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© 2012 Pearson Education, Inc.
Lecture Presentation
Chapter 9 and
10
Atmosphere and
Severe Weather

Energy, cont.
 The ability to do work
 Types
 Potential - stored energy
 Kinetic - energy of motion
 Heat - energy of random motion of atoms and
molecules

Heat Transfer
 Conduction
 Transfer through atomic or molecular
interactions
 Two bodies in contact with one another
 Convection
 Transfer through mass movement of a fluid
 Hot air rises displaces cool air which falls
 Creates a convection cell
 Radiation
 Transfer through electromagnetic waves

Earth’s Energy Balance
 A general equilibrium between incoming and outgoing
radiation
 Earth intercepts only a fraction of the sun’s radiation
 Sun’s energy drives hydrologic cycle, ocean waves
and currents, and global atmospheric circulation
 Nearly all of the energy available at Earth’s surface
comes from the sun
 Exception, heat from Earth’s core that drives plate
tectonics

Heat Energy
 Energy transferred between two objects at
different temperatures
 Sensible heat
 Heat that is monitored by a thermometer
 Latent heat
 Energy necessary to cause a change in state
 Example: latent heat of vaporization is energy
necessary to change liquid water into water vapor

Energy Behavior
 Redirection
 Reflection back to space by clouds, water, land
 Scattering disperses energy is many directions
 Transmission
 Energy is passed through atmosphere
 Absorption
 Alters molecules or causes them to vibrate
 Some of this may be re-emitted to space

Energy Behavior, cont.
 Temperature depends on amount of energy
absorbed or reflected
 Reflection depends on albedo
 Describes the reflectivity of surfaces
 Dark woodlands reflect 5 percent to 15 percent
 Light grasslands reflect 25 percent
 Absorption
 Energy that is not reflected is absorbed
 Different objects absorb different wavelengths
 Hotter objects radiate energy more rapidly and at
shorter wavelengths

Weather/Climate
 Weather- an areas short term atmospheric
condition (hours and days)
 Parameters use to determine atmospheric condition
 Temperature
 Pressure
 Moisture content
 Precipitation
 Sunshine
 Cloud cover
 Wind speed and direction

Climate
 Climate- a region’s long-term atmospheric
conditions (averages over decades)
 Temperature and precipitation are the two main
factors that influence climate
 These two factors are influence by air circulation,
latitude, altitude, and ocean currents

The Atmosphere
 Gaseous envelope that surrounds Earth
 Composed mostly of nitrogen and oxygen
 Smaller amounts of water vapor, argon, carbon dioxide
 Water vapor
 Important for cloud formation and circulation
 Comes from evaporation off of Earth’s surface
 Humidity describes amount of moisture in atmosphere at
particular temperature
 Relative humidity is the ratio of water vapor present to the
amount that saturates the air

Structure of the Atmosphere
 Troposphere
 All of Earth’s surface
is within this layer
 Upper boundary is
tropopause
 Temperature
decreases with
increasing altitude
 Clouds are present at
the tropopause
Figure 9.7

Clouds
 Made from very small water droplets or ice
crystals that condense from the atmosphere
 Cumulus – puffy fair weather clouds
 Cumulonimbus – Tall, dark storm clouds

Weather Processes: Atmospheric Pressure
and Circulation
 Atmospheric pressure also called barometric
pressure
 Weight of a column of air above a given point
 Force exerted by molecules on surface
 In the atmosphere, pressure decreases with
increasing altitude
 Nearly all of the weight of the atmosphere is in
the lower atmosphere

Weather Processes: Atmospheric
Pressure and Circulation, cont. 1
 Changes in air temperature and air movement are
responsible for horizontal changes in pressure
 Temperature influences pressure because cold air
is more dense and exerts greater pressure on
surface
 Global variations in temperature cause global winds
 At equator, air is warm and low in density
 Creates low pressure zones at the equator
 Air rises, condenses, forms clouds and rain
 Cooler, drier air sinks at latitudes around 30°
causing deserts

Weather Processes: Atmospheric Pressure
and Circulation, cont. 2
 Air movement can cause changes in pressure
 Convergence occurs when air flows in increasing pressure
 Divergence occurs when air flows out decreasing pressure
 At surface, air moves from surface high pressures (H) to low
pressures (L)
 Air at low rises into atmosphere and then diverges in the upper
atmosphere
 A surface low is often associated with a high aloft and vice
versa
 Jet streams
 Narrow, fast moving jets of air caused by low pressures near
the top of the troposphere

Air Circulation

Tropical rain forest
Tropical dry forest
Tropical savanna Temperate woodland
and shrubland
Desert
Temperate grassland
Boreal forest
(Taiga)
Northwestern
coniferous forest
Temperate forest
Mountains and
ice caps
Tundra
Section 4-3
Figure 4-11 The World’s Major Land Biomes

