•Basic physical and functional unit of heredity
•Made up of DNA
•Each person has 2 copies of each gene, one inherited from each parent.
•Deoxyribonucleic Acid or DNA
•Contains the instructions an organism needs to develop, live, and reproduce
•Found inside every cell
•Passed down from parents to children (offspring)
•Thread-like molecules that carry heredity information
•Made of protein and one molecule of DNA
•Most have arranged pairs within the nucleus of the cell
•Each person has one pair of sex chromosomes in each cell.
•Females have two X chromosomes.
•Males have an X and Y chromosome.
•The Y chromosome contains a gene, which triggers embryonic development to become a male.
•Smallest structural, functional, and biological unit of all living organism
•Often called the “building blocks of life”
•Nucleus – organelle present in most eukaryotic cells – contains genetic material
•Type of reproduction by which offspring arise from a single organism
•Produced by mitosis
•Offspring inherit the genes of only one parent
•The offspring is genetically identical (uniform)of the parent.
•Type of cell division
•Results in two daughter cells having the same number and kind of chromosomes as the parent cell
•Cell goes through different phases before becoming clones of parent
Types of Asexual Reproduction
•Common in prokaryotes (organisms with no nucleus)
•Occurs in some single-celled eukaryotes (with a nucleus)
•Fully grown parent cell splits into two halves, producing two new cells
•Examples: bacteria and amoeba, and euglena
•Offspring grows out of the body of the parent (buds)
•The body of the parent breaks into distinct pieces
•Each piece can produce an offspring
•If a piece of a parent is detached, it can grow and develop into a completely new offspring.
•Example – some starfish
•A process by which new organisms arise without production of seeds or spores
•Example: some plants, potatoes
Advantages of Asexual Reproduction
•Good for organisms that are not mobile and cannot look for a mate
•Numerous offspring without “costing” the parent great amount to energy
Disadvantages of Asexual Reproduction
•Lacks genetic variation
•Because organisms are the same, they share the same weaknesses
•If the environment changes, there may not be time to adapt quickly enough to survive.
•Type of reproduction by which offspring arise from two parents.
•The testis in the male produces male gametes or sperm.
•The ovary in the female produce female gametes or ovum.
•Gametes (sex cells) are formed by meiosis.
•A type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.
•Goes through many phases
•Required to produce egg and sperm cells for sexual reproduction
Mitosis vs Meiosis
•Mitosis produces 2 diploid cells, which are identical to the parents. (uniform)
•Meiosis produces 4 haploid cells, which contain some characteristics of the parent cell but are not identical. (diverse)
•A sperm enters an ova during fertilization.
•Each gamete contains 23 pairs of chromosomes.
•The two fuse to form a zygote with 46 pairs of chromosomes.
•Offspring appearance vary due to new combinations of genes.
•The zygote then divides by mitosis
•It passes through different developmental phases to transform into a multicellular individual
•The offspring are genetically different (not identical) to their parents
Advantages of Sexual Reproduction
•Leads to genetic variations in new generations, which is fundamental for environmental adaptation
•Organism is more protected – does not necessarily have the weakness of parent
•Removes bad genes from the population
Disadvantages of Sexual Reproduction
•Organism must find a mate
•Takes longer time to reproduce
•Can prevent favorable genes from being passed down
•Produces fewer offspring
Some Organisms Reproduce Both Ways
•Some plants and animals can reproduce both ways
•There are benefits to this adaptation. What might they be?
•Plant examples: fungi, strawberries, daffodils
•Animal examples: starlet sea anemone, jellyfish, sponges
•A change that occurs in the DNA sequence
•Causes changes in an organism – its appearance, how it behaves, and how it functions
•Mutations are essential to evolution – the raw material of genetic variation
•Sickle cell anemia
•Wingless Fruit Fly
•Sickle cell anemia is a genetic disease with severe symptoms
•Caused by a the mutation of the gene that helps make hemoglobin (carries O2 to red blood cells)
•The “R” is the dominate gene and the “r” recessive
Earthquakes, also called temblors, can be so tremendously destructive, it’s hard to imagine they occur by the thousands every day around the world, usually in the form of small tremors.
WHERE DO MOST EARTHQUAKES OCCUR?
