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Below are definitions of important terms related to forces of nature.

• active volcano: a volcano erupting or liable to erupt
• advection fog: a cloud of water droplets formed when warm, moist air travels over a cold surface and condenses
• aftershocks: smaller quakes that follow a major earthquake
• air current: a movement of air
• air mass: a very large area of air with the same temperature throughout
• air pressure: the force with which air presses down on the ground below
• angle of rest: angle at which material will remain stable
• angle of repose: angle at which material will remain stable
• artificial levee: raised banks that increase a river’s water capacity
• asthenosphere: the hot top part of the earth’s mantle, just under the crust
• atmosphere: the thin blanket of air that surrounds the earth
• atmospheric pressure: local atmospheric conditions that send the barometer up or down; up foretells good weather; down means bad weather
• avalanche: a large mass of snow, ice, soil or rock, which detaches from a mountain slope and slides or falls suddenly downward; the sliding or falling of rocks, snow or other materials down the side of a mountain
• avalauncher: a cannon, powered by compressed nitrogen, that can hurl a two-pound projectile 2,000 yards. It is used to break up unstable snow, allowing it to avalanche and leaving more stable snow in place
• backdraft: inside buildings, when the oxygen in a room is almost used up, the fire begins to die down from lack of the substance; the flames lower and the room fills with smoke; but if you open the door to the room at that time, the fire sucks oxygen in so hard that fire gases explode
• Bannwalder: an area of woodland in an avalanche zone, which cannot be cut or disturbed because it is a natural barrier to avalanches
• banned woods: an area of woodland in an avalanche zone, which cannot be cut or disturbed because it is a natural barrier to avalanches
• barometer: device for measuring air pressure
• barometric pressure: another term for air pressure
• barrier beach: a strip of sand running along a coast, which protects the mainland from waves and erosion
• Beaufort scale: a way to measure wind strength
• black blizzard: a popular term for a dust storm
• body wave: a vibration from an earthquake that travels deep underground
• breakaway zone: the area where an avalanche is most likely to start or break away
• bush: open forest land
• bypass channels: special structures that redirect overflowing dam water
• caldera: a large, more or less circular, more-than-a-mile-in-diameter depression formed either by collapse or by explosion, usually at the summit of a volcano
• cinders: nut-sized pieces of red or black lava containing bubble-like cavities known as vesicles
• climate: the weather pattern in a region over a long period of time
• coastal flooding: seashore flooding caused by high tides usually brought about by storms
• complex faults: a series of faults that may tilt rocks in many different directions
• concrete dam: a man-made dam erected across a river valley to hold back water
• condensation: the process by which water vapor turns into liquid water
• conflagration: when a fire burns a large area, sometimes an entire town
• continental drift: the breakup of the earth’s original single landmass into continents that then separated; they are still drifting apart today
• crater, volcanic: a steep-walled depression at the top or on the flank of a volcanic cone, out of which the volcanic materials are ejected
• creeping: the slow, gradual movement of a hill’s upper layers caused by the pull of gravity on loose stones, gravel, and soft topsoil
• crust: the outside solid part of the earth’s surface
• cumulonimbus: a very large thundercloud that brings heavy rain, hail, or snow
• cyclone: a violent, spiraling tropical storm with fierce rotating winds and a calm central eye; usually develops over warm tropical seas; these storms are known as cyclones in the Indian Ocean
• dam: a barrier made of any material, which stops the flow of rivers and streams
• death toll: the total number of people who die as a result of a disaster
• debris basin: a concrete, bowl-shaped pit built on a mountainside to catch debris from mud and rock slides
• delta: a triangular area of swampy land created here the mouth of a river branches into several streams
• dendrochronology: the science of studying tree rings
• depth hoar: a layer of snow made up of round or cup-shaped crystals that act like ball bearings, allowing the layer of snow on top to slide of easily; also known as sugar snow
• dew point: the point at which water vapor condenses into droplets of water
• dike: a barrier usually made of earth, which runs alongside a river to keep it from overflowing at high water
