The Earth

Chapter 5

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Unit Organization:

• Plate Tectonics
• Rocks
• More on Climate
• ENSO
• Natural Hazards
  • Geologic
  • Weather

Reading:

Textbook: Chapter 5

Ancillary Reading:

• 5.38,

Plate Tectonics

In the 1960's, geology moved from a static view of the Earth's surface to a dynamic view, due to several new sources of data

• One source was the magnetic striping of the ocean floor due to Geomagnetic Reversals
• Geomagnetic Reversal - a flip-flop in the polarity of the Earth's magnetic field (N become S and vice versa)

Now geologists believe that the building (in Latin, tectonicus) of the Earth's surface features is due largely to the motion of part of the upper mantle that breaks the solid crust into plates and moves them, resulting in Continental Drift

• The mantle motion is not uniform, resulting in Hot Spots, where the mantle may cause volcanism on land or sea, sometimes building islands, or may cause intrusions of magma under the continental rock, which can cause faulting and mountain building (Orogeny)
• The movement in the mantle results in boundaries (edges) between plates that can be
  • Static Boundaries (not moving)
  • Faulting also called Strike-Slip Boundaries (sliding past one another)
    • San Andreas Fault in California
  • Spreading Boundaries of two plates moving away from one another -
    • Mid-Oceanic Ridge
    • Rift Lake
  • Colliding Boundaries cause overlap of the colliding plates, with one plate riding over the other
    • Subduction Zone
    • Oceanic Trench
    • Orogeny
• The large amount of kinetic and heat energy associated with collisions and hot spots causes Volcanism and Earthquakes there

Plate motion is slow, ranging from 1 to 12 inches per year but, given millions of years, the surface of the Earth was very different in the past and will be different from today's configuration in the future

• At 1 inch per year, it would take about 1.5 bya to go around the world at the Equator
• Continents have been aggregated into Supercontinents in the past
  • Most recent supercontinent is Pangea (or Pangaea), which formed about 300 mya and broke in two (Laurasia and Gonwanda) from about 200 mya to 140 mya
• Fossil (and some living organisms) distributions are evidence of the existence of the supercontinent

Rocks

Rocks are mixtures of minerals

• Minerals are crystals of consistent chemical composition (like Quartz, which is Silicon dioxide)

Kinds of Rocks

Igneous - formed by cooling Magma or Lava, this is the parent rock from which other rock types are made

• Some (Gabbro and Basalt) are very similar in composition to the mantle materials
• Plutonic - formed by the intrusion of magma close to the surface, where it can cool slowly and produce large mineral crystals within the rock
• Volcanic - formed by the rapid cooling of lava ejected from volcanoes, these form fine-grained rocks as large crystals do not have time to form

Sedimentary- rock that forms in three ways

• Clastic Sedimentary Rock - pieces of rock (from weathering of other rock) collect in beds which are buried below newer sediments and are cemented (chemically connected) by the pressure and heat
• Biogenic Sedimentary Rock - from shells, etc. - mostly CaCO₃ (Limestone and Dolomite) but also SiO₂ -
• Chemical Precipitates - stalactites and stalactites in caves, travertine terraces at geysers,
• Diagenesis is the process of change that begins with the buildup of sediments and ends when the rock is under such high temperature and pressure that metamorphosis begins (see below)
  • Lithification is the part of diagenesis that cements the particles together
• Sedimentary rock has pores in it (spaces between the sediments) and originally filled with fluids
  • Diagenesis reduces pore size through pressure but does not eliminate the fluids
  • Usually water but can be natural gas or petroleum

Metamorphic rock - when sedimentary rock is taken deep enough into the Earth to experience temperatures and pressures that alter the rock structure

• Time, heat, and pressure are the three elements of metamorphosis
• Foliated Metamorphic Rock is layered as the slow squeezing of the original rock flattens and aligns the crystals into visible layers (Gneiss)
• Non-Foliated Metamorphic Rock has no obvious layering, although it often has patches of different color or shade in it (Marble )

The Rock Cycle

• The Rock Cycle begins with molten magma that cools to form igneous rocks
  • Igneous rocks are broken apart by Weathering
• The pieces of rock that are produced collect as sediment beds and are buried by later sediments
• As more sediments collect above, the particles are subjected to both pressure and heat
• Sediments are turned into rock once they are buried, where they are cemented together (lithified)
  • Subduction takes sediments down into areas of great pressure and heat where one of two things happens:
    • metamorphosed by pressure and heat
    • melted - start cycle over as new igneous rocks
• Uplift (mountain building or Orogeny) may bring metamorphosed or sedimentary rocks to surface, where they too are weathered

More on Climate

Climate and Weather differ in scale in both space and time.