Some 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire" because of the preponderance of volcanic activity there as well. Most earthquakes occur at fault zones, where tectonic plates—giant rock slabs that make up the Earth's upper layer—collide or slide against each other.
These impacts are usually gradual and unnoticeable on the surface; however, immense stress can build up between plates. When this stress is released quickly, it sends massive vibrations, called seismic waves, often hundreds of miles through the rock and up to the surface. Other quakes can occur far from faults zones when plates are stretched or squeezed.
EARTHQUAKE MAGNITUDE RATINGS AND THEIR IMPACTS
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.
Earthquake Safety TipsEarthquakes are a common occurrence, rumbling below Earth's surface thousands of times every day. But major earthquakes are less common. Here are some things to do to prepare for an earthquake and what to doonce the ground starts shaking.
SAFETY TIPS• Have an earthquake readiness plan.
• Consult a professional to learn how to make your home sturdier, such as bolting bookcases to wall studs, installing strong latches on cupboards, and strapping the water heater to wall studs.
• Locate a place in each room of the house that you can go to in case of an earthquake. It should be a spot where nothing is likely to fall on you, like a doorframe.
• Keep a supply of canned food, an up-to-date first aid kit, 3 gallons (11.4 liters) of water per person, dust masks and goggles, and a working battery-operated radio and flashlights in an accessible place.
• Know how to turn off your gas and water mains.
IF SHAKING BEGINS• Drop down; take cover under a desk or table and hold on.
• Stay indoors until the shaking stops and you're sure it's safe to exit.
• Stay away from bookcases or furniture that can fall on you.
• Stay away from windows. In a high-rise building, expect the fire alarms and sprinklers to go off during a quake.
• If you are in bed, hold on and stay there, protecting your head with a pillow.
• If you are outdoors, find a clear spot away from buildings, trees, and power lines. Drop to the ground.
• If you are in a car, slow down and drive to a clear place. Stay in the car until the shaking stops.
Uncontrolled blazes fueled by weather, wind, and dry underbrush, wildfires can burn acres of land—and consume everything in their paths—in mere minutes.
On average, more than 100,000 wildfires, also called wildland fires or forest fires, clear 4 million to 5 million acres (1.6 million to 2 million hectares) of land in the U.S. every year. In recent years, wildfires have burned up to 9 million acres (3.6 million hectares) of land. A wildfire moves at speeds of up to 14 miles an hour (23 kilometers an hour), consuming everything—trees, brush, homes, even humans—in its path.
HOW THEY ARE FORMED
There are three conditions that need to be present in order for a wildfire to burn, which firefighters refer to as the fire triangle: fuel, oxygen, and a heat source. Fuel is any flammable material surrounding a fire, including trees, grasses, brush, even homes. The greater an area's fuel load, the more intense the fire. Air supplies the oxygen a fire needs to burn. Heat sources help spark the wildfire and bring fuel to temperatures hot enough to ignite. Lightning, burning campfires or cigarettes, hot winds, and even the sun can all provide sufficient heat to spark a wildfire.
Although four out of five wildfires are started by people, nature is usually more than happy to help fan the flames. Dry weather and drought convert green vegetation into bone-dry, flammable fuel; strong winds spread fire quickly over land; and warm temperatures encourage combustion. When these factors come together all that's needed is a spark—in the form of lightning, arson, a downed power line, or a burning campfire or cigarette—to ignite a blaze that could last for weeks and consume tens of thousands of acres.
These violent infernos occur around the world and in most of the 50 states, but they are most common in the U.S. West, where heat, drought, and frequent thunderstorms create perfect wildfire conditions. Montana, Idaho, Wyoming, Washington, Colorado, Oregon, and California experience some of the worst conflagrations in the U.S. In California wildfires are often made worse by the hot, dry Santa Ana winds, which can carry a spark for miles.
HOW TO CONTROL OR STOP THEM
Firefighters fight wildfires by depriving them of one or more of the fire triangle fundamentals. Traditional methods include water dousing and spraying fire retardants to extinguish existing fires. Clearing vegetation to create firebreaks starves a fire of fuel and can help slow or contain it. Firefighters also fight wildfires by deliberately starting fires in a process called controlled burning. These prescribed fires remove undergrowth, brush, and ground litter from a forest, depriving a wildfire of fuel.
