cover an area of between 311 and 497 miles (500 and 800 kilometers), Tsuboki said.

But to the researchers’ surprise, the predicted supertyphoons will be smaller, stretching only 249 miles (400 kilometers).

However the storms will pack a far higher concentration of energy, wind speed, and overall destructive power.

Widespread Damage

The tempests would cause a great deal of damage across Japan, which is unprepared for such violent weather systems, Tsuboki said.

Ferocious winds would level homes and damage infrastructure such as bridges and power lines. Severe floods would also inundate low-lying areas.

The most destructive typhoon to strike Japan to date was Typhoon Vera, which barreled across the country in September 1959.

Known in Japan as the Isewan Typhoon, the storm came ashore in Ise Bay near Nagoya and killed 5,238 people.

Lahars can be best described as volcanic mudflows. They may not necessarily be caused by volcanic activity, but at the very least do originate from some type of volcanism. Lahars have the consistency of concrete: fluid when moving, then solid when stopped. Lahars can be huge: the Osceola lahar produced 5,600 years ago by Mount Rainier in Washington produced a wall of mud 140 metres (460 ft) deep in the White River canyon and covered an area of over 330 square kilometres (130 sq mi) for a total volume of 2.3 cubic kilometers (0.55 cubic miles).

Lahars can be deadly because of their energy and speed. Large lahars can flow several dozen meters per second and can flow for many kilometres, causing catastrophic destruction in their path. The lahars from the Nevado del Ruiz eruption in Colombia in 1985 caused the Armero tragedy, which killed an estimated 23,000 when the city of Armero was buried under 5 metres (16 ft) of mud and debris. New Zealand’s Tangiwai disaster in 1953, where 151 people died after a Christmas Eve express train fell into the Whangaehu River, was caused by a lahar.

Lahars usually travel down valleys. They have a wide range of velocities varying from 1 m/s to 40 m/s. The velocity of a lahar depends on the channel width, channel slope, volume of the flow, and grain size composition (Scott, 1989). Lahars can travel long distances. Some lahars have traveled hundreds of kilometers from their source (Scott, 1989). The deposits of a lahar that traveled 60 km from its source at Mount Rainier can be found near the large city of Seattle, Washington (Pierson et al., 1992). The lahar’s origin at Mount Rainier helped make that volcano a decade volcano.

Lahars have been known to transport very large boulders. At Mount Pinatubo, boulders measuring 1.5 m long were not uncommon in lahar deposits (Pierson et al., 1992). The lahars from Nevado del Ruiz transported a boulder with a volume of 208 cubic meters, 300 m downstream (Mileti, 1991).

When a lahar travels down valley, the high point of the lahar is usually marked by the mudline it leaves on trees, valley walls, and buildings. This mudline marks the upper limit of how high a lahar will go. This upper limit is important because it defines how high people must go to be out of danger from the lahar. The small eruption of Nevado del Ruiz in 1987, produced large lahars that destroyed the city of Armero. Unfortunately, the 30,000 people who lost their lives might have been saved had they established an appropriate line of communication and evacuated to higher ground (Francis, 1993).

In 1991, Mount Pinatubo erupted. Some of the pyroclastic flows initiating from this eruption were transformed into lahars as they moved downslope through river valleys. Secondary lahars were formed when rain mixing with ash from the eruption became unstable. The formation of these lahars often occured within 30 minutes of as little as 10-15 mm of precipitation falling on the loose ash near the summit of Pinatubo (Primer, 1992). Secondary lahars are still forming today from the unconsolidated ash.

Lahars are extremely dangerous especially to those living in valley areas near a volcano. Lahars can undercut banks and cause houses on those banks to be destroyed. Lahars can bury and destroy manmade structures including roads and bridges. At Nevado del Ruiz, lahars destroyed an entire city; filling the first floor of a hospital with mud, breaking windows, floating cars, and leaving debris in the tops of trees (Mileti, 1991).