• Average conditions are important descriptors of climate but averages are not sufficient for the purpose
• Maximum and minimums are also important.
• Extreme events may also be important.
  • Hurricanes are very important components of the climate of southern Florida but they do not affect long-term
    averages very much.

Solar Radiation and the Earth

Electromagnetic radiation

• Basic unit is the photon, a packet of energy that travels as a wave
  • waves have lengths (distance from successive peak to peak) and frequency (number of peaks passing a fixed point per unit time)
• since the speed of light is a constant, the frequency is related to the wavelength in that shorter wavelengths have greater frequencies
• the wavelength (and frequency) of electromagnetic radiation is determined by the amount of energy in the photon - greater energy causes shorter wavelengths
• all bodies with a temperature above absolute zero (just zero on the Kelvin scale) radiate energy
  • the higher the temperature, the shorter the wavelength

Consider an iron bar in a blacksmith's furnace.  As it heats it begins to give off visible radiation.  It starts to glow red and, as it heats more, it glows with a white light.  There are some lessons to be learned here.

Why red first?  Heating is not uniform throughout the rod, and each region emits photons directly related to the temperature at that spot.  Red light has the longest wavelength of visible light and so, as the iron heats, the hottest areas will appear red, the first wavelengths of electromagnetic spectrum we can detect with our eyes.  As the rod continues to heat, more areas are hot enough to emit visible radiation but, once again, the actual temperature is not uniform and so we get a wide range of photons with wavelengths in the visible spectrum and we perceive the overall effect as white light, since there is no one wavelength of light that is white.

Second Lesson - Objects with the temperature of our sun emit most of their radiation in the visible spectrum.   This is not a coincidence.  We evolved eyes to detect at these wavelengths as they dominate the spectrum available to us because we are close to the sun.  Note that we do not detect all of the common wavelengths given off by the sun.  We do not see ultraviolet, which has wavelengths shorter than the shortest we can see.  Other organisms (not just animals!) can detect these but we name the visible spectrum for what humans can see, not for what all living organisms can see.  A collective name for the radiation given off by the sun is shortwave radiation (from about 100 to 2000 nanometers).

Third lesson - radiation is emitted by the rod before we can see it glow.  In fact, radiation is emitted from all objects, including you.  However, the wavelengths emitted by you are too long for our eyes to detect.  When special cameras that do detect at our wavelengths are used, we do glow.  This is the basis of night vision.  Images are made from the glow of objects that emit in the Infrared range of wavelengths and translated into wavelengths that we can see by the night vision apparatus.

Fourth lesson - infrared means below red.  Thus, it is radiation with wavelengths below our power to detect as the photons do not have sufficient energy to initiate the physiochemical reactions we call vision.  Infrared is divided into two types, near and far.  Near Infrared has wavelengths near to visible (from about 700 to 4000 nanometers).  Far Infrared is even longer wavelength (about 4000 to 1 million nm).  This is the range emitted by objects at the temperatures we have on Earth and so we also refer to this as Thermal Radiation.

The radiation discussed here is not the entire spectrum.  Gamma ray and X-ray radiation have shorter wavelengths than the radiation discussed here and can have wavelengths as short as a millionth of a nanometer.  Radar, radio, TV, and cell phone use radiation with wavelengths longer than far infrared (up to hundreds of meters long).  To see the entire spectrum, go to this site at the Laboratory for Atmospheric and Space Physics.

Heat and Radiation

Recall that heat is kinetic energy, the energy of moving and vibrating atoms and molecules.  Temperature is determined by the average speed of that motion (e. g. hotter gasses and liquids have faster moving molecules).  Electromagnetic radiation is converted to heat when photons are absorbed by atoms and the atoms move or vibrate faster as a result (we measure this as an increase in temperature).  Heat energy is converted to electromagnetic variation when a photon is emitted by a moving atom that slows as a result of the loss of energy (we measure this as cooling).