Although often harmful and destructive to humans, naturally occurring wildfires play an integral role in nature. They return nutrients to the soil by burning dead or decaying matter. They also act as a disinfectant, removing disease-ridden plants and harmful insects from a forest ecosystem. And by burning through thick canopies and brushy undergrowth, wildfires allow sunlight to reach the forest floor, enabling a new generation of seedlings to grow.
Unlike many natural disasters, most wildfires are caused by people—and can be prevented by people, too. Meteorologists are not yet able to forecast wildfire outbreaks, so people in fire-prone areas should plan ahead and prepare to evacuate with little notice.
Here are some tips on how to prevent wildfires and what to do if you're caught in the middle of one.
HOW TO PREVENT A WILDFIRE
BEFORE YOU LEAVE, PREPARE YOUR HOUSE
IF CAUGHT IN A WILDFIRE
Volcanoes are manifestations of the fiery power contained deep within the Earth. These formations are essentially vents on the Earth's surface where molten rock, debris, and gases from the planet's interior are emitted.
Lava began flowing from the Mayon volcano in the Philippines January 15, and thousands have evacuated the area. A "hazardous eruption is possible within weeks or even days," a bulletin from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) warned. Mayon has erupted many times over the past few centuries, and sometimes has been deadly—more than 70 were killed in a 1993 eruption, and an entire town was destroyed in 1814.
HOW VOLCANOES ARE MADE
When thick magma and large amounts of gas build up under the surface, eruptions can be explosive, expelling lava, rocks and ash into the air. Less gas and more viscous magma usually mean a less dramatic eruption, often causing streams of lava to ooze from the vent.
The mountain-like mounds that we associate with volcanoes are what remain after the material spewed during eruptions has collected and hardened around the vent. This can happen over a period of weeks or many millions of years.
A large eruption can be dangerous for people living near a volcano. Flows of searing lava, which can reach 2,000 degrees Fahrenheit (1,250 degrees Celsius) or more, can be released, burning everything in their path. Boulders of hardening lava can rain down on villages. Mud flows from rapidly melting snow can strip mountains and valleys bare and bury towns.
Ash and toxic gases can cause lung damage and other problems, particularly for infants and the elderly. Scientists estimate that more than 260,000 people have died in the past 300 years from volcanic eruptions and their aftermath.
WHERE ARE EARTH'S VOLCANOES?
Volcanoes tend to exist along the edges between tectonic plates, massive rock slabs that make up Earth's surface. About 90 percent of all volcanoes exist within the Ring of Fire along the edges of the Pacific Ocean.
About 1,900 volcanoes on Earth are considered active, meaning they show some level of activity and are likely to explode again. Many other volcanoes are dormant, showing no current signs of exploding but likely to become active at some point in the future. Others are considered extinct.
Volcano Safety Tips
A volcanic eruption can be an awesome and destructive event. Here are some tips on how to avoid danger and what to do if you're caught near an eruption.
SAFETY TIPS• As much as possible, stay away from active volcanoes.
• If you live near an active volcano, keep goggles and a mask in an emergency kit, along with a flashlight and a working, battery-operated radio.
• Know your evacuation route. Keep gas in your car.
IF A VOLCANO ERUPTS IN YOUR AREA• Evacuate only as recommended by authorities to stay clear of lava, mud flows, and flying rocks and debris.
• Avoid river areas and low-lying regions.
• Before you leave the house, change into long-sleeved shirts and long pants and use goggles or eyeglasses, not contacts. Wear an emergency mask or hold a damp cloth over your face.
• If you are not evacuating, close windows and doors and block chimneys and other vents, to prevent ash from coming into the house.
• Be aware that ash may put excess weight on your roof and need to be swept away. Wear protection during cleanups.
• Ash can damage engines and metal parts, so avoid driving. If you must drive, stay below 35 miles (56 kilometers) an hour.
Floods are among Earth's most common–and most destructive–natural hazards.
LATEST NEWSWest Japan is experiencing widespread flooding after Typhoon Lan touched down in late October 2017. The powerful storm resulted in landslides, torrential rain, and massive waves that injured over ninety people and left three dead.
HOW FLOODS DEVELOP
A flood occurs when water overflows or inundates land that's normally dry. This can happen in a multitude of ways. Most common is when rivers or streams overflow their banks. Excessive rain, a ruptured dam or levee, rapid ice melting in the mountains, or even an unfortunately placed beaver dam can overwhelm a river and send it spreading over the adjacent land, called a floodplain. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.