Causes of Lahar

Lahars have several possible causes:

• Snow and glaciers can be melted by lava or a pyroclastic flow during an eruption
• A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar, also called glacier run or jokulhlaup
• Water from a crater lake, combined with volcanic material in an eruption

In particular, although lahars are typically associated with the effects of volcanic activity, lahars can occur even without any current volcanic activity, as long as the conditions are right to cause the collapse and movement of mud originating from existing volcanic ash deposits.

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.

These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.

A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.

Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas.

The best defense against any tsunami is early warning that allows people to seek higher ground. The Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed to protect coastal areas worldwide.

This enormous electrical discharge is caused by an imbalance between positive and negative charges. During a storm, colliding particles of rain, ice, or snow increase this imbalance and often negatively charge the lower reaches of storm clouds. Objects on the ground, like steeples, trees, and the Earth itself, become positively charged—creating an imbalance that nature seeks to remedy by passing current between the two charges.

A step-like series of negative charges, called a stepped leader, works its way incrementally downward from the bottom of a storm cloud toward the Earth. Each of these segments is about 150 feet (46 meters) long. When the lowermost step comes within 150 feet (46 meters) of a positively charged object it is met by a climbing surge of positive electricity, called a streamer, which can rise up through a building, a tree, or even a person. The process forms a channel through which electricity is transferred as lightning.

Some types of lightning, including the most common types, never leave the clouds but travel between differently charged areas within or between clouds. Other rare forms can be sparked by extreme forest fires, volcanic eruptions, and snowstorms. Ball lightning, a small, charged sphere that floats, glows, and bounces along oblivious to the laws of gravity or physics, still puzzles scientists.

Lightning is extremely hot—a flash can heat the air around it to temperatures five times hotter than the sun’s surface. This heat causes surrounding air to rapidly expand and vibrate, which creates the pealing thunder we hear a short time after seeing a lightning flash.

Lightning is not only spectacular, it’s dangerous. About 2,000 people are killed worldwide by lightning each year. Hundreds more survive strikes but suffer from a variety of lasting symptoms, including memory loss, dizziness, weakness, numbness, and other life-altering ailments.

Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring 3 to 5 is considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major; and 8 or more is great.

On average, a magnitude 8 quake strikes somewhere every year and some 10,000 people die in earthquakes annually. Collapsing buildings claim by far the majority of lives, but the destruction is often compounded by mud slides, fires, floods, or tsunamis. Smaller temblors that usually occur in the days following a large earthquake can complicate rescue efforts and cause further death and destruction.

Loss of life can be avoided through emergency planning, education, and the construction of buildings that sway rather than break under the stress of an earthquake.

Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year. “Tornado Alley,” a region that includes eastern South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.

A tornado forms when changes in wind speed and direction create a horizontal spinning effect within a storm cell. This effect is then tipped vertical by rising air moving up through the thunderclouds.

The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in late afternoon, when thunderstorms are common, and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.

Tornadoes’ distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm’s moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.

Tornadoes move at speeds of about 10 to 20 miles (16 to 32 kilometers) per hour, although they’ve been clocked in bursts up to 70 miles (113 kilometers) per hour. Most don’t get very far though. They rarely travel more than about six miles (ten kilometers) in their short lifetimes.

Tornadoes are classified as weak, strong, or violent storms. Violent tornadoes comprise only about two percent of all tornadoes, but they cause 70 percent of all tornado deaths and may last an hour or more.

People, cars, and even buildings may be hurled aloft by tornado-force winds—or simply blown away. Most injuries and deaths are caused by flying debris.

Tornado forecasters can’t provide the same kind of warning that hurricane watchers can, but they can do enough to save lives. Today the average warning time for a tornado alert is 13 minutes. Tornadoes can also be identified by warning signs that include a dark, greenish sky, large hail, and a powerful train-like roar.