Unstable Air
 Tendency of air is to remain in place
 Atmospheric stability
 Air parcels resist movement or return to original
spot after they move
 In unstable air, parcels are rising until they
reach air of similar temperature and density
 Air is unstable when lighter, warm or moist air is
overlain by denser cold or dry air
 Some air sinks and some air rises

Fronts
 Air masses do not mix, Fronts are the boundary
between cooler and warmer air masses
 Cold front when cold air is moving into warm air
 Warm front when warm air is moving into cold air
 Stationary front where boundary shows little movement
 Occluded front where rapidly moving cooler air
overtakes another cold air mass wedging warm air in
between

Rain Shadow

Hazardous Weather: Thunderstorms
 Most occur in equatorial regions
 Most common in the afternoon or evening hours
in spring or summer
 Three conditions necessary
 Warm and humid air in lower atmosphere
 Steep vertical temperature gradient such that the
rising air is warmer than the air above it
 Cold air over warm air
 Updraft must force air up to the upper
atmosphere

Thunderstorm Development
 Moist air is forced upwards, cools and water vapor
condenses to form cumulus clouds
 Cumulus stage
 Moisture supply and updrafts continue, clouds grow
 A continuous release of latent heat from
condensation warms the surrounding air causing the
air to rise further
 Expanding the cloud into colder air causes water
droplets to freeze
 Larger snowflakes fall and melt as raindrops
 Large droplets grow until they cannot be supported
by updrafts

Thunderstorm Development, cont.
 Mature stage
 Downdrafts and falling precipitation leave the base of the
cloud
 Updrafts and downdrafts are present
 Cloud continues to grow until it reaches the top of unstable
atmosphere (tropopause)
 Storm produces heavy rain, lightning and thunder, and
occasionally hail
 Dissipative stage
 Upward supply of moist air is blocked by downdrafts
 Thunderstorm weakens, precipitation decreases, and the
cloud dissipates
 Most are air mass thunderstorms and do little damage

Severe Thunderstorms
 National Weather Service, classified severe if
 winds > 93 km (58 mi.) per hour, or
 hailstones > 1.9 cm (0.75 in), or
 generates a tornado
 Necessary conditions
 Large changes in vertical wind shear
 differences in wind speed and direction
 Greater the wind shear, the more severe the storm
 High water vapor content in lower atmosphere
 Updraft of air
 Dry air mass above a moist air mass

Severe Thunderstorm Types
 Mesoscale convective systems (MCSs)
 Most common type
 Very large clusters of self-propagating storms in which
downdrafts from one creates a new storm
 Downdrafts come together to form outflow boundaries
 curved lines of thunderstorms that may travel long distances
 Squall lines
 Long lines of individual storm cells common along cold
fronts
 Updrafts form anvil-shaped clouds extending ahead of the
line
 Downdrafts surge forward as gust front in advance of
precipitation
 Can develop along drylines
 Fronts with differing moisture content

Severe Thunderstorm Types, cont.
 Supercells
 Smaller than MCSs and squall lines, but more damaging
 Extremely violent and spawn most tornadoes
 Last from 2 to 4 hours
 Downbursts from thunderstorms can create:
 Derechos
 Strong, straight-line windstorms
 Wind gusts can be tornado strength
 Cause fallen trees, power outages, injuries, fatalities
 Microbursts
 Hazard for aviation

Hail
 Hard, round, irregular pieces of ice originating
from thunderstorms
 Hail moves up and down in lower part of the
storm adding layers of liquid water which then
freezes
 Cause mostly property damage
 http://www.youtube.com/watch?v=HPDzy5q-s1g

El Nino

Lightning
Lightning requires the separation of different
charges into different regions of a cloud.
How does charge separation in clouds occur?
We don’t know for certain, but we observe this:
Lightning only occurs in cold clouds with
supercooled droplets and temps below 5o
F.
Thus, the ice crystal processes responsible for
precipitation in cold clouds likely plays an critical
role in charge separation.

Charge Separation: One Theory
Hailstones are covered by a layer of liquid water.
The thin layer of liquid is positively charged.
When hailstones and ice crystals collide, some of
liquid molecules stick to the ice crystals.
Along with the mass transfer, positive ions transfer
from the hailstones to the ice crystals.
The heavier, negative hail falls to cloud bottom.
The lighter, positive ice crystals drift to cloud top.
Produces negative lower, positive upper cloud.