Global Heat Budget

The sun contributes almost all of the energy that drives climate and, ultimately, living systems.  The sun's contribution is in the form of electromagnetic radiation  Earth receives energy as photons from the Sun. 

• The energy balance at the surface of the Earth is result of losses to space and the incoming radiation from the Sun.  Thus, we can do a budget, just like a household budget is the record of income and expenditures.

Heat energy is transported either through:

• Conduction (transfer of heat energy from molecule to molecule in a solid),
• Radiation (conversion to photons and loss through space) or
• Convection (transfer to molecules in a fluid - either gas or liquid).

Budget Figures

As sunlight reaches the Earth (at an energy density of 100% = 342 Watts/square meter)

A watt is one joule of energy produced or consumed per second, recall that 624 billion visible photons add up to one erg of energy, and that there are 10 million ergs in a joule, so the input of the Sun is 2 billion trillion (2,000,000,000,000,000,000,000) photons per second striking a square meter above the atmosphere at the Equator

However, only 48% is absorbed by the surface or the Earth -- 52% is never absorbed

• 23% is absorbed by the atmosphere, 23% is reflected , and 6% is reflected by the surface of the Earth

At the surface of the Earth, energy is received from two sources

• the sun (48 percentage units)
• thermal radiation from the atmosphere (100 units)
  • The 100 units from the Atmosphere is the famous Green House Effect as gasses in the atmosphere absorb thermal radiation from the Earth and return most of it to the surface.  Our effect on the concentration of greenhouse gasses is the basis of the worries about global warming.

and energy is lost through three processes

• evaporation (25 units) - evaporation is the loss of faster moving water molecules from water.
• convection by the atmosphere (6 units)
  • Air is heated by the Earth and expands, which decreases its density, and it rises as cooler, denser air displaces it at the surface.  This convection constitutes a loss of energy from the surface of the Earth,
• thermal radiation (117) - The infrared "glow" of the Earth's surface is a loss of energy

As you can see, the largest flow of energy is thermal radiation (117units from the Earth, 100 back from the atmosphere

• Thus, changing the effectiveness of the greenhouse effect can significantly alter the budget and trap more heat at the surface
• Note that the budget is balanced at the surface (48 + 100 = 148 units of energy gained and 25 +6 + 117 = 148 units lost)

Climate - A general description of how much heat and moisture characterize a region and the timing of the minima and maxima for both.

Partially results from the curvature of the Earth on density of solar insolation

• curve spreads density of solar radiation over greater surface area at poles than equator
• sunlight has to travel through more atmosphere at poles than equator due to decreased angle
• poles are colder because of this, but this is not the reason for

Seasonality in temperature is a result of the tilt of Earth's axis, which changes both the day length and intensity of the sun in a regular way each year

• the tilt is 23.5° which enables us to divide the earth into latitudinal regions
  • Arctic and Antarctic Circles - dividing line between those regions that get at least one day of 24 hr of sunlight at height of summer, 24 hr of darkness in depths of winter and those regions that get some dark and light periods every day
  • Equator - Circumference midway between N and S poles
  • Tropics - regions of earth where the sun is directly overhead at least once a year
    • go from 23.5° N (Tropic of Cancer) to 23.5° S (Tropic of Capricorn)
• tilt divides up the year as well as the Earth
  • Solstices (Summer and Winter) - shortest and longest days of the year
  • Equinoxes (Autumnal and Vernal) - days on which there are 12 hrs of sunlight and 12 hrs of darkness

Wind patterns are caused by the effects of insolation and the rotation of the Earth