Safety Tips

• Prepare for tornadoes by gathering emergency supplies including food, water, medications, batteries, flashlights, important documents, road maps, and a full tank of gasoline.
• When a tornado approaches, anyone in its path should take shelter indoors—preferably in a basement or an interior first-floor room or hallway.
• Avoid windows and seek additional protection by getting underneath large, solid pieces of furniture.
• Avoid automobiles and mobile homes which provide almost no protection from tornadoes.
• Those caught outside should lie flat in a depression or on other low ground and wait for the storm to pass.

At midday the barrier island in Vero Beach has the weird feel of a place quickly and angrily abandoned. Poking around debris-lined streets among spray-painted signs all denigrating a certain Jeanne—I’m hoping not to be mistaken for a looter by a cop—or, worse yet, by an armed Floridian homeowner. Wind gusts drive intermittent sheets of rain as a few stragglers throw a last suitcase or heirloom into a car before scurrying over the causeway to the mainland. Jeanne, you see, is a major hurricane, already a killer of thousands in Haiti. This thin strip of land is surely not the place to be when she arrives in full force tonight.

The ghost town air of shuttered, boarded-up, duct-taped houses and condos prevails everywhere but the oceanfront, where visitors keep arriving. Police officers try to shoo them away. “We’re under an evacuation order here, folks,” they announce. “Don’t even think about it!” a patrolman bellows at a pair of daredevils who drive up with surfboards strapped to their car. But as soon as the officers move off to another public beach, more hurricane pilgrims appear. Mostly they’re locals who’ve come to curse at fate, or to ponder nature’s cruel sense of humor, or maybe just to wearily accept what’s coming.

Again.

Three weeks ago a cyclone named Frances tore into central Florida on a nearly identical path. That hurricane, the second to hit the state in the unlucky summer of 2004, left several feet of water in Greg McIntosh’s house. Turning his back to a hard blast of wind, he wonders what Jeanne—storm number four—has in store.

“It’s like in basic training when you get into your bivouac and then the sergeant blows the whistle and you have to go another 10 miles (16 kilometers) in the dark,” says McIntosh.

David Mitchell, a recent transplant to Florida, leans on the railing above a seawall and stares over the rising sea. Monstrous swells crash into a breakwater a few hundred yards offshore.

“I’ve only been here four months, and in that time, two hurricanes,” he says. He’ll soon retreat to his apartment, where he has taped over the windows, even though that’s not likely to offer much protection from windblown debris. “I guess when you live in Florida, it’s just something you have to get used to.”

Not just Floridians—anyone in the eastern coastal regions of the United States and Central America, as well as the entire Caribbean, is getting used to it. Since 1995 the Atlantic has been producing powerful hurricanes at a hyperactive pace, doubling that of the previous quarter century. If few in the U.S. noticed at first, it was because atmospheric conditions mostly kept the abundant storms out at sea or headed elsewhere. But the winds shifted in 2004.

“The whole East Coast was very lucky for the past 30 or 40 years,” says William Gray, of Colorado State University, a pioneer in long-range hurricane research and forecasting. Gray had predicted a damaging storm season for last year. “We’d been saying things would change, but nobody expected anything like the 2004 season.”

The coming hurricane seasons might make last year’s estimated 40 billion dollars’ worth of U.S. damage look small. Warns Gray: “There’s no way—if we see a return to the type of landfall activity we saw in the thirties, forties, and fifties—no way but that economic losses will double, triple, quadruple, or worse.”

That’s because societal risk—roughly defined as the number of people and value of property vulnerable to hurricanes—has exploded. The southeastern coastal population has grown dramatically, with Florida’s alone more than tripling since 1960. The economic risk has also multiplied. About 1.1 trillion dollars’ worth of at-risk property was insured in 1980; the total now is an estimated 5.5 trillion dollars’ worth. And the latest census figures show that between 2000 and 2004, 29 of the 50 fastest-growing U.S. counties were in East Coast and Gulf Coast states.

The hurricane glut is happening at the same time sea levels continue to rise—the result of global warming that most scientists blame in part on human activity. A recent study using the latest computer climate models predicts warming of the tropical sea surface will strengthen hurricane winds and rainfall by the end of the 21st century. However, some experts, including Gray, argue that climate change due to human activity will not significantly affect hurricanes.

