Natural Disasters Week 3

Wind
The atmosphere is in constant motion because air flows due to pressure, moisture, and
temperature differences. Wind moves and mixes water as well as nutrients. This constant motion
also brings precipitation and storms.
Wind Direction
With no spin, wind would flow directly towards the lowest pressure, converge, and rise. With
spin, The Coriolis Force deflects wind to the right , producing an overall counter-clockwise
rotation of the wind. But with friction slowing the flow, which diminishes the Coriolis Effect,
wind actually spirals towards the center and up. System is reverse for high pressure.
Three forces act to influence the direction and speed of the wind at a location:
1) Pressure gradient: variations in temperature and water content of air over the Earth’s
surface result in variation of air density and pressure.
o Fluids move from high pressure to low pressure
o Velocity depends on the size of difference: larger difference = faster movement
2) Frictional force: Air feels friction as a volume of faster air shears past a slower volume
of air OR as it flows over the Earth’s surface.
o Air is denser at the Earth’s surface
o More stuff at the surface to flow over and around.
o This boundary layer, where air feels more friction, varies from 200m over the
ocean to around 2,500m over cities and mountains.
3) Coriolis force: the apparent east-west deflection of air currents of the convective cells
o As Earth rotates, equatorial regions spin faster resulting in:
▪ Divergence being to the right in the Northern Hemisphere
▪ Divergence being to the left in the Southern Hemisphere
Geostrophic Wind
The Coriolis force and pressure gradient force become equal and the wind flows along lines of
equal pressure rather than perpendicular to them (geostrophic).
• The Coriolis Effect, and resulting deflection, increase as air volume accelerates
• The pressure gradient force decreases once you leave the boundary layer
• Common above the boundary layer over much of Earth
o When we deviate from geostrophic we get big storms
Convective Circulation
Less dense, warmer air rises, creating vertical currents
• Air rises because it is warm and lower density (expands)

• Once it is away from the surface, it cools, and increases density
• Precipitation makes cooler dry air that sinks
• It then warms and rises again, continuing the process
Global Circulation
1) Vertical and north-south circulation in the hemispheres
2) Hadley Cells
o Equator gets the most solar radiation
o Heated, warm air rises over equator and cools
o Water vapor condenses, precipitates and dries
o Air moves away from the convergence
o Descends again as dry, cold air near 30 degrees latitude
Global circulation patterns create rain forests at the equator where warm, wet, rising air
produces a lot of rain. These patterns also create deserts at 30 degrees where cold, dry, falling air
flows back towards the equator.
These patterns repeat twice more further north and south
• Wetter, warmer, (OK, snowier) Taiga forests (Ferrel cell)
• Cold, dry polar regions (Polar Cell)
Global Winds
At the equator where there is little wind is a region know as the doldrums. Curving global wind
patterns from the Coriolis Effect cause the trade winds between the equator and 30 latitude,
and the westerlies between 30 and 60. These wind patterns facilitated ocean travel by wind-
powered sailing ships.
These global winds are historically important because they aided in global “trade”:
• North America exported sugar, tobacco, and cotton to Europe
• Europe exported textiles, rum, and manufactured goods to Africa
• Slaves taken from Africa and brought to America
Southern Oscillation and El Nino
Walker Circulation patterns are east-west air movements related to ocean temps:
• Flow of air pulls warmer water towards New Zealand and Australia
• Cold water rises to replace it enforcing the circulation
• When Walker Circulation weakens, wind and upwelling cease and warm water pours
back towards Peru
• Trade winds weaken or reverse and storms shift east