• unequal heating causes winds as warm air rises and colder air move in to fill space vacated by heated air or vise versa, warm air moves in to fill space vacated by sinking, cooling air
  • hot air rises at equator and moves north where it sinks and then moves toward equator again
  • such circulating movement is a Convection Cell
    • zone of heating is where hot air rises
      • rising air condenses the water it holds and rain results
      • air tends to be moist because it has been in contact with surface of Earth
    • Subsidence Zone is where air in upper atmosphere cools, becomes more dense, and sinks Earthward
      • air is dry as there is no source of moisture and what the air contained was lost by rains when air first rose to upper levels of troposphere
  • The circulation is not a single cell in each hemisphere, but is complicated by cooling of air before it reaches the poles
    • hot air rises at equators and moves to about 30° N and S, where it cools and subsides
    • hot air also rises at about 60° N and S and moves both N and S
    • Movement toward equator cools at 30° and subsides with air from equatorial heating
    • Movement toward pole cools at pole and subsides there
  • this results in three convection cells in each hemisphere (from north to equator):
    • Polar Cell - 60° N or S rising air to polar subsidence
    • Ferrel Cell- 60° N or S rising air to 30° N or S subsidence
    • Hadley cell- equator to to 30° N or S subsidence
• Coriolis effect
  • caused by moving N or S on rotating Earth
    • earth is rotating from left to right, if you look at globe on a page with N pole facing top of page
  • person standing motionless on equator revolving faster than is person standing at polar circle
    • so as one moves toward equator, one enters a faster region from a slower one
      • objects in motion seem to be deflected to left as they are moving slower (to the right) than their surroundings
    • moving away from equator, one enters a slower region from a faster one
      • objects in motion seem to be deflected to right as they are moving faster (to the right) than their surroundings
• Regions of Winds result from combination of convection cells and coriolis effect
  • Horse Latitudes - 30° N or S subsidence zones - no winds as air is sinking but not rushing to fill a void
  • Doldrums- equatorial zone of rising air - no winds as air is rising but not rushing to fill a void
  • Trade Winds - zone between horse latitudes and doldrums where air is moving toward equator and is deflected to the right in the Northern Hemisphere (left in the Southern), so it appears to come from the northeast (southeast in southern hemisphere)
    • called trade winds because they were much used by trade shipping to get from Europe to Americas
  • Westerlies - zone between horse latitudes and 60° (N and S) - zones of rising air where air is moving away from equator and is deflected to the right in the Northern Hemisphere (left in the Southern), so it appears to come from the southwest (northwest in southern hemisphere)
• Ocean Currents - Steady winds cause water to move in large masses called currents
  • because moving water is replaced by surrounding water, surface currents pushed by wind tend to form large circular movements called Gyres that circulate around an oceanic basin
    • pushed by westerlies the gyres move counterclockwise in southern hemisphere clockwise in northern hemisphere
  • results in moving warm waters northward or southward where, when they contact land, they can warm the climate of the land
    • example is the Gulf Stream in the North Atlantic, which so warms western Europe that palm trees grow in southern Ireland
    • can also move cool water south (must do so to replace water moving north) with the opposite effect on the land's climate
      • California Current is responsible for the coolness of weather in central and northern California

ENSO

El Niño Southern Oscillation (ENSO) is the periodic (meaning recurring at somewhat regular intervals - 3 to 7 years in the case of ENSO) change in atmospheric conditions over the Equatorial Pacific Ocean that results in widespread changes in weather patterns worldwide

Heating the Pacific

Most years, a convection cell is created when hot water rises over heating Pacific ocean and moves eastward at high altitude

• Air cools and descends in Americas and flows back to the hot spot in the eastern Pacific
• Air moving from east to west over surface of the Ocean pushes water in that direction
• As water is pushed west, it is replaced by upwelling water along the western coast of South America

Effects:

• Upwelling produces algal blooms and fish fatten on the abundant productivity

El Niño

hot spot migrates eastward, hot air rises over eastern Pacific and sinks over western Pacific

• Warmer, drier winters in mid and western US, wet and cool in southern US
• Dry summers in much of Southeast Asia - can cause failure of the Monsoon rains important to the water budgets for the area
• May suppress Hurricanes in Atlantic and probably do in the Pacific
• Surface current is reversed and water piles up on eastern edge of the Pacific so upwelling off of the coast of South America is suppressed and South American fisheries suffer

La Niña

L Niña years - hot spot located farther west and rainy region moves further west during summers

• Cool and wet winters for northern US, dry and warm for southern US
• Heavy rains and sometimes flooding for Southeast Asia
• Dry conditions in East Africa

The Southern Oscillation is the change in pressures associated with rising hot air (low pressure) and sinking cool air (high pressure) that one observes as the heated air (the low pressure area) follows the moving warm seawater in the Equatorial Pacific Ocean