• disaster: a manifestation of an interaction between extreme physical or natural phenomena and a vulnerable human group that results in general disruption and destruction, loss of life and livelihood and injury (Tilling, Volcanic Hazards, fig. 1.1, p. 3)
• doldrums: periods of calm winds and light breezes in the tropics
• dormant: when a volcano remains inactive for a long time
• drought: a period when it rains less often than usual
• dry line: the boundary where winds from the Gulf of Mexico and Canada meet and clash
• dust devil: a small, short-lived dust vortex that whirls along at 20 mph (3 kph) and about 1,640 feet (500 m) up in the air; created by ground heating
• dysentery: an infection of the intestine caused by drinking polluted water
• earthen dams: dams made up of earth and stone
• earthflow: when the wet ground breaks up and falls down a mountainside, making a rounded, tongue-like shape; usually occurs on clay, silt, or sand; the slowest, driest flow of weathered material down a hillside
• earthquake: a trembling and shaking of the earth’s surface; a vibration or trembling of the ground caused by movement of the subsurface
• El Niño: an unusually warm current of water that appears periodically in the Pacific Ocean, along the equator
• El Viejo: another term for La Niña
• embankment: raised banks that increase a river’s water capacity
• epicenter: the point on the earth’s surface directly above the focus of the quake
• erosion: the natural wearing away or weathering of rocks or soil
• eruption: the sudden emergence of hot material from the earth’s interior
• ESSA: Environmental Science Service Administration, replaced by NOAA (National Oceanic and Atmospheric Administration) in 1970
• evacuation: the movement of people out of an area because of war or disaster
• evaporation: the process by which liquid water turns into water vapor
• extinct volcano: volcanoes no longer liable to erupt
• eye: the calm, cloudless center of a hurricane around which the storm winds whirl
• famine: a severe shortage of food, often brought on by drought
• fault: a crack in a layer of rock, usually caused by an earthquake or other movement in a rock layer; great splits between masses of rock at the earth’s surface
• fire fountain: a jet of magma spouting from a narrow volcanic vent
• fireline: a strip of land from which all brush and debris have been cleared to rob a wildfire of its fuel
• firestorm: huge fires caused by enormous numbers of separate fires all burning together
• fissure volcano: a long crack in the Earth’s crust through which magma erupts
• flash flood: the flooding of an area that occurs in a matter of hours
• flashover: when burning objects heat walls and other objects in an enclosed area to their ignition temperature, causing them to flame; marked by large increase in flame volume and a sudden, marked rise in gas temperature
• flash point: the temperature at which something ignites
• flood: the covering or submerging by water of land that is usually dry
• floodgates: locks or gates built across a river, which can be opened or closed to prevent flooding downstream
• flood plain: the plain next to a river where flooding occurs
• focus: the point where the earthquake begins below the earth’s surface
• foreshock: a small tremor before a major earthquake
• fossil: the impression of an ancient animal or plant left in a rock deposit, or the animal or plant itself
• front: the edge of an air mass
• funnel: the spinning cloud that reaches down to earth from the storm
• gallery: a wooden, steel or concrete barrier or bridge built in known avalanche paths; a gallery allows cascading snow to pass over highways and railroad racks
• geologist: a scientist who studies the origin, history, structure, and processes of the earth
• global warming: the theory that average temperatures will rise throughout the world
• gravitational pull: the force that draws all bodies in the universe toward one another
• greenhouse effect: the process by which the heat of the sun becomes trapped in our planet’s atmosphere, raising the global temperature
• hailstones: small pellets of ice that fall from cumulonimbus clouds
• haze: dry dust particles in the air
• high-pressure system: an area in which the air pressure is high
• horst: a block of rock thrown up between normal faults
• hurricane: an extremely violent, whirling storm that forms over the oceans in the tropics; a severe tropical storm with winds over 75 miles an hour; in the Atlantic, the storm is called a hurricane; in the Pacific, it is known as a typhoon
• hurricane warning: the next stage of a weather alert after a hurricane watch; it means the storm may hit within 24 hours
• hurricane watch: the first weather alert advising people that a hurricane is in the general area