Most floods take hours or even days to develop, giving residents ample time to prepare or evacuate. Others generate quickly and with little warning. These flash floods can be extremely dangerous, instantly turning a babbling brook into a thundering wall of water and sweeping everything in its path downstream.
Disaster experts classify floods according to their likelihood of occurring in a given time period. A hundred-year flood, for example, is an extremely large, destructive event that would theoretically be expected to happen only once every century. But this is a theoretical number. In reality, this classification means there is a one-percent chance that such a flood could happen in any given year. Over recent decades, possibly due to global climate change, hundred-year floods have been occurring worldwide with frightening regularity.
Moving water has awesome destructive power. When a river overflows its banks or the sea drives inland, structures poorly equipped to withstand the water's strength are no match. Bridges, houses, trees, and cars can be picked up and carried off. The erosive force of moving water can drag dirt from under a building's foundation, causing it to crack and tumble.
In the United States, where flood mitigation and prediction is advanced, floods do about $6 billion worth of damage and kill about 140 people every year. A 2007 report by the Organization for Economic Cooperation and Development found that coastal flooding alone does some $3 trillion in damage worldwide. In China's Yellow River valley, where some of the world's worst floods have occurred, millions of people have perished in floods during the last century.
When floodwaters recede, affected areas are often blanketed in silt and mud. The water and landscape can be contaminated with hazardous materials, such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures. Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera.
But flooding, particularly in river floodplains, is as natural as rain and has been occurring for millions of years. Famously fertile floodplains like the Mississippi Valley in the American Midwest, the Nile River valley in Egypt, and the Tigris-Euphrates in the Middle East have supported agriculture for millennia because annual flooding has left millions of tons of nutrient-rich silt deposits behind.
Here are some safety tips to prepare for rising water—and what to do once a flood has begun.
BEFORE A FLOOD
• Avoid building in a floodplain.
• Construct barriers (levees, beams, floodwalls) to stop floodwater from entering your home.
• Seal walls in basements with waterproofing compounds to avoid seepage.
• If a flood is likely in your area, listen to the radio or television for information.
• Know the difference between a flood watch and a flood warning. A watch means flooding is possible. A warning means flooding is occurring or will occur soon.
WHEN A FLOOD IS IMMINENT• Be prepared! Pack a bag with important items in case you need to evacuate. Don't forget to include needed medications.
• If advised to evacuate your home, do so immediately.
• If there is any possibility of a flash flood, move immediately to higher ground.
• If possible, bring in outdoor furniture and move essential items to an upper floor.
• Turn off utilities at the main switches or valves if instructed to do so. Disconnect electrical appliances.
DURING A FLOOD
• Do not walk through moving water. As little as 6 inches (15 centimeters) of moving water can make you fall.
• If you have to walk in water, wherever possible, walk where the water is not moving. Use a stick to check the firmness of the ground in front of you.
• Do not drive into flooded areas. If floodwaters rise around your car, abandon the car and move to higher ground if you can do so safely.
• Do not touch electrical equipment if you are wet or standing in water.
AFTER A FLOOD
• Listen for news reports to learn whether the community's water supply is safe to drink.
• Avoid floodwaters; water may be contaminated by oil, gasoline, or raw sewage. Water may also be electrically charged from underground or downed power lines.
• Avoid moving water.
• Be aware of areas where floodwaters have receded. Roads may have weakened and could collapse under the weight of a car.
• Stay away from downed power lines, and report them to the power company.
• Return home only when authorities indicate it is safe.
• Stay out of any building if it is surrounded by floodwaters.
• Service damaged septic tanks, cesspools, pits, and leaching systems as soon as possible. Damaged sewage systems are serious health hazards.
• Clean and disinfect everything that got wet. Mud left from floodwater can contain sewage and chemicals.
Lightning is an electrical discharge caused by imbalances between storm clouds and the ground, or within the clouds themselves. Most lightning occurs within the clouds.
"Sheet lightning" describes a distant bolt that lights up an entire cloud base. Other visible bolts may appear as bead, ribbon, or rocket lightning.
During a storm, colliding particles of rain, ice, or snow inside storm clouds increase the imbalance between storm clouds and the ground, 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.