Natural Hazards

Disturbance and Landscapes

• Disturbances are short-term events that disrupt communities and may even alter the composition of landscapes
  • Examples: fire, drought, windstorms and tornados, cold spells, floods, epidemics (happens in both animals and plants as well as in human populations), volcanic activity, rock and mud slides, avalanches, ice storms
  • Individual events have two landscape properties:
    • Intensity of the event - measured in terms of the loss of individuals (biomass) or habitat
    • Scale of the event - the area affected by the disturbance relative to the size of the landscape under consideration
• Disturbance Regime - the recurring pattern of a particular type of disturbance (i. e. fire regime, hurricane regime, etc.)
  • Regimes have both intensity and scale but add the dimension of frequency - how often disturbances occur

Hazards of Geological Origin

Earthquakes

• Caused by sudden slippage of one mass of rock past another mass of rock
  • Sudden conversion of tension energy into kinetic energy
• Shock waves propagate from center and can shake surface many miles away
  • Can change surface abruptly and cause seismic waves (tsunamis)
  • Tsunamis are not regular waves as the surface of the ocean does not return to the same level after the wavefront passes a point
    • When they arrive on land, cubic miles of water may suddenly flood coastal regions
• Measured by either a logarithmic scale based on the energy released (Richter scale or the similar Moment Magnitude Scale) or on the Mercalli scale, which measures the intensity of shaking
  • Earthquakes centered closer to surface cause more shaking, so two quakes of the same MMS magnitude might have very different Mercalli scores
  • The actual center of the slipping rock is the Hypocenter, which can be deep under the surface
  • The point on the surface directly above the hypocenter is the Epicenter, which is what is usually reported in the press

Volcanoes

Volcanoes are ruptures in the crust where three things from the mantle escape:

• magma -molten rock, called lava when it escapes to the surface
• volcanic ash - when expanding gasses explosively throw magma into air, it cools as ash
• gasses (H₂O, CO₂, SO₂, HCl, HF are the majority)

Volcanoes are associated with Hot Spots, Spreading Zones between diverging plates and Collision Zones of converging plates

• There are lots of types of volcanoes, from simple fissures to large mountains
• Volcanoes are located under water (submarine volcano) and under icecaps
• Volcanoes can be active, dormant or extinct

Volcanism (the level of volcanic activity) has varied over geological time

• Huge eruptions have covered large areas (thousands of square miles) with Flood Basalts (remember that basalt forms from rapidly cooling lava)

Hazards occur both locally (ash covers crops and cities, lava flows destroy fields and buildings) and at great distances (ash clogging jet engine intakes, weather changes thousands of miles away)

• Volcanic Winters have followed large eruptions that eject SO₂ aerosols into the Stratosphere, where they persist as rain rarely forms as high as the stratosphere (rain washes the droplets out of the atmosphere)
  • The droplets change the reflectivity of the atmosphere (its Albedo) so more sunlight is reflected and not absorbed so the Earth cools
  • Can last for up to 5 years
  • Pinatubo in 1991 cooled Earth for 2 years
  • 1883 - Krakatoa (Indonesia) exploded - 4 cold years, record snowfalls worldwide
  • 1815 - Mt. Tambora (Indonesia) - "Year without a Summer" in US - June Snow in New England's, July frost in New York

Land Instability

Landslides come in many types: mudslides, debris flows, land slumps, rockfalls and others

• Triggered by heavy rains that saturate the soil and create surface runoff

Avalanches are collapses of unstable layers of snow and ice on steep mountains slopes that may be triggered by new snowfall or by skiers!

Exploding Lakes

Gas (CO₂ or Methane or even H₂S) may accumulate and saturate the water of a large lake when the lake does not mix deep and surface waters

• Gasses come from volcanic leakage into bottom of lake or from decomposition
• A thermocline divides the lake into dense, cool water and less dense, warmer surface water (the thermocline is the narrow band of water between the warm upper and cool deeper waters)
• thermoclines are seasonal in temperate climates but can be virtually permanent in tropical lakes
• Gasses build up due to pressure of water above and cooler temperature, like a bottle of champagne
• When something happens (earthquake, windstorm, rainstorm) to shock the gasses out of the water, they expand as bubbles and the bubbles cause the water to move toward the surface, where the lowered pressure allows further degassing and the gasses erupt over the lake
• Cloud of CO₂ passes over adjacent land downwind

Three lakes identified as potential problems: Kivu (border between DR Congo and Rwanda), Nyos, and Monoun (both in Cameroon)

Two know explosions: Monoun in 1984 (37 people asphyxiated  and Nyos in 1986 (over 1700 people asphyxiated)