Charge Separation
• Top of cloud top has a
positive charge.
• Lower and middle of cloud
has a negative charge.
• Charge separation in cloud
maintains the earth’s fair
weather electric field
denoted by the arrow E
• E points toward positive
polarity
Williams, The Weather
PolarizationPolarization
––
++
E

Lightning Stroke
Ground strikes are usually
negative, that is electrons
flow from cloud to ground.
Williams, The Weather

www://thunder.msfc.nasa.g
Types of Discharges

http://video.nationalgeographic.co
m/video/environment/environme
nt-natural-disasters/landslides-
and-more/lightning/

Lightning Safety
Williams, The Weather Book

Hazardous Weather: Tornadoes
 Usually spawned by severe
thunderstorms
 1992–2002, killed 57
people/year
 Defined by vortex extending
downward from the cloud
and touching the ground
 Called funnel clouds when
it does not touch ground
 Form where there are large
differences in atmospheric
pressure over short
distances
Figure 9.18b

Tornado Development—Organizational Stage
 Vertical wind shear causes rotation to develop within
the storm
 Strong updrafts in advance of the front tilt the
horizontally rotating air vertically
 Known as a mesocyclone
 Updrafts at rear of the storm lower part of the cloud
 Wall cloud
 Wall cloud rotates and funnel descends

Tornado Development—Mature Stage
 Visible condensation
funnel extends to ground
 Moist air drawn upward
 In stronger tornadoes,
smaller whirls may
develop within tornado
 Suction vortex
 Responsible for the
greatest damage
Figure 9.18d

Tornado Development—Shrinking
and Rope Stage
 Shrinking stage
 Supply of warm air is reduced and tornado
begins to thin
 More dangerous because wind speeds increase
as diameter decreases
 Rope stage
 Downdrafts cause tornado to move erratically
and disappear

Video:
http://video.nationalgeographic.com/video
/environment/environment-natural-
disasters/tornadoes/tornadoes-101/
Last two weeks:
http://abcnews.go.com/US/wireStory/weathe
http://www.weather.com/video/see-and-hear

Tornado Classification
 Classified according to damage that they
produce using Enhanced Fujita Scale (EF)
 Waterspouts
 Tornadoes that form over water
 Develop beneath fair weather cumulus clouds
as a result of wind shear

Hazardous Weather: Blizzards
 Severe winter storms with
 large amounts of falling or blowing snow,
 High winds
 Low visibilities for extended period of time
 Whiteout – Extremely low visibility
 In United States: winds > 56 km (35 mi.) per hour, visibilities
< 0.4 km (0.25 mi.) for at least 3 hours
 In Canada: winds > 40 km (25 mi.) per hour, visibilities
< 1 km (1.6 mi.) for at least 4 hours
 Wind chill – wind cools skin, evaporates moisture, reduces
time it takes for frostbite to form

Ice Storms
 Prolonged periods of freezing rain
 Upon contact with cold objects, rain immediately freezes
to form a coating of ice
 Develop during winter on the north side of a stationary
or warm front
 Three conditions for freezing rain
1.Ample source of moisture
2.Warm air over shallow layer of cold air
3.Objects on land close to or at freezing

Fog
 A cloud in contact with ground
 Form by air cooling to condensation or adding water to
cooled air through evaporation
 Cooling
 At night heat radiates from land
 Warm air blows over cold water
 Humid air rises up a mountain side
 Evaporation
 Cold air flows over warm body of water
 Warm rain falls through cool air

Drought
 Extended period of low precipitation
 Produces a shortage of water for people,
animals and plants
 Regional food shortages
 Affects more people than any other natural
hazard
 Causes water and power shortages and
agricultural problems

Mountain Windstorms
 Develop seasonally on the downwind side of
mountain ranges or glacial ice fields
 Mountains block prevailing winds and can, under
specific conditions, cause winds to move quickly
down slopes
 Chinooks, east of Rocky Mountains
 Santa Ana, in Southern California
 Can cause roof and tree damage, blow cars off
highways, contribute to large wildfires

Dust Storms
 Strong windstorms in
which dust reduces
visibility for significant
amount of time
 Can be several hundred
kilometers in diameter and
carry 100 million tons of
dust
 Safety hazard for travel
 Affect climate and human
health
Figure 9.26a

Sandstorms
 Desert phenomenon where sand transported in
a cloud
 Rarely extends > 2m (6.5 ft.) above land
 Along with dust storms, occur mostly in
midlatitude, semiarid, and arid regions

Heatwaves
 Prolonged periods of extreme heat that are both longer
and hotter than normal
 Associated with long areas of high pressure, called
ridges
 Wet conditions to the west of ridge
 Dry conditions to the east of ridge
 Accompany either severe humidity or extreme dryness
 Heat index
 Body’s perception of air temperature