That debate will continue, but many scientists agree that the present hurricane surge is likely part of a 60-to-70-year cycle that changes the strength of ocean currents distributing heat around the globe. Researchers have used tree rings and ice cores to track this variability back hundreds of years. We’re now in a fast-flowing mode of this up-and-down cycle, named the Atlantic Multidecadal Oscillation (AMO), during which Atlantic sea-surface temperatures and wind conditions favor hurricane generation. Ten years from now, or perhaps thirty (the timetable is difficult to predict), the cycle should reverse, tending to suppress major hurricanes.

Why the variation? “Frankly, no one can say with 100 percent certainty, but it appears to be a natural effect,” says Thomas Delworth, a climate modeler at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Delworth is part of a major scientific effort to develop accurate computer climate models, and much of his work focuses on thermohaline circulation—that is, the way ocean currents, and consequently such cycles as the AMO, are driven by heat and salinity.

Thermohaline circulation runs the Atlantic conveyor belt, part of a global ocean system in which a continuous flow of upper-level water is drawn from the tropical Atlantic north toward the Pole. There the water cools, sinks, and cycles back to the southern oceans in deepwater currents.

As the conveyor belt speeds up, tropical surface water is drawn north more quickly, and temperatures in the North Atlantic are as much as 2°F (1°C) warmer. That’s good for hurricanes. “A hurricane is essentially an engine that runs on heat,” says Chris Landsea, a meteorologist at NOAA’s Hurricane Research Division in Miami. “The warmer the sea-surface temperature [it must be at least 80°F (27°C) for a hurricane to start] and the more warm, moist air that’s available, the stronger a hurricane can become.”

How and where does the conveyor belt speed up, increasing the overturning circulation of warm and cold water? It’s at the point where cold surface water sinks that the acceleration of the Atlantic conveyor belt probably happens, Delworth says. Cold, dry air coming off Canada extracts heat from the water. When these winds blow stronger and colder than average over a number of years, increasingly chilled water sinks faster because it is more dense, intensifying the flow rate. Years of weaker and warmer winds have the opposite effect, slowing the conveyor belt.

Climate records indicate a correlation between a pattern of increased cold winds and the 1995 upswing in hurricane formation. “From the late sixties to the mid-nineties, westerly winds strengthened,” Delworth says. “The overturning circulation probably increased in that period in response.”

Increased circulation brings mighty storms—born as air spirals into a low-pressure zone charged with warm, humid air over warmer sea surfaces. The winds meet and ascend, causing clouds to billow upward, further lowering air pressure and causing winds to barrel even faster toward the center. The Earth’s rotation lends spin to the gathering cyclone. When water vapor in the ascending clouds cools and falls as rain, the amount of heat energy released dwarfs the amount of electricity consumed daily by all of humanity. The energy warms the eye, further lowering the pressure and strengthening the storm.

The cyclone can continue to strengthen if atmospheric currents guide it over warm water, and if it is not destroyed by vertical wind shear—the differential between wind speeds at lower and higher altitudes. Strong wind shear can dissipate a storm, but the warm phase of the AMO tends to weaken vertical wind shear in the Atlantic.

The combined effect of changes in the AMO and the Atlantic conveyor belt has been dramatic. In the Caribbean, production of cyclones skyrocketed 400 percent. In the entire Atlantic Basin, major hurricanes, with sustained winds of 111 miles (179 kilometers) an hour or higher, increased 150 percent. The intensifying is most pronounced in powerful storms like Ivan, whose winds at times exceeded 155 miles (249 kilometers) an hour as it smashed past Jamaica and headed for landfall near Pensacola.

It was just after midnight on September 16, 2004. Residents of Grande Lagoon who chose to ignore evacuation warnings and ride Ivan out in their upscale homes flanking the Intracoastal Waterway west of Pensacola passed time reading, playing cards with their children, wondering if they had miscalculated.