Today, attempts are being made to bring enough water to surface in a controlled manner to keep gas pressure low enough at deeper layers to prevent explosion

• Simply a pipe that goes from surface to bottom, started with a pump but degassing process in the pipe makes it self-operating after start
• Water spurts out of the pipe to a height of 120 feet
• Monoun has been degassed and Nyos is being degassed

Hazards Related to Weather

Wildfire

• Wildfires are uncontrolled fires that burn naturally occurring fuels (vegetation, coal, peat)
• Wildfire frequency in an ecosystem is negatively correlated with rainfall, so areas or seasons with low rainfall are fire-prone
  • El Niño, La Niña, droughts, and dry winds (Santa Ana Winds in California) are all weather phenomena that can alter the probability of wildfire occurrence
• Fires are initiated by:
  • natural events, usually lightning strikes but also volcanic activity, sparks from rock falls, or even spontaneous combustion
  • human activity - in some regions, arson and carelessness (campfires, cigarette butts) are the prevalent human causes but in much of the world (Mexico, Central and South America, Africa, Southeast Asia) wildfire is used to either clear forested land for grazing or planting (Slash and Burn Agriculture) or cleared areas are burned to keep the forest from returning and to promote grass growth for grazing domestic animals
• Some landscapes have fires so-frequently and of such scale and intensity that the species that live there have adapted to a fire regime that prevents other species from invading the community - the Fire-Adapted Community is sometimes called a fire-climax community
• Types of wildfires (there are other classification schemes)
• Underburns are fires of low intensity and small scale burn off the litter and singe the lower trunks of trees but do not burn the foliage of mature trees
• Ground fires use subterranean fuel such as dead roots, buried leaf litter, peat, and coal
• Brush fires are intense fires in shrublands and grasslands
• Crown Fires reach the tops of trees and spread from tree crown to tree crown
  • These fires spread faster as the wind speeds are greater above the canopy
  • Greater heat is released and these fires kill trees
• Firestorms are the most intense fires that produce gale-force winds along the surface as air rushes in to replace the air rising above the fire zone
  • Only occur where a large amount of fuel has accumulated
• Coal Seam Fires - fires fueled by underground coal deposits, can burn for many years, can cause environmental hazard (toxic gasses, land subsidence, CO2 release to the atmosphere) - there are thousands of these fires burning right now
• Intense crown fires, brush fires and firestorms can permanently alter the landscape as they can burn off the organic content of the soil which can make the soil unable to support new tree growth
• Humans influence fire regimes
• Some cultures have historically initiated fires to maintain grasslands (Northern Australia, Western USA)
• In the USA, we have historically suppressed fires
  • This can lead to fewer fires, but can increase the intensity when they do occur if fuels accumulate
  • In our western forests, the climate is so dry that decomposition does not, on average, consume the leaf and branch litter that falls each year, which leads to fuel accumulation over the years
• International Aspect - Africa, Brazil and the drought-fires in southeast asia
• Fire management in the US now includes periodic, low intensity burns to reduce fuel accumulation
  • Questions remain about what to do about natural fires
    • Suppressing them can lead to worse fires in the future but, when the fires are frequent (in dry or drought years) or threaten human habitation or activity (smoke can make it hard to work outside and can harm those with respiratory problems), suppression may be the proper course
    • Controversy also surrounds the practice of removing dead tree trunks after fire has killed them.  These can be valuable in the short term to logging companies but may delay the recovery of the forest as they may promote new tree growth as they decay

Storms: Tropical Cyclones (Hurricanes and Typhoons), Ice Storms, and Thunderstorms with Tornados

Tropical Cyclones

Can be called either hurricanes (Atlantic) or typhoons (Pacific) Storms that are generated by warm ocean temperatures causing water-saturated air to rise

• The expanding air causes low pressure and some of the lowest barometric readings have been taken during tropical cyclones
• They spin because of the Coriolis Effect, so they spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere
• As it rises, the air cools and the water vapor condenses and precipitates
• When enough energy is available, the wind speeds reach hurricane strength

Tropical cyclones are important parts of the heat budget as they transfer heat out of the tropics

Tropical cyclones can cause damage along coast from both winds and flooding

• Wind damage may be worsened due to the thunderstorms and tornados spawned by the cyclone
• Flooding is due to heavy rain and to the storm surge
• Tropical cyclones can also cause flooding inland due to heavy rains

