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1999 Oklahoma Tornado Outbreak

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1

1999 Oklahoma Tornado Outbreak

152 tornadoes over 7 days the U.S. Great Plains

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On average 1,300 tornadoes/year in the U.S.

Number of Tornadoes State in the U.s.

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1999 and 2013 Tornadoes Moore, Oklahoma

City struck by two extremely powerful tornadoes

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1999 oklahoma tornado

Highest-ever recorded wind speed of 484 km/hour

Killed 36, injured 295, and damaged/destroyed ~ 10,000 homes

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2013 oklahoma tornado of comparable destructiveness

Followed remarkably similar path

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6

Energy

a quantifiable physical property describing the state of a system

Ability of something to do work

Stored in many forms

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Force

Push or a pull

Magnitude measured by how much a body is accelerated

Measured in Newtons (N)  - Force necessary to accelerate 1 kg mass

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Work

Done when energy is expended

Physics: force applied to an object and it moves a certain distance in the direction of the applied force

Work = force x distance over which it is applied

Measured in Joules (J)

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Power

Atmospheric processes are concerned with large amounts of energy in atmospheric processes

Measured in exajoules (EJ) 1018 to describe global energy

Rate at which work is done = energy/time

Measured in joules/second = Watts (W)

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Potential energy

Stored energy

Example: water held behind a dam

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Kinetic energy

Energy of motion

Example: water flowing from the dam

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Heat energy

Energy of random motion of atoms & molecules

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Sensible heat

heat sensed or measured by thermometer

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Latent heat:

amount of heat absorbed or released when a substance changes phase

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Latent Heat

Amount of heat absorbed or release when a substance changes phase

No change in temperature

Liquid – gas – latent heat of vaporization

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Conduction

Transfer through atomic/molecular interactions

Two bodies in contact with one another

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Convection

Transfer through mass movement of a fluid

Hot air rises displaces cool air which falls

Creates a convection cell

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Radiation

Transfer through electromagnetic waves

Emitted by any substance that possesses heat

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Earth’s Energy Balance

energy received from the Sun balances the energy lost by the Earth back into space

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Electromagnetic Energy

Everything above absolute zero (0˚ Kelvin = -273˚C ) emits electromagnetic energy

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Electromagnetic Energy

form of energy that propagates as both electric and magnetic waves (𝜆)

travel in packets of energy called photons

aka light, radiation, electromagnetic waves/radiation

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Electromagnetic Energy

Wavelength

Frequency

Energy

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Sun

Nuclear fusion (H ⇢ He) creates electromagnetic (solar) radiation

Temperature

150 million km from Earth

Speed of light 300,000 km/s

~ 8 min to reach Earth

Earth intercepts a very small amount of all solar radiation

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Surface

6,000 oC

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Internal

16 million oC

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Electromagnetic Energy

Most of the sun’s energy is electromagnetic

Described by the wavelength

Distance from one crest to the next

All wavelengths are part of electromagnetic spectrum

Only a small part of this is visible

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Infrared radiation

Earth radiation is entirely (thermal) infrared (long wave)

~45% of the solar energy infrared (short wave)

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Visible Light

(0.4 to 0.7 µm)

47% of the solar energy

~ short Wave

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Ultraviolet (UV)

0.01 to 0.4 µm

~8% of solar energy

Most filtered by ozone layer

Sort Wave

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Absorption

gases & particulates interrupt the flow of radiation by absorbing specific wavelengths and gain heat

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Reflection

redirected radiation returning to space and has no heating effect

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Scattering

solar radiation bounces of an object in a variety of directions and has no heating effect

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Energy Behavior

Temperature depends on amount of energy absorbed or reflected

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Reflection depends on albedo

Describes the reflectivity of surfaces

Dark woodlands reflect 5 - 15%

Light grasslands reflect 25%

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Absorption

Energy that is not reflected is absorbed

Different objects absorb different wavelengths

Hotter objects radiate energy more rapidly and at shorter wavelengths

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The Atmosphere

Thin gaseous envelope that surrounds Earth

Gas molecules

Suspended particles of solid and liquid

Falling precipitation

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Atmosphere

Causes weather experienced every day

Responsible for trapping heat that keeps the Earth warm

Knowledge of structure and dynamics critical to understand severe weather

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Permanent (Constant) Gases

Nitrogen (N2) - 78%

Oxygen (O2)  - 21 %

Argon (Ar) – 0.93%

Neon (Ne), Helium (He), Hydrogen (H2)