The cacophony of wind and debris pelting their houses covered up any sound that might signal the approach of the real enemy—not wind, but a wide dome of seawater Ivan had piled up and was pushing toward them in the dark. This was the storm surge, the deadliest part of a hurricane for those living near water. Two men who survived the sudden flooding related this same sequence of events: They looked down first at a wet floor, then at a few inches of water around their feet. Each then opened the front door to a waist-deep onslaught of dirty seawater.

Three others who refused to evacuate the area died when the sea invaded their homes. The search for bodies delayed the return of residents who evacuated and then came back wanting to know what they had lost. For many, that turned out to be everything they didn’t take with them when they fled.

In the chaos of the aftermath, one couple in the neighborhood seems somewhat at home. Al and Dean Hoffman have set up camp in a tent trailer outside their devastated ranch house. At the moment, there’s enough wood debris heaped against the house, which backs water, to rebuild several docks. A motorboat has pierced a side room. The interior is muck-coated and smells of rotting fish. But the retired couple will not be pushed from the coastline a second time.

“I came back to a concrete pad after Hugo hit South Carolina in 1989, so I can handle this,” says Dean Hoffman. “We sure know how to pick ‘em, don’t we?” adds her husband, Al.

A big topic of conversation in hurricane-flooded communities is FEMA’s “50 percent rule.” If inspectors from the federal disaster agency determine that a house has sustained more than 50 percent damage, the structure must be rebuilt to the latest state and local codes. This rule protects the government-run National Flood Insurance Program—which pays up to $250,000 for reconstruction—from repeatedly covering repairs to the same house. For Dean and Al the latest codes could require a new house on 10- or 15-foot (3- or 4.5-meter) pilings. “My God—can you imagine how ridiculous that would be?” Al asks, glancing up, perhaps imagining a ranch house among the trees. There’s no way his waterlogged home is more than half done for, he declares.

According to the latest National Hurricane Center updates, which I’m monitoring the morning of September 25 in my Vero Beach hotel room, Jeanne has traversed luxuriously warm water and is now a major hurricane. As I study satellite images of the rotating cloud mass on the NOAA website, I’m perversely pleased that my first hurricane won’t be of the garden variety.

I might be less sanguine were I not planning to ride out the storm in an absolute bunker of a house a few miles inland. I’ll be at the home of Jonathan Gorham, coastal engineer for Indian River County. His house was built in 2003 to all the latest hurricane codes. Gorham and his wife have also invited two other families to weather the hurricane with them. Their houses were seriously damaged by Frances and might not stand up to Jeanne’s pounding.

Our group, assembled and under assault by early evening, has been waiting for the calm of the storm’s eye. But sometime after midnight we realize there will be no letup-the eye is passing just south of us. “I’ve heard you can see the stars come out in the eye,” says Mike Bresette, dejected, as he turns in for the night. I’m on a bedroll near the front door, which seems to bow inward with each hard gust. With the rumbling that’s coming through the thick walls of the Gorham house, I begin to feel in my bones a measure of the fear many millions have known for as long as humans have lived near oceans.

So I’m deflated to learn some weeks later that for all its fury, the storm, where I was, was perhaps not all it was advertised to be. This news comes from Tim Reinhold, vice president of engineering at the Institute for Business and Home Safety, a research and education organization funded by the insurance industry. Reinhold, a former civil engineering professor, rode out each 2004 Florida hurricane while tending wind gauges and other instruments he set up at houses in the paths of the storms.

Jeanne’s maximum sustained wind speed at landfall was 120 miles (193 kilometers) an hour. But slightly inland, the best Reinhold’s instruments could offer was 87 miles (140 kilometers) an hour—a piddling Category One event. Charley, in August, remained genuinely powerful over land; the others lost significant force.

So what, exactly, is a major hurricane? “There’s more than one kind of measurement, especially when we’re trying to measure what people went through in the places they live,” Reinhold says. But one thing is certain: Last season’s devastation could easily pale in seasons to come before the current cycle reverses.