• hydrologic cycle: the continuous process by which water evaporates and enters the air as water vapor, then falls back out as rain, only to evaporate again as water vapor
• hypocenter: the place below ground where an earthquake starts
• inferno: a huge, roaring fire
• jet streams: high-speed currents of air circling the world at heights of about 6 miles
• lahars: volcanic mudflows and debris flows
• landslides: the falling of masses of earth or rock
• La Niña: an unusually cold current of water that appears periodically in the Pacific Ocean, near the equator
• lava: liquid, molten rock
• levee: a barrier usually made of earth or clay, which runs alongside a waterway to keep it from overflowing
• Love wave: a type of surface wave where motion is horizontal
• low-pressure system: an area in which the air pressure is low
• magma: hot molten rock created by the partial melting of the crust and mantle at high temperatures far below the ground
• magma chamber: a cavity beneath a volcano where magma collects
• magnitude: the size of an earthquake as measured on the Richter scale
• mantle: fiercely hot rock 1,800 miles deep beneath the crust
• mass movement: the downhill movement of rock and weathered material
• Mercalli Scale: a measure of an earthquake’s intensity, ranging from 1 to 12
• mesocyclone: the column of storm winds stretching upward and downward through the storm clouds, and from which the tornado funnels drop
• metamorphose: scientific term to describe the change in snow as it settles
• meteorologist: a scientist who studies and predicts the weather
• monsoon: a wind that blows northward from the Indian Ocean toward the Asian continent, carrying moisture-laden air with it
• mudflow: when a slope is so heavily saturated with water that it rushes downhill as a muddy river, carrying down debris and spreading out at the base of the slope; the fastest, wettest flow of weathered material down a hillside
• National Oceanic and Atmospheric Administration (NOAA): a government agency that in part is concerned with weather and flooding
• natural disaster: any terrible event, not caused by human activity, that results in deaths, injuries, or damage to property
• normal faults: a fault in which blocks of rock slip straight down; also called dip-slip faults
• oblique-slip fault: a fault in which blocks of rock slip up or down, and then past each other diagonally; when it happens on a large scale, they are known as transtension or transpression faults
• paleontologist: a scientist who studies fossils to determine facts about prehistoric plants and creatures
• plates: giant slabs of underground rocks, often bigger than continents
• plug: a tower of hard rock left by a volcano
• primary wave: fast-moving body waves that push and pull on rocks laterally; they move at 3 miles (5 km) per second
• probe line: a line of 20 to 30 people, standing elbow to elbow, who advanced up a slope, poking into the snow with long poles in an effort to find victims buried by an avalanche
• pulaski: a combination ax and hoe, used to dig a fireline
• pyroclastics: volcanic solids like rock, cooled lava, etc.
• quicksand: a mass of loose, wet sand that becomes fluid when suddenly vibrated; heavy objects will sink into it
• radar: a system that locates distant objects by sending out radio waves and detecting them when they bounce back off the objects
• radiation fog: a cloud of water droplets formed when the cooling ground lowers the temperature of the air above it to its condensation point
• Rayleigh wave: a type of surface wave that moves vertically
• relative humidity: the ratio of the current amount of vapor in the air to how much vapor the air can hold at saturation
• reservoir: a body of water stored in an artificial or natural pond or lake
• reverse fault: a fault in which one block of rock slides up over another
• Richter scale: the system invented by Dr. Charles F. Richter to measure the strength of earthquakes
• rift valley: a huge, trough-shaped valley created by faulting
• Ring of Fire: the earthquake and volcano belt in countries that rim the Pacific Ocean
• rock fall: when fragments break away from the face of a steep slope and fall
• rock slide: similar to rock falls, except they take place on lower slopes, closer to inhabited areas, and are thus more dangerous
• rooster tails: long, flimsy clouds seen as a hurricane first approaches an area; they indicate where the hurricane is coming from
• satellite tornadoes: smaller tornadoes that spin off from major tornadoes and may go their own way
• scoria: bubble-filled stones ejected from a volcano
• secondary waves: a type of body wave, slightly slower than a primary wave, that moves rocks vertically or horizontally; they move at 2 miles (3 km) per second
• seismographs: instruments that make an automatic record of the time, duration, direction, and intensity of earthquakes