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Variable (trace) Gases

Water (H2O) - 1 – 4 %

Carbon dioxide (CO2) - 0.04%

Methane (CH4)

Nitrous Oxide (N20)

Ozone (O3)

CFCs and HFCs (anthropogenic)

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Thermal layers of the Atmosphere

distinguished by temperature changes and gases

differs in density &  composition

“sphere” vs “pause”

Troposphere

Stratosphere

Mesosphere

Thermosphere

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The Troposphere

Most active zone

Majority of nonmarine living organisms

Weather occurs in this layer

Vertical mixing is common

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Temperature decreases with increasing altitude

Environmental lapse rate (ELR)

Temperature decreases at an average rate of 6.5oC per 1000m

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Humidity

Amount of water vapour in the atmosphere

Varies spatially and temporall

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Warm air can hold much more than cold air

Cold dry air can have close to 0%

Warm tropical air may have 4-5%

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Measures of humidity

Vapour Pressure

Specific humidity

Dew-Point Temperature

Relative humidity

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Clouds

composed of tiny water  or ice droplets that have condensed/frozen onto condensation nuclei

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four Weather Processes

Atmospheric pressure and circulation patterns

Vertical stability of the atmosphere

Coriolis effect

Interaction of different air masses

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Atmospheric Air Pressure Changes

Atmospheric pressure (barometric pressure)

Weight of a column of air above a given point

Force exerted by molecules on surface

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Relationship

Pressure decreases with increasing altitude

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Thermal Pressure

Results from changes in temp.

Equatorial low, polar high

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Dynamic Pressure

Caused by air movement

Subtropical High Pressure, polar lows

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Warm air is associated with

low pressure

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Cold air is associated with

high pressure

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Relationship between air temp. and air pressure

not constant

dynamic pressure can override thermal pressure

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How a pressure gradient creates wind

Any fluid under the influence of gravity will move until the pressure is uniform

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Differential Heating

Changes in air temperature and air movement are responsible for horizontal changes in pressure

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pressure belts

\n

A region on the earth that is dominated by either high pressure cells or low pressure cells

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Low- and High-Pressure Centers

Surface, air moves from surface high pressures (H) to low pressures (L)

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Atmospheric Stability

Tendency of air is to remain in place

Atmospheric stability describes what happens to a vertically rising parcel of air

Air parcels resist movement or return to original spot after they move

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Atmospheric Stability 2

The stability of the atmosphere depends on

Relative temperature

Density

Temperature of a parcel of air compared with surrounding atmosphere2

n 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

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Fronts

Boundary between cooler and warmer air masses

Air masses do not mix

Warmer air will always be lifted by the colder, denser air mass

Air masses also have different humidity levels, densities, wind patterns, and stability

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Stationary front

boundary shows little movement

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Occluded front

rapidly moving cooler air overtakes another cold air mass wedging warm air in between

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Severe weather refers to

Thunderstorms

Tornadoes

Hurricanes (Chapter 10)

Blizzards

Ice storms

Mountain windstorms

Heat waves

Dust storms

Hazardous due to the energy they release and damage they are capable of causing

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Thunderstorm Occurrence

Most occur in equatorial regions

Most common in the afternoon or evening hours in spring or summer

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Thunderstorms

Three conditions necessary

Moisture

Instability

Lifting Mechanism

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Moisture

Warm &  humid air available in lower atmosphere

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Instability

Steep vertical temperature gradient such that the rising air is warmer than the air above it

Colder air over warmer, moist air

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Updraft must force air up to the

upper atmosphere

Lifting Mechanism

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Formation of a thunderstorm

Moist air is forced upwards, cools and water vapor condenses to form cumulus clouds

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Air-mass-thunderstorms (Single Cell)

Most individual thunderstorms Last less than 1 hour and do little damage

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Severe Thunderstorms

Classified as severe by National Weather Service

Winds > 93 km per hour, or

Hailstones > 1.9 cm , or

Generates a tornado

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Formation conditions

Large changes in vertical wind shear

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

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Mesoscale convective systems (MCS)

circular clusters of storms

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Squall lines

linear belts of thunderstorms

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Supercells

large cells with single updrafts

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Mesoscale convective systems (MCS)