The east-west seesaw in surface air pressure across equatorial Pacific accompanies changes in
Walker Circulation in called El Nino – Southern Oscillation (ENSO). These changes in the
Pacific Walker Circulation patterns impact the globe:
• El Nino: drought and fires in Australia and Indonesia, flooding in Peru, California floods,
and Midwest blizzards and droughts
o El Nino in 1983 was catastrophic: droughts, dust storms, brush fires, encephalitis
outbreak from an increased population of mosquitoes, bubonic plague in New
Mexico, etc.
• La Nina: drought in Somalia, wildfires in the Rockies, floods in SE Asia, weakened
India monsoons
Monsoons
Monsoons are a seasonal change in air circulation directions over a region that results in
seasonal changes in rainfall. Usually, produced by summer winds blowing off a body of water
(typically the ocean) and winter winds blowing from land to sea.
In the summer, the land heats faster than the sea. Warmer land heats the air causing lift, and
cooler, wet sea air comes to replace it. Water laden air heats and rises producing rain.
In the winter, the land cools more than the sea. Warmer air rises over the sea and colder air flows
off the land to replace it. Colder, drier air produces clear skies and dry weather.
Mid-Latitude Weather
Mid-latitudes are the battleground between cold polar air and warm equatorial air. This battle is
between air masses along fronts. An air mass is a broad body of air where temperature and
humidity are relatively uniform. Incoming air either absorbs heat from the warm surface
(and takes on water) or incoming air loses heat to the colder surfaces.
Where cold, the air sinks and semi-permanent high pressure systems dominate. High pressure
usually involves sinking, cooler air, producing clear skies. Where warm and/or wet, the air rises
and semi-permanent low pressure systems dominate.
Fronts
1) Cold front: boundary where faster moving cold air lifts warm air in front of it
o Warm air cools and precipitation occurs
2) Warm front: boundary where cold air retreats and warm air advances to fill the space
o Warm air cools and precipitation occurs
o Less severe storms, but big storms can develop
o Ice and blizzards
3) Occluded front: boundary where faster moving cold air overtakes and lifts warm air
between fronts
o Heavy rain and blizzards
4) Stationary front: boundary where adjacent air masses aren’t moving
o Warm air cools over the cooler mass

o Can bring rain for days
o Heavy snow, floods over time
5) Polar front: divides warm air originating in the Tropics and polar air originating in the
Arctic and Subarctic
Jet Streams
Jet streams are formed when large temperature changes occur over relatively short distances
across the front, producing steep pressure gradients. Large pressure gradients induce high
velocity geostrophic winds at altitude. This pattern forms jet streams.
Jet streams are typically wavey depending on the air mass position and speed. This forms ridges
of warm air and troughs of cold air which strongly influence the weather.
Cyclones
Cyclones are broad areas of precipitation and strong winds due to rising warm air and steep
pressure gradients. The head of the storm is an occluded front and warm front that can
produce blizzards and ice storms, or long lasting rain. The tail of the storm is the cold front
producing a line of precipitating clouds that can produce thunderstorms, tornadoes, and
windstorms.
Steps to forming a cyclone:
1) Warm air flows upward over the front
2) Cold air rapidly comes in to replace warm air
o Storms along this front
3) Warm air continues to pass over the cold front and cold air continues to pass under the
warm air
o Everything begins to speed up
4) Eventually, the cold front catches the warm front, forming an occluded front
o Cooler air over cold air which is capped by warm air
Mid-latitude cyclones (what the Midwest experiences) formation:
1) Air deficit at divergence zones (air thins where it speeds up) creating a low pressure
center
2) Air rises in the low pressure area and is replaced by lower level air
3) The whole thing feeds itself as it spins, often creating heavy storms or multiple storms
over a period of days