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.
TYPES OF LIGHTNING
Cloud-to-ground lightning bolts are a common phenomenon—about 100 strike Earth’s surface every single second—yet their power is extraordinary. Each bolt can contain up to one billion volts of electricity. A typical cloud-to-ground lightning bolt begins when a step-like series of negative charges, called a stepped leader, races downward from the bottom of a storm cloud toward the Earth along a channel at about 200,000 mph (300,000 kph). 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. When the two connect, an electrical current flows as negative charges fly down the channel towards earth and a visible flash of lightning streaks upward at some 200,000,000 mph (300,000,000 kph), transferring electricity as lightning in the process.
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.
About one to 20 cloud-to-ground lightning bolts is "positive lightning," a type that originates in the positively charged tops of stormclouds. These strikes reverse the charge flow of typical lightning bolts and are far stronger and more destructive. Positive lightning can stretch across the sky and strike "out of the blue" more than 10 miles from the storm cloud where it was born.
THE IMPACT OF A LIGHTNING STRIKE
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. Strikes can cause cardiac arrest and severe burns, but 9 of every 10 people survive. The average American has about a 1 in 5,000 chance of being struck by lightning during a lifetime.
Lightning's extreme heat will vaporize the water inside a tree, creating steam that may blow the tree apart. Cars are havens from lightning—but not for the reason that most believe. Tires conduct current, as do metal frames that carry a charge harmlessly to the ground.
Many houses are grounded by rods and other protection that conduct a lightning bolt's electricity harmlessly to the ground. Homes may also be inadvertently grounded by plumbing, gutters, or other materials. Grounded buildings offer protection, but occupants who touch running water or use a landline phone may be shocked by conducted electricity.
Lightning Safety Tips
Lightning kills as many as 2,000 people worldwide every year. Hundreds more people are struck but survive, usually with lingering and debilitating symptoms.
Here are some things you can do to avoid electrical storms and decrease your chances of getting struck.
WHEN IT HITS• If you are outside, seek refuge in a car or grounded building when lightning or thunder begins.
• If you are caught outside away from a building or car, stay clear of water bodies and tall objects like trees. Find a low spot or depression and crouch down as low as possible, but don't lie down on the ground. Lightning can move in and along the ground surface, and many victims are struck not by bolts but by this current.
• If you are inside, avoid taking baths or showers and don't wash dishes. Also avoid using landline phones, televisions, and other appliances that conduct electricity.
ONCE IT'S OVER• Stay inside for 30 minutes after you last see lightning or hear thunder. People have been struck by lightning from storms centered as far as 10 miles (16 kilometers) away.
Tornadoes are vertical funnels of rapidly spinning air. Their winds may top 250 miles (400 kilometers) an hour and can clear a pathway a mile (1.6 kilometers) wide and 50 miles (80 kilometers) long.
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 the area in the eastern state of 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 is a violently rotating column of air that extends from a thunderstorm to the ground. It's often portended by a dark, greenish sky. Black storm clouds gather. Baseball-size hail might fall. A funnel suddenly appears, as though descending from a cloud. The funnel hits the ground and roars forward with a sound like that of a freight train approaching. The tornado tears up everything in its path.
Some of Earth's most violent events, nearly a thousand tornadoes—many of them deadly—touch down every year in the United States. Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120. Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States.
Related to tornadoes, waterspouts are weak twisters that form over warm water. They sometimes move inland and become tornadoes.
Dust devils are small, rapidly rotating columns of air that are made visible by the dust and dirt they pick up. Dust devils are not associated with thunderstorms.
WHAT CAUSES TORNADOES?
The most violent tornadoes come from supercells, large thunderstorms that have winds already in rotation. About one in a thousand storms becomes a supercell, and one in five or six supercells spawns off a tornado.
Tornado season begins in early spring for the states along the Gulf of Mexico. The season follows the jet stream—as it swings farther north, so does tornado activity. May generally has more tornadoes than any other month, but April's twisters are usually more violent.
Although they can occur at any time of the day or night, most tornadoes form in the late afternoon. By this time the sun has heated the ground and the atmosphere enough to produce thunderstorms.
Tornadoes form when warm, humid air collides with cold, dry air.
The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction.