Minimizing Severe Weather Hazards:
Forecasting and Prediction
 Difficult to forecast weather events
 Doppler radar
 Detects clouds, rain, ice particles, etc
 Used to make short term predictions
 Watch
 Possibility of severe weather developing
 Warning
 Severe weather has been spotted, take action

Forecasting and Prediction, cont.
 Not able to predict tornado intensity
 Predicting amounts of snow and ice is difficult
 Nowcasting makes real time predictions
 Using radar, satellites, weather station data once
storms have formed

Preparedness and Personal Adjustments
 Know your climate and when hazards are most
likely
 Prepare your home for likely hazards
 Get information
 NOAA, National Weather Service, FEMA
 Wear proper clothing
 Protection from heat or cold
 Know the signs of hypothermia

Introduction to Cyclones
 An area or center of low pressure with rotating
winds
 Counter-clockwise in Northern Hemisphere
 Clockwise in Southern Hemisphere
 Tropical or extratropical
 Based on origin and core temperature
 Characterized by intensity
 Sustained wind speeds and lowest atmospheric
temperature

Tropical and Extratropical Cyclones
 Tropical Cyclones
 Form over warm tropical or subtropical ocean water (5°–20°)
 Have warm central cores
 Tropical depressions, tropical storms, hurricanes
 High winds, heavy rain, surges, and tornadoes
 Derive energy from warm ocean water and latent heat
 Extratropical Cyclones
 Form over land or water in temperate regions (30°–70°)
 Associated with fronts and cool central cores
 Strong windstorms, heavy rains, surges, snowstorms,
blizzards
 Most do not produce severe weather
 Derive energy from temperature contrasts along fronts

Classification
 Nor’easter
 Extratropical cyclone that moves along northward along East
Coast U.S.
 Hurricanes
 Tropical cyclones in Atlantic and eastern Pacific Oceans
 Typhoons
 Tropical cyclones in Pacific Ocean west of International
Dateline and north of the equator
 Cyclones
 Tropical cyclones in Indian Ocean
 Saffir-Simpson Scale classifies hurricanes based on wind
speed

Naming
 Extratropical storms are sometimes named after their
origins
 Example: Alberta Clipper
 Hurricanes named by international agreement through
World Meteorological Organization
 Named once winds exceed 63 km (39 mi.) per hour
 Names assigned sequentially each year from list for each
origin
 Male/Female names alternated
 Names are reused every 6 years
 Names of big storms are retired (example: Katrina)

Tropical Depressions and Tropical Storms
 Tropical Depression
 Tropical disturbance wind speeds increase and
begins to spin
 A low pressure center is formed
 Tropical Storm
 Winds increase to 63 km (39 mi.) ph
 Storm is given a name
 Wind speeds are not at hurricane strength, but
rainfall can be intense

Hurricanes
 Not all tropical storms develop into hurricanes
 Classified when winds reach 119 km (74 mi.) per hour
 Environmental conditions
 Thick layer of warm ocean water
 Steep vertical temperature gradient
 Atmosphere must cool quickly with increasing altitude
 Weak vertical wind shear
 Strong winds aloft prevent hurricane development.

Hurricane Structure
 Rain bands
 Clouds that spiral inward around center
 Counterclockwise in Northern Hemisphere
 Increase in intensity towards the center of the hurricane
 Eyewall
 Innermost band of clouds
 Contain the greatest winds and rainfall
 Eye
 Area of calm at center of the hurricane
 Narrow at surface and wider at top

Hurricane Structure, cont.
 Warm, moist air spirals upward around eyewall
 Air rises, it loses moisture
 Upward rotation draws air from eye, causing dry
air to sink back into center
 Upward rotation also causes air to flow out the
top of the storm concentrated in exhaust jets
 Allows additional warm air to feed bottom of the
storm

Hurricane Paths and Demise
 Movement is controlled by the Coriolis effect and
steering winds
 In Northern Hemisphere storms deflect to the right
 Track west in trade winds and curve northwest and
then northeast
 Hurricanes can make a loop
 In North Atlantic, steered by Bermuda High
 As hurricane moves over land, it loses energy
(warm water)
 Can become extratropical cyclone

Atlantic Hurricane Paths
 West toward East coast of
Florida, sometimes passing
over Caribbean
 Move out into the Atlantic
Ocean to the northeast
 Westward over Cuba and
into the Gulf of Mexico to
strike the Gulf Coast
 Westward to the Caribbean
and then northeastward
skirting the East Coast
 May strike the continent
from central Florida to New
York
Figure 10.17

Effects on Storm Surge Magnitude
 Largest effect from stress exerted by wind on water
 Fetch refers to the area over which the wind blows
 Larger fetch results in larger storm surge
 Smaller effect from low atmospheric pressure in
storm pulling up on water surface
 Also depends on shape of coastline
 Water level tends to increase continually as storm
approaches

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