Page

of 9

Zoom

EES:1400 – Natural Disasters
Lecture 6: Storms
Mid-Latitude Weather
Mid-latitudes are the battleground between cold polar air and warm equatorial air. This battle is
between air masses along fronts. An air mass is a broad body of air where temperature and
humidity are relatively uniform. Incoming air either absorbs heat from the warm surface
(and takes on water) or incoming air loses heat to the colder surfaces.
Where cold, the air sinks and semi-permanent high pressure systems dominate. High pressure
usually involves sinking, cooler air, producing clear skies. Where warm and/or wet, the air rises
and semi-permanent low pressure systems dominate.
Fronts
1) Cold front: boundary where faster moving cold air lifts warm air in front of it
o Warm air cools and precipitation occurs
2) Warm front: boundary where cold air retreats and warm air advances to fill the space
o Warm air cools and precipitation occurs
o Less severe storms, but big storms can develop
o Ice and blizzards
3) Occluded front: boundary where faster moving cold air overtakes and lifts warm air
between fronts
o Heavy rain and blizzards
4) Stationary front: boundary where adjacent air masses aren’t moving
o Warm air cools over the cooler mass
o Can bring rain for days
o Heavy snow, floods over time
5) Polar front: divides warm air originating in the Tropics and polar air originating in the
Arctic and Subarctic
Jet Streams
Jet streams are formed when large temperature changes occur over relatively short distances
across the front, producing steep pressure gradients. Large pressure gradients induce high
velocity geostrophic winds at altitude. This pattern forms jet streams.
Jet streams are typically wavey depending on the air mass position and speed. This forms ridges
of warm air and troughs of cold air which strongly influence the weather.
Cyclones
Cyclones are broad areas of precipitation and strong winds due to rising warm air and steep
pressure gradients. The head of the storm is an occluded front and warm front that can
produce blizzards and ice storms, or long lasting rain. The tail of the storm is the cold front

producing a line of precipitating clouds that can produce thunderstorms, tornadoes, and
windstorms.
Steps to forming a cyclone:
1) Warm air flows upward over the front
2) Cold air rapidly comes in to replace warm air
o Storms along this front
3) Warm air continues to pass over the cold front and cold air continues to pass under the
warm air
o Everything begins to speed up
4) Eventually, the cold front catches the warm front, forming an occluded front
o Cooler air over cold air which is capped by warm air
Mid-latitude cyclones (what the Midwest experiences) formation:
1) Air deficit at divergence zones (air thins where it speeds up) creating a low pressure
center
2) Air rises in the low pressure area and is replaced by lower level air
3) The whole thing feeds itself as it spins, often creating heavy storms or multiple storms
over a period of days
Thunderstorms
A thunderstorm is a towering cloud that produces thunder and lightning, and is accompanied by
strong winds, heavy rains, and sometimes hail.
Features of thunderstorms:
• Can reach from 2 km above ground surface to altitudes of 12-20 km
• Usually less than 30 km in diameter
• Travel at rates up to 80 km/h
• Usually occur in lines – can experience one after the another
• Occur worldwide
o Equator has more storms
o Southeast and southern Midwest have many storms
Three requirements for a thunderstorm:
1) The lower atmosphere must hold moist air
o Most of the water comes from the ocean – carried by regional winds
o In North America, water comes from the Gulf of Mexico and Atlantic Ocean
o Also from lakes and plant transpiration
2) A lifting mechanism must be present to initiate and updraft that can get the moist air to
higher altitudes
o Lifting behind cold fronts
▪ Cold air advances under slower moving warm air

o Lifting due to gust fronts
▪ Once precipitation begins, cold downdrafts occur
▪ These cold pools spread from the base of the storm
▪ Act as local cold fronts
o Orographic lifting over mountains
o Convective lifting over hot surfaces
3) Atmospheric instability must be present to allow that air to rise buoyantly to the top of
the troposphere
o For thunderstorms, the lifted air has to become, and remain, less dense than the air
around it to keep it rising
o It has to get warmer than the surrounding air or it will stop rising and start sinking
again
Atmospheric Instability
Stability depends on:
1) The environmental lapse rate – the rate at which the air surrounding our air parcel
decreases with altitude
2) The moisture content of our air
3) Dry adiabatic lapse rate – the rate a parcel of dry (unsaturated) air cools as it expands at
increasing elevations (~10C)
o If the dry adiabatic lapse rate > environmental lapse rate then the parcel stops
rising
▪ Parcel is cooling faster than the surrounding air until it is as cold or colder
4) Moist adiabatic lapse rate – the rate a parcel of wet (saturated) air cools as it expands at
increasing elevations (~6C)
o Moist adiabatic lapse rate (~6C) < Dry adiabatic lapse rate (~10C)
o Water in saturated air condenses as it cools with altitude
o Condensation of water vapor to liquid water gives off heat, keeping the parcel
warm and buoyant
o If the moist adiabatic lapse rate is < environmental lapse rate then our parcel
keeps rising
▪ SYSTEM UNSTABLE
Unstable air produces an updraft, where air blows vertically upward. Strong, saturated updrafts
produce towers of cumulonimbus clouds. Moisture at the surface is drawn up these updrafts,
further fueling the storm.
“Flow Chart” for Atmospheric Instability
If a rising air parcel is unsaturated:
• It cools more quickly than the surrounding air
• Reaches the same temperature/density as surrounding air
• Then it stops rising.