“When you looked at images of Jeanne, Frances, or Ivan, you didn’t see a complete doughnut-shaped eye wall like you see in a really powerful hurricane,” Reinhold says. “You see that in images of Andrew in ’92. Andrew looked like a buzz saw.” William Gray and his colleagues, who predicted last year’s above average probability of destruction, have done so again for this year. It may turn out the hurricanes of 2004 were but a wake-up call. The buzz saws might already be winding up far out at sea.

The Atlantic Ocean’s hurricane season peaks from mid-August to late October and averages five to six hurricanes per year.

Hurricanes begin as tropical disturbances in warm ocean waters with surface temperatures of at least 80 degrees Fahrenheit (26.5 degrees Celsius). These low pressure systems are fed by energy from the warm seas. If a storm achieves wind speeds of 38 miles (61 kilometers) an hour, it becomes known as a tropical depression. A tropical depression becomes a tropical storm, and is given a name, when its sustained wind speeds top 39 miles (63 kilometers) an hour. When a storm’s sustained wind speeds reach 74 miles (119 kilometers) an hour it becomes a hurricane and earns a category rating of 1 to 5 on the Saffir-Simpson scale.

Hurricanes are enormous heat engines that generate energy on a staggering scale. They draw heat from warm, moist ocean air and release it through condensation of water vapor in thunderstorms.

Hurricanes spin around a low-pressure center known as the “eye.” Sinking air makes this 20- to 30-mile-wide (32- to 48-kilometer-wide) area notoriously calm. But the eye is surrounded by a circular “eye wall” that hosts the storm’s strongest winds and rain.

These storms bring destruction ashore in many different ways. When a hurricane makes landfall it often produces a devastating storm surge that can reach 20 feet (6 meters) high and extend nearly 100 miles (161 kilometers). Ninety percent of all hurricane deaths result from storm surges.

A hurricane’s high winds are also destructive and may spawn tornadoes. Torrential rains cause further damage by spawning floods and landslides, which may occur many miles inland.

The best defense against a hurricane is an accurate forecast that gives people time to get out of its way. The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will make landfall within 24 hours.

Avalanches are classified by their morphological characteristics, and are rated by either their destructive potential, or the mass of the downward flowing snow. Some of the morphological characteristics used to classify avalanches include the type of snow involved, the nature of the failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation. Avalanche size, mass, and destructive potential are rated on logarithmic magnitude scales, typically made up of 4 to 7 categories, with the precise definition of the categories depending on the observation system or forecast region.

Formation and Occurrences

Avalanches only occur when the stress on the snow exceeds the shear, ductile, and tensile strength either within the snow pack or at the contact of the base of the snow pack with the ground or rock surface. A number of the forces acting on a snow pack can be readily determined, for example the weight of the snow is straightforward to calculate, however it is very difficult to know the shear, ductile, and tensile strength within the snow pack or with the ground. These strengths vary with the type of snow crystal and the bonding between them. The thermo-mechanical properties of the snow crystals in turn depend on the local conditions they have experienced such as temperature and humidity. One of the aims of avalanche research is to develop and validate computer models that can describe the time evolution of snow packs and predict the shear yield stress. A complicating factor is the large spatial variability that is typical.

Classification and Terminology

All avalanches share common elements: a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path along which the avalanche flows, a run out where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. As well avalanches have a failure layer that propagates the failure and the bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. Additionally slab avalanches have a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a shallow staunch fracture at the bottom of the start zone. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.

The nature of the failure of the snow pack is used to morphologically classify the avalanche. Slab avalanches are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab. Loose snow, point release, and isothermal avalanches are generated when a stress causes a shear failure in a weak interface, either within the snow pack, or at the base. When the failure occurs at the base they are known as full depth avalanches. Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.

Loose snow avalanches occur in freshly fallen snow that has a lower density and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. This is in contrast to a slab avalanche.

Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, cohesive layer of snow known as a slab. These are usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving almost instantaneously.

A third starting type is a wet snow avalanche or isothermal avalanche, which occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.

Among the largest and most powerful of avalanches, powder snow avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even up hill for short distances. A powder snow avalanches is a powder cloud that forms when an avalanche accelerates over an abrupt change in slope, such as a cliff band, causing the snow to mix with air. This turbulent suspension of snow particles then flows as a gravity current.

Terrain

Terrain affects avalanche occurrence and development through three factors: First, terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack. Second, terrain affects the stability of the snow pack, through the geometry and ground composition of the slope. Third, the down slope features of the terrain affects the path and consequences of a flowing avalanche.

For a slope to generate an avalanche it must be simultaneously capable of retaining snow, and allowing snow to accelerate once set in motion. The angle of the slope that can hold snow depends on the ductile and shear strength of the snow, which is determined by the temperature and moisture content of the snow. Drier and colder snow, with lower ductile and shear strength, will only bond to lower angle slopes; while wet and warm snow, with higher ductile and shear strength, can bound to very steep surfaces. In particular, in coastal mountains, such as the Cordillera del Paine region of Patagonia, deep snow packs collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on the shear strength of the snow. Snow that has been water saturated to the point of slush can accelerate on shallow angled terrain; while a cohesive snow pack will not accelerate on very steep slopes, such as the typical snow pack in the Chugach Mountains of Alaska.

The snow pack on slopes with sunny exposures are strongly influenced by sunshine. Daily cycles of mild thawing and refreezing can stabilize the snow pack by promoting settlement, strong freeze thaw cycles will result in the formation of surface crusts during the night, and the formation of unstable isothermal snow during the day. Slopes in the lee of a ridge or other wind obstacle accumulate more snow and are more likely to include pockets of abnormally deep snow, wind slabs, and cornices, all of which, when disturbed, may trigger an avalanche. Conversely a windward slope will be bare of snow.

The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally convex slopes are less stable than concave slopes, because of the disparity between the tensile strength of snow layers and their compressive strength. The composition and structure of the ground surface beneath the snow pack influences the stability of the snow pack, either being a source of strength or weakness. Vegetation, such as heavy timber, can anchor a snow pack; however, boulders and sparsely distributed vegetation will create weak areas deep within the snow pack, through the formation of strong temperature gradients. Full-depth avalanches (avalanches that sweep a slope virtually clean of snow cover) are more common on slopes with smooth ground cover, such as grass or rock slabs.

Avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line these drainages are well defined by vegetation boundaries where the avalanches have prevented the growth of large vegetation. Engineered drainages, such as the avalanche dam on Mount Stephen in Kicking Horse Pass, have been constructed to protect people and property, by redirecting the flow of avalanches. Deep debris deposits from avalanches will collect in catchments at the terminus of a run out, such as gullies and river beds, .

Slopes flatter than 25 degrees or steeper than 60 degrees typically have a lower incidence of avalanche involvement, likewise slopes with windward and sunny exposure have a lower incidence of avalanche involvement . Human triggered avalanches have the greatest incidence when the snow’s angle of repose is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. But when the incidence of human triggered avalanches are normalized by the rates of recreational use hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found. The rule of thumb is: A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle.

Snow Structure And Characteristics

The snow pack is composed of deposition layers of snow that are accumulated over time. The deposition layers are stratified parallel to the ground surface on which the snow falls. Each deposition layer indicates a distinct meteorological condition during which the snow was accumulated. Once deposited a snow layer will continue to evolve and develop under the influence of the meteorological conditions that prevail after deposition.

For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below a slab of cohesive snow. In practice the mechanical and structural determinants of snow pack stability are not directly observable outside of laboratories, thus the more easily observed properties of the snow layers (e.g. penetration resistance, grain size, grain type, temperature) are used as proxy measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snow pack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snow pack vary widely within small areas and time scales, resulting in an inability to extrapolate point observations of snow layers. Second, the understanding of the relationship between the readily observable snow pack characteristics and the snow pack’s critical mechanical properties has not been completely developed.