How much damage a storm will do is very variable and depends on the strength of the storm in both wind speed and amount of rain and what land it passes over

• Only 3 storms were category 5 (the strongest storm level) when they made landfall:   (1935), Camille (196), and Andrew (1992)
• Barometric pressure is a measure of storm strength and the lowest for a hurricane was Hurricane Wilma (2005, 882 millibars) and Typhoon Tip (1979, 870 millibars) was the strongest in the Pacific
• For comparison, Hurricane Katrina happened in the same year as Hurricane Wilma.  
  • Katrina made its second landfall on the Gulf Coast (Florida was first) as a category 3 storm but at its strongest, out over the Gulf, reached a low pressure of 902 millibars. 
  • Katrina did far more damage than Wilma

Thunderstorms

Also called electrical storms due to associated lightening

• Lightening causes the thunder that warns of the storm’s approach

Occur where warm, moist air is rapidly carried aloft at either a front where cool air pushes under the warm or where the Sun’s heating of the land causes the moist air to rise rapidly

• If associated with a front, storms often from a long, thin line of storms along the edge of the front
• Rapid Adiabatic Cooling (cooling caused by expansion of air as pressure drops) leads to condensation of a cumulonimbus cloud and to heavy precipitation
• Coriolis effect and other forces may lead to rotation of storm
• Rising air causes strong, in-line winds as cooler air rushes in to fill the space

Cause damage from winds, flash flooding from heavy rain, and hail

• Can spawn more dangerous winds: tornados

Tornados

Violent rotating downdrafts that reach from cloud to ground and are associated with thunderstorms

• Wind speeds up to 300 mph, up to 3 miles wide, can stay on the ground for many miles (record is over 200 miles)

Can occur on any continent but most common in certain areas (US, Canada, Europe, etc.

• Farming and tornadoes go hand-in-hand
• The majority of tornadoes occur in the US
• Can occur over water as a waterspout
• Can have multiple funnels (vortexes or vortices) from one thundercloud

Most commonly used scale is the Fujita (F0 to F5 – theoretically an F6 is possible but has never been measured, as the wind speed would have to exceed 320 mph)

• Strongest tornados form in Supercells
  • These are thunderstorms, often found in front of an advancing front, with a large layer of rotating winds at midlevel in the storm system (a mesocyclone)
• Formation of the funnel cloud is not well understood
• Weather patterns that lead to tornado formation include warm, moist air from the Gulf of Mexico moving northeast and colliding with cold, dry air from the western US moving southeast

Tornado deaths vary greatly with year as incidence, intensity, and circumstance must all conspire to create the truly tragic disasters that are deadly tornadoes

• Since 1925, deaths per million of population have fallen at a steady rate
• The April 25-28 2011 Super Outbreak consisted of 359 confirmed tornadoes in 21 states and Canada
  • There deaths in six states (AL, MS, TN, AR, GA, and VA), with Alabama most severely affected
  • At least on F05 tornado was detected
  • It was the fourth most deadly outbreak in US history

Ice Storms

Ice storms form when a layer of warm (just over freezing) air is trapped between two colder (below freezing) layers

• Precipitation starts in upper layer and is frozen
• Then falls through the warm layer melts
• Supercooled as it falls through lowest layer
  • Supercool water is liquid but below the freezing point
  • When supercooled droplets contact any surface at or below the freezing temperature, crystallization is almost instantaneous
• So, ice forms on any cold surface

Worst ice storm: 8 inches of ice in northern Idaho in 1961

• Locally, the February 4, 1994 ice storm caused over $1 billion of damage in 3 states (Al, TN, and MS)
• Worst in North America was up to 4 inches in 1998 in NE US and Quebec
  • 1.25 million homes without heat for up to a month

Floods

Floods are the rapid rise of body of water (lake or river) caused by rainfall somewhere in the body’s watershed

• Flash floods are local rises in water level due to a single storm’s precipitation
• Floods are normal events in that all rivers flood when too much rain falls
• Floods can be periodic if rainfall is periodic
• Floodplain is a flat area adjacent to a river that is flooded by periodic floods
  • Floodplains are valuable land for agriculture, industrial, and residential purposes
• By looking at historical records, statisticians can predict how often a periodic flood will reach a given level
  • We use these predictions to draw 10-, 50-, 100-, and 500-year flood levels on contour maps