If an air parcels is saturated:
• Water vapor condenses as it rises
• The parcel cools slower than an unsaturated air parcel
Then, if:
• The rising saturated air parcel cools faster than surrounding air
• Water condenses into clouds but no thunderstorms
o SYSTEM STABILIZES
Or:
• Rising saturated air parcel stays warmer than surrounding air
• It keeps rising and thunderstorms can occur!
Types of Thunderstorms
1) Ordinary thunderstorms/single-cell thunderstorms
o Associated with: convective cells (warm ground) or orographic lifting
o Short lived
o Rarely produce hail, strong winds, or tornadoes
o Ordinary thunderstorm formation:
▪ Updrafts form
▪ Air parcels rise to the top of the troposphere and spread out
▪ Ice particles at high altitudes become snow and (or) graupel and start
falling
▪ Ice melts on the way down, becoming rain
▪ Rain starts to evaporate on its way down
▪ Rain-pushed, drier air becomes a downdraft
▪ Downdraft becomes stronger than updraft and shuts of “fuel”
▪ Gust front may produce new cell
2) Squall line thunderstorms/multicell thunderstorms
o Storms cover a large area and can last hours to days
o Responsible for most of the rain in the Midwest and Southern US
o Develop from combining gust fronts of disorganized ordinary thunderstorms or
along cold fronts with strong pressure gradients
3) Supercell thunderstorms: contain a particularly strong, rotating, updraft
o Severe storms
o Strong winds – up to 100 mph
o Tornadoes
o Hail
o Usually an isolated storm – 30 mi wide
o Develop when strong vertical wind shear exists close to the ground surface and
strong winds blow in the upper troposphere

▪ Vertical wind shear: condition where winds near differ in speed and
direction
o Vertical wind shear increases storm organization which increases the longevity of
the storm as well as the chance of severe weather
Supercell Outbreak
From April 25th to April 28th, 2011 waves of violent thunderstorms moved across the
southeastern United States. These supercell storms produced 360 tornadoes, demolishing the old
record. Despite proper warning, 346 people were killed and $10.2 billion in damage occurred.
Hazards of Thunderstorms
Reminder: a thunderstorm is a towering cloud that produces thunder and lightning, and is
accompanied by strong winds, heavy rains, and sometimes hail.
The main hazards associated with thunderstorms are:
1) Lightning
2) Strong winds
3) Heavy rains
4) Hail
Lightning
An analogy for lightning: shock from rubbing your feet on the carpet and then touching a door
knob or someone else!
• Charge separation: electrons from the carpet’s atoms flow into your body, making your
skin negatively charged relative to the object you touch
• When you get closer enough, electrons flow from your skin to the doorknob
• Electrostatic discharge: the small spark that results after touching the doorknob or
someone else
Lightning is a huge electrostatic discharge following a large charge separation:
• As ice particles in clouds move around each other, electrons move from small ice
particles to large ice particles
• Charge separation: small, positively charged particles move up by updrafts, large,
negatively charged particles fall
• Electrostatic discharge: electrostatic potential grows until it drives electrons across the
air