As the rotating updraft, called a mesocycle, draws in more warm air from the moving thunderstorm, its rotation speed increases. Cool air fed by the jet stream, a strong band of wind in the atmosphere, provides even more energy.
Water droplets from the mesocyclone's moist air form a funnel cloud. The funnel continues to grow and eventually it descends from the cloud. When it touches the ground, it becomes a tornado.
CHARACTERISTICS OF TORNADOES
Twisters are usually accompanied or preceded by severe thunderstorms and hlgh wlnds. Hail is also common.
Once a tornado hits the ground, it may live for as little as a few seconds or as long as three hours.
The average twister is about 660 feet (200 meters) wide and moves about 30 miles (50 kilometers) an hour. Most don't travel more than six miles (ten kilometers) before dying out.
Massive tornadoes, however—the ones capable of widespread destruction and many deaths—can roar along as fast as 300 miles (480 kilometers) an hour.
These measurements are scientists' best estimations. Anemometers, which measure wind speed, cannot withstand the enormous force of tornadoes to record them.
Using units F0 to F5, the Fujita scale measures a tornado's intensity by analyzing the damage a tornado has done and then matching that to the wind speeds estimated to produce comparable damage.
What Damage Do Tornadoes Do?
Every year in the United States, tornadoes do about 400 million dollars in damage and kill about 70 people on average.
Extremely high winds tear homes and businesses apart. Winds can also destroy bridges, flip trains, send cars and trucks flying, tear the bark off trees, and suck all the water from a riverbed.
High winds sometimes kill or injure people by rolling them along the ground or dropping them from dangerous heights. But most tornado victims are struck by flying debris—roofing shingles, broken glass, doors, metal rods.
The number of average deaths per year in the United States used to be higher before improved forecasting and warning systems were put into place.
HOW ARE TORNADOES FORECASTED?
Meteorologists at the U.S. National Weather Service use Doppler radar, satellites, weather balloons, and computer modeling to watch the skies for severe storms and tornadic activity.
Doppler radars record wind speeds and identify areas of rotation within thunderstorms. Since Doppler radar has been in use, the warning time for tornadoes has grown from fewer than five minutes in the 1980s to an average of 13 minutes today.
When weather conditions are conducive for tornado formation, the National Weather Service issues a tornado watch. When a tornado has been sighted or indicated on radar, a tornado warning is issued.
Some scientists, meteorology buffs, and adrenaline junkies hit the road during tornado season to chase storms. Researchers race to place sensors in tornadoes' paths. The sensors measure data such as wind speed, barometric pressure, humidity, and temperature.
The challenge for researchers is being in the right place at the right time, a nearly impossible feat. Every morning they study weather conditions and head for the area that seems most likely to spawn a twister. They drive through severe storms, dodge lightning, face flash floods, and get pounded by hail—sometimes for years—before ever spotting a tornado.
Tornado Safety Tips
Tornadoes are one of nature's most powerful and destructive forces. Here's some advice on what a tornado is, how to prepare for one, and what to do if you're caught in a twister's path.
HOW DOES A TORNADO FORM?
A tornado is a violently rotating column of air that extends from a thunderstorm to the ground. It's often portended by a dark, greenish sky. Black storm clouds gather. Baseball-size hail might fall. A funnel suddenly appears, descending from a cloud. The funnel hits the ground and roars forward with a sound like that of a freight train approaching. The tornado tears up everything in its path.
Some of Earth's most violent events, nearly a thousand tornadoes—many of them deadly—touch down every year in the United States. Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120. Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but most tornadoes occur in the United States.
HOW TO STAY SAFE• 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.
Hurricanes are giant, spiraling tropical storms that can pack wind speeds of over 160 miles (257 kilometers) an hour and unleash more than 2.4 trillion gallons (9 trillion liters) of rain a day. These same tropical storms are known as cyclones in the northern Indian Ocean and Bay of Bengal, and as typhoons in the western Pacific Ocean.
The Atlantic Ocean’s hurricane season peaks from mid-August to late October and averages five to six hurricanes per year.
Centuries ago, the Spanish used huracan, an indigenous word for evil spirits and weather gods, to name the storms that sank their ships in the Caribbean. Today "hurricane" is one of three names for a rotating tropical storm with winds of at least 74 miles (119 kilometers) an hour. These storms are called hurricanes when they develop over the Atlantic or eastern Pacific Oceans. They are cyclones when they form over the Bay of Bengal and the northern Indian Ocean, and they are typhoons when they develop in the western Pacific.