Most common type of thunderstorm

Large clusters of self-propagating storms

downdrafts from one cell leads to formation of another

Continued growth - storms can last for 12 hours or more

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Squall lines thunderstorm

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 also develop along drylines

Fronts with differing moisture content

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Damaging Winds

Begins with downdrafts

Can generate strong, straight-line windstorms

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Down Bursts

Strong winds produced by a downdraft over horizonatl area up to 10 km

Classified as

Micro Bursts

Macroburts

Derechos

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Microbursts

small downburst <4 km horizontal dimeter

Last for 2-4 minutes up to 270 km/h

Hazard for aviation

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Macroburst

Wind gusts can be tornado strength

Cause fallen trees, power outages, injuries, fatalities

dozen strike North America /year

Most in eastern two-thirds of U.S. & Canada

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Derechos

family of downburst clusters

At least 400 km in length

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Lightning flash per year 1995-2002

More lightning occurs over land

More lightning occurs near the Equator

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Lightning

Flashes of light produced by discharge of millions of joules of electricity

Common occurrence during thunderstorms

Extreme heat from discharge causes air to rapidly expand

Produces thunder

Most is lighting is cloud-to-cloud

Cloud-to-ground is less common

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us lighnight

us Kills ~ 100 and injures m > 300 each year

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Canada lighitng kills

10  injured an estimated 120 - 190 each year (Red Cross)

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Development of Cloud-to-Ground Lightning

25 million ground strikes in U.S./year

Complex process (simplified here)

Difference in electrical charges between cloud & earth

Narrow column of high-speed electrons toward Earth

+ve stepped leader & –ve streamers join by a spark creating a return stroke

Additional leaders & return strokes follow path creating lightning flash

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Types of Lightning

Males are five times more likely than females to be struck by lightning; around 85% of lightning fatalities are men (CDC)

People aged 15-34 years account for almost half of all lightning strike victims (41%). The majority (89%) of lightning deaths occur among whites (CDC)

~ 1/3 (32%) of lightning injuries occur indoors (CDC)

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90

Staying Safe in Lightning

To determine distance of a lightning bolt -  count number of seconds till thunder occurs:

divide by 3 to get distance in kilometers

30/30 rule

During a thunderstorm

Always shelter indoors if possible.

Outdoors - do not be the highest object or stand under highest trees.

Crouch low, with as little of your body touching the ground as possible

Cars are safe.

Wait 30 minutes after storm has passed

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Hail

Hard, round, irregular pieces of ice originating from thunderstorms

Contain rings due to adding coatings during updrafts

Hail moves up and down in lower part of the storm adding layers of liquid water which then freezes

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Hail

Cause mostly property damage

Averages $41 billion per year in the United States

Most common locations

North America: Great Plains in United States, Calgary region of Alberta, Canada

Other regions: North-central India, Bangladesh, Kenya, and Australia

Deaths not uncommon in Bangladesh and India because of poorly constructed dwellings

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93

Tornadoes

Usually spawned by severe thunderstorms

One of nature’s most violent natural processes

1992 to 2002, killed average of 62 people per year

Variety of shapes

Rope

Funnel

Cylinder

Wedge

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

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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

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Tornadoes mature stage

Visible condensation funnel extends to ground

Moist air drawn upward

In stronger tornadoes, smaller whirls may develop within tornado

Suction vortices

Responsible for the greatest damage

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Shrinking stage

supply of warm air is reduced and tornado begins to thin

More dangerous because wind speeds increase as diameter decreases

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Rope stage

Downdrafts cause tornado to move erratically and disappear

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Classification of tornadoes

Classified according to most intense damage that they produce

Assigned value on Enhanced Fujita (EF) Scale

Survey determines levels of damage experienced by 26 types of buildings, towers, and poles and hardwood and softwood trees

Estimate the maximum 3-second wind gusts

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99

Waterspouts

Tornadoes that form over water

Develop beneath fair weather cumulus clouds as a result of wind shear

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Occurrence of tornadoes

Found throughout the world, but much more common in the United States

Has the just the right combination of weather, topography, and geographic location

Most U.S. tornadoes occur in midwestern states between Rocky Mountains and Appalachians

Spring and summer in late afternoon and evening are most common times

Highest risk is in “Tornado Alley” – stretches from north to south through the Great Plains states

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