• seismologists: scientists who study earthquakes and their causes and results
• shear waves: waves that radiate out from an earthquake’s epicenter rocking from side to side at a fast pace
• shock waves: earthquake waves that travel along the surface with a swift up and down motion
• sinkholes: rapid and sudden manifestations of subsidence most frequently experienced in areas underlain by gypsum or limestone substrata
• SKYWARN: ESSA’s (now NOAA’s) network of volunteer tornado spotters
• slab: a layer of snow that breaks loose and avalanches
• sleet: falling snow or hail that has partly melted; partly frozen rain
• slumping: the slow and gradual slippage of all layers of soil so that the bottom of a hillside becomes level with the lowlands or even tipped upward
• soil creep: the gradual downhill slide of weathered material; the slow, gradual movement of a hill’s upper layers caused by the pull of gravity on loose stones, gravel, and soft topsoil
• spillway: a specially built structure to catch overflowing water from dams
• stabilized snow: a layer of snow that has metamorphosed; snow that has been packed down so that it will not avalanche; rangers stabilize a ski run, for example, by blowing up an unstable layer of snow, leaving only packed, safe snow for skiing
• storm: violent disturbance of the atmosphere accompanied by thunder, lightning, rain, snow or hail
• storm surge: a rise in sea level along the coast as a result of high storm winds
• stress: the intensity of two forces working against each other
• subduction zones: areas where the crustal layer descends into the mantle
• subsidence: the vertical fall of earth, usually caused by the loss of underground water or gas support, sometimes resulting from man-made pumping and sometimes from seismic action
• sunspots: dark, relatively cool areas on the surface of the sun
• supercells: the storm clouds that give birth to mesocyclones and their tornadoes
• surface wave: a vibration from an earthquake that travels at ground level
• tectonic plates: 70-mile-thick blocks of the earth’s crust on which the continents ride
• tephra: fragments thrown into the air by a volcanic eruption
• theory: a belief explaining something, backed by certain facts
• thrust fault: when a reverse fault has an angle of 45º or less
• thunderhead: the top part of a thundercloud tornado: a powerful column of winds spiraling violently around a center of atmospheric low pressure
• tornado warning: the next stage of a weather alert after a tornado watch; advises people in an area that one or more tornadoes are likely to occur
• tornado watch: the first weather alert advising people that tornadoes may strike
• TOTO: a device dropped in the path of a tornado to measure its strength, speed, and direction
• tremors: the shaking caused by earthquakes
• tropical storm: an ocean storm that forms in the tropics
• tropics: the very warm, humid region lying north and south of the equator; the sun’s rays are strongest here
• troposphere: the lower layer of the atmosphere where most weather occurs
• tsunami: a large ocean wave usually caused by an underwater earthquake or a volcanic explosion
• turbine: a machine that uses fluids to turn a wheel or cylinder to create energy, usually electricity
• typhoid: a dangerous infectious fever caused by drinking polluted water
• typhoon: a hurricane that arises over the Pacific in he tropics
• vegetation: all the trees and plants that grow in a particular area
• vent: the single hole from which magma erupts
• vibration: a continuous shaking movement
• visibility: the distance over which objects can be seen
• volcanic cone: a conical mountain built up by volcanic eruptions
• volcanic crater: a steep-walled depression at the top or on the flank of a volcanic cone, out of which the volcanic materials are ejected
• volcanic sunset: a colorful sunset following a volcanic eruption
• volcano: a gap in the earth’s crust where molten rock and other materials escape onto the Earth’s surface
• vortex: a rapidly whirling spiral
• wall clouds: heavy clouds that form around the eye of a hurricane
• waterspout: a moving column of water drawn up by a whirlwind at sea and meeting a descending, funnel-shaped cloud
• water vapor: the invisible gaseous form of water
• weather satellites: unmanned spaceships in orbit up to 22,000 miles above the earth; they have cameras that photograph the planet and send signals to receiving stations on earth
• wetlands: land, such as a bog or marsh, that has wet and spongy soil
• wildfires: a fire that occurs near shrub, grass, forests, or other natural areas with low human population densities.
• wrench fault: a fault in which blocks of rock slip sideways past each other; also known as tear faults, strike-slip faults, or, when especially large, transcurrent faults

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