While the deterministic relationship between snow pack characteristics and snow pack stability is still a matter of ongoing scientific study, there is a growing empirical understanding of the snow composition and deposition characteristics that influence the likelihood of an avalanche. Observation and experience has shown that newly fallen snow requires time to bond with the snow layers beneath it, especially if the new snow falls during very cold and dry conditions. Shallower snow, that can lie above or around boulders, plants, and other discontinuities in the slope, will weaken from the presence of a stronger temperature gradient. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering process that forms bonds within the snow pack will also cause the snow crystals to become smaller and rounder. Consolidated snow is less likely to slough than either loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and will mask persistent instabilities within a snow pack. The empirical understanding of the factors influencing snow stability only places broad predictive bounds on the stability of the snow, consequently a conservative use of avalanche terrain, well within the recommended guidelines of the local avalanche forecasts and bulletins, is always recommended.

Weather

Avalanches can only occur in a standing snow pack. Typically winter seasons and high altitudes have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a snow pack. The evolution of the snow pack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snow pack. Among the critical factors controlling snow pack evolution are: heating by the sun, radiational cooling, vertical temperature gradients in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snow pack; and conversely very cold, windy, or hot weather will weaken the snow pack.

At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place. The melting and refreezing of water in the snow strengthens the snow pack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point of water, may cause a slope to avalanche, especially in the spring.

Persistent cold temperatures can either prevent the snow from stabilizing or destabilize a snow pack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snow pack is close to freezing; unless the snow pack is standing on glaciated terrain, in which case the temperature at the base of the snow pack can be significantly below freezing. When a temperature gradient greater than 10oC change per vertical meter of snow is sustained for more than a day depth hoar will form in the snow pack, through the thermal transport of moisture away from the depth hoar along the temperature gradient, from bottom to top. This layer of depth hoar becomes a persistent weakness in the snow pack, characterized by faceted grains forming either above or below crusts and slabs. When a slab lying on top of this persistent weakness is loaded by a force above the tensile and ductile strength of the slab and the shear strength of the persistent weak layer, the persistent weak layer will fail and generate an avalanche.

Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind pressure at a favorable angle can stabilize other slopes. A “wind slab” is a particularly fragile and brittle structure which is heavily-loaded and poorly-bonded to its underlayment. Even on a clear day, wind can quickly shift the snow load on a slope. This can occur in two ways: by top-loading and by cross-loading. Top-loading occurs when wind deposits snow perpendicular to the fall-line on a slope; cross-loading occurs when wind deposits snow parallel to the fall-line. When a wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading, from the top to the bottom of that lee slope. When the wind blows across a ridge that leads up the mountain, the leeward side of the ridge is subject to cross-loading. Cross-loaded wind-slabs are usually difficult to identify visually.

Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snow pack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.

Daytime exposure to sunlight will rapidly destabilize the upper layers of a snow pack. Sunlight reduces the sintering, or necking, between snow grains. During clear nights, the snow pack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snow pack, the heat stored in the snow is re-radiated into the atmosphere.

Triggers

Avalanches are always caused by an external stress on the snow pack, they are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, radiative and convective heating, rock fall, ice fall, and other sudden impacts; however, even a snow pack held at a constant temperature, pressure, and humidity will evolve over time and develop stresses, often from the downslope creep of the snow pack. Human triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress load can be either localized to the failure point, or remote. Localized triggers of avalanches are typified by point releases from solar heated rocks. Remotely triggered avalanches occur when a tensile stress wave is transmitted through the slab to the start zone, once the stress wave reaches the start zone a fracture initiates and propagates the failure. Of exceptional note is that avalanches can not only entrain additional snow within the failing slab, but can also, given the sufficient accumulation of overburden due to a smaller avalanche, step down and trigger deeper slab instabilities that would be more resilient against smaller stresses. The triggering of avalanches is an example of critical phenomena.