The Amazon is an excellent example of seasonal (periodic) flooding

• Largest river by volume (20% of all river flow) draining the largest watershed (2.7 million square miles)
• The river is from 1 to 6 miles wide during the dry season but can expand to over 30 miles wide during the wet season
• Amazon Flood maps show three regions: not flooded, occasionally flooded, and regularly flooded
• In some regularly flooded regions, the flood can reach 40 ft high yearly

Drought

Drought is:

• an extended period of lower-than-average precipitation such that both the ecosystem and human systems are stressed by lack of water
• set at different levels in different ecosystems
• unavoidable in almost all ecosystems
• caused by both global (ENSO) and local (land use practice, chance) processes

Drought may result in:

• local plant death (millions of trees have died in the Texas drought)
• dust storms and, if regional and prolonged, dust bowls
• famine, if the larger system does not supply water and food
• long term ecosystem damage
• long term agrisystem damage
• Social unrest, including war and mass migration

Drought is currently affecting or has recently affected:

• the southern edge of the Sahara, from Chad to the Horn of Africa, has had persistent droughts over the last several decades
  • Desertification is producing desert from shrub and grassland
  • currently, a terrible drought is affecting the Horn, where millions are at risk (the UN is appealing for aid for the region)
• southern England (2011 – today)
• Minnesota (2012)
• Georgia (2006 – 2008)
• from 1995 until 2009, parts of Australia experienced the country’s worst recorded drought
  • Global effects – rice shortages, higher prices for beef and lamb
  • Worst drought ended with record-breaking floods
  • Australia is under the influence of ENSO and drought/flood cycles are normal (and still terrible)
• Texas and parts of most of the South, record setting drought 2010 - 2012

The historic trend has been an increase in drought conditions

• The Palmer Drought Severity Index is based on soil moisture, with a negative number indicating a deficit and a positive a surplus (in the graph, the negative of the PDSI has been used, so that the line goes up with a greater likelihood of drought)
• Climate models predict that wet areas will get wetter and dry areas drier, so that the world will experience both more floods and more droughts if the prediction is true

Dust Storms

Dust Storms are most common to arid and semi-arid lands

• Larger particles are found in sandstorms
• Dust is transported farther than sand

Regular phenomenon in some regions

• China pours dust into the Pacific, North America and Africa pour dust into the Atlantic
• Some dust can be deposited continents away - dust from China is detectable in the western US

Causes are drought and poor farming practices

• Mechanism
  • Loose, dry particles on surface
  • Wind blowing across surface
  • Particles begin to vibrate, then jump (saltation), eventually they become suspended in the air
  • Electrostatic force greatly enhances the amount of suspended material

Dust Bowl in the US

• From 1932 until 1936 (in places, 1940), farm and range land in the South, Midwest, and even pars of Canada was subject to extensive erosion of soil by wind
  • Over 1 million acres (400,000 km2) were seriously eroded
  • Displaced many farmers, many of whom migrated west, many to California
• Caused by a series of events
  • Replacement of native plants, with deep, drought resistant root systems by shallow rooted, annual crops
  • Failure to practice soil conservation (crop rotation, cover crops, wind rows)
  • Drought

There are two lessons to be learned from the Dust Bowl, one relating to individual versus group action and one relating to the idea of Scale in environmental science

1. People, when not acting cooperatively, may collapse the environmental systems upon which their lives depend (The Tragedy of the Commons – Garrett Hardin)
• The situation was corrected because the collapsed system was embedded in a larger system
2. Systems are embedded within systems, so every system has its own scale
   • Disasters in smaller scale systems are often rectified by using resources available in the larger system

What happens when the changes affect the largest system?

• The Aral Sea is a dust bowl happening today

Extreme Temperatures

Extreme temperature has to have a local definition

• A dangerous extreme cold temperature in Nashville is not an extreme in Nome, Alaska
• Continental interiors are subject to the greatest extremes between high and low temperature for the year
• Oceanic effect buffers coasts from extremes
  • Miami has recorded only 1 day of 100 (in July 1942) and none higher ever with an average of 62 days a year 90 °F or greater
  • Memphis, 657 miles north of Miami has a record high temperature of 108 °F with an average of 67 days a year 90 °F or greater

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Last updated February 22, 2012