Whatever their names, these storms are capable of annihilating coastal areas and causing massive death tolls.
HOW ARE HURRICANES FORMED?
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.
A DAMAGING STORM
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.
Hurricane Safety Tips
BEFORE IT HITS
• Coastal residents should form evacuation plans before a warning is issued to identify a safe shelter and a route to get there.
• Prepare for a hurricane by stocking up on emergency supplies including food, water, protective clothing, medications, batteries, flashlights, important documents, road maps, and a full tank of gasoline.
DURING THE STORM
• As a storm unfolds, evacuees should listen to local authorities on radio or television. Evacuation routes often close as a storm develops. Dedicated professionals and improved technology have made hurricane forecasting more accurate than ever before—but it’s far from precise.
• If forced to weather a storm, get inside the most secure building possible and stay away from windows.
• Remember that a lull often signifies the storm’s eye—not its end. Anyone riding out a hurricane should wait for authorities to announce that the danger has passed.
While avalanches are sudden, the warning signs are almost always numerous before they let loose.
Yet in 90 percent of avalanche incidents, the snow slides are triggered by the victim or someone in the victim's party. Avalanches kill more than 150 people worldwide each year. Most are snowmobilers, skiers, and snowboarders.
TYPES OF AVALANCHES
Many avalanches are small slides of dry powdery snow that move as a formless mass. These "sluffs" account for a tiny fraction of the death and destruction wrought by their bigger, more organized cousins.
Disastrous avalanches occur when massive slabs of snow break loose from a mountainside and shatter like broken glass as they race downhill. These moving masses can reach speeds of 80 miles (130 kilometers) per hour within about five seconds. Victims caught in these events seldom escape.
Avalanches are most common during and in the 24 hours right after a storm that dumps 12 inches (30 centimeters) or more of fresh snow. The quick pileup overloads the underlying snowpack, which causes a weak layer beneath the slab to fracture. The layers are an archive of winter weather: Big dumps, drought, rain, a hard freeze, and more snow. How the layers bond often determines how easily one will weaken and cause a slide.
Storminess, temperature, wind, slope steepness and orientation (the direction it faces), terrain, vegetation, and general snowpack conditions are all factors that influence whether and how a slope avalanches. Different combinations of these factors create low, moderate, considerable, and high avalanche hazards.
WHAT TO DO IN AN AVALANCHE
If caught in an avalanche, try to get off the slab. In most instances, this is not easy. Skiers and snowboarders can head straight downhill to gather speed, then veer left or right out of the slide path. Snowmobilers can punch the throttle to power out of harm's way. No escape? Reach for a tree. No tree? Swim hard. The human body is three times denser than avalanche debris and will sink quickly. As the slide slows, clear air space to breathe. Then punch a hand skyward.
Once the avalanche stops, it settles like concrete. Bodily movement is nearly impossible. Wait—and hope—for a rescue. Statistics show that 93 percent of avalanche victims survive if dug out within 15 minutes. Then the survival rates drop fast. After 45 minutes, only 20 to 30 percent of victims are alive. After two hours, very few people survive.
Avalanche Safety Tips
Avalanches can occur without warning, sending thousands of tons of debris and ice downhill at breakneck speeds.
Every year, hundreds of people—usually skiers, snowboarders, or snowmobilers—get caught in avalanches. Here are some key steps you can take to avoid avalanches and actions to take if you or someone you're with gets caught in a snowslide.
•Wear an avalanche rescue beacon that signals your location.
• Learn how to use the rescue equipment.
• Practice using the rescue equipment.
• Constantly evaluate avalanche conditions.
• Areas with fresh accumulations of wind-driven snow are particularly vulnerable.
• Extremely steep slopes particularly in shaded areas near a ridge are also risky.
• Always travel with a partner. Descend risky areas one by one and watch for avalanche signs.
WHAT TO DO IF CAUGHT
• If caught in a slide, try to get off the slab or grab a tree.
• If swept away, swim to the surface.
• Carry a small shovel and a long probe to locate a buried partner.
• Evaluate the avalanche hazard before attempting a rescue.
Erik E. Mason