Dangers of lightening:
• Evaporates sap in trees, leading to explosions and fires
• Passes through frame of a house and can cause fires
• Strikes power lines, blowing transformers and lines and causing blackouts
Lightning and humans:
• Lightening strikes don’t generally kill humans
o The current moving outward from the strike is what kills humans
• Causes severe burns and wounds
• Air and water expand and heat up, causing serious internal injuries
• Paralysis of limbs and organs
• Breathing and/or heart failure
• Heat and light may ruin your eyes
Lightning avoidance:
• GO INSIDE!
• Avoid open areas and tall objects
• Get off the water!
• If you feel your hair standing up, get low or get in a car!
o Cars and airplanes conduct electricity on the outside but not the inside
o Assume a stooping position with your feet together
o Minimize contact with the ground by rocking from toes to heels
o Do NOT lie down
Hail
Spherical or irregularly shaped lumps of solid ice formed at high altitudes are called hail. It
forms in nearly all thunderstorms but don’t always precipitate out.
Hail hazards:
• Dent or damage cars, roofs and siding
• Shatter windows
• Knock down power lines

• Kill flocks of birds
Hail and humans:
• An orange sized hailstone is more than enough to kill you
• Hail rarely kills people
o GO INSIDE!
Hail and agriculture:
• Flattens crops
• Destroys gardens
• Kills livestock
Damage done by hail causes an annual loss of $1.3 billion to the U.S. government in crop
insurance each year.
Tornadoes
A tornado is a violently rotating vortex (funnel) of air that extends from the base of a severe
thunderstorm to the ground. Tornado Alley has the most tornadoes in the world with a
whopping 5-9 per year. More tornadoes in the Mississippi Valley have occurred in the last 2
decades and fewer in Oklahoma and Texas. However, Oklahoma and Texas still have more
overall.
Tornado formation:
1) Supercell formation with perpendicular lower atmosphere and upper atmosphere winds
(lower and upper winds are in opposition)
2) Upper wind tilt the supercell and an updraft starts to form a rear downdraft
3) A horizontal roll of rotating air forms at edge of rear-flank downdraft
4) The roll under the updraft gets sucked up, thins, and spins faster
5) Breaks free, elongates, and spins
o If it doesn’t touch down = funnel cloud
o If it does touch down = tornado
Tornados spin faster than the supercell, similar to a skater pulling their arms in tighter to make
them spin faster.
Multiple suction vortices in the same tornado vortex is called a multiple-vortex tornado. These
type of tornadoes are the worst with long-lived, wide swaths and intense winds. Multiple-
vortex tornado are formed in the following way:

1) Downdraft forms in the center of the tornado as it widens
2) Central downdraft reaches the ground, tornado is much wider
3) Downdraft interacts with spinning tornado vortex updraft and breaks system into smaller
vortices.
4) A number of short-lived intense suction vortices form that contain the most intense wind
The intensity and damage of a tornado is dependent upon the wind speed. However, this
type of data is hard to find because humans are not near the tornado to make these measurements
(evacuated or in basement) and equipment to measure tornadoes would get destroyed
immediately. Instead, we measure the amount of damage that occurs, similar to the Mercalli
Intensity Earthquake Scale.
Tornadoes can develop anywhere there is a strong horizontal wind shear (non-supercell
tornadoes). These types of tornadoes can occur anywhere air on one side of the front is moving
horizontally in a direction opposite to air on the other side, such as: waterspouts, landspouts, dust
devils, squall line tornadoes, and hurricane tornadoes.
Squall Line Storms
Squall line tornadoes occur when shearing in the atmosphere causes the air moving in opposing
directions to spin around a vertical axis forming new, weak vortices. These vortices get sucked
up by an updraft causing narrowing and lengthening becoming a short-lived tornado.
Study Guide Questions
1) What are the five kinds of fronts and how do they differ?
2) How do jet streams form?
3) What are the steps in forming a cyclone?
4) What is a thunderstorm and what are the three requirements to get one?
5) What is the source of moist air in thunderstorm formation?
6) Understand the four lifting mechanisms in thunderstorm formation.
7) What does atmospheric stability depend on?
8) What is the difference between the moist and dry adiabatic lapse rate?
9) What are the three different types of thunderstorms and how are they different? Which
kind is the most dangerous?
10) What are the four main hazards associated with thunderstorms?
11) What is lightning and how does it happen? Include the words: charge separation and
electrostatic discharge.
12) What part of lightning kills humans: the lightning strike itself or the electrical current?
13) What is hail?
14) What is the best way to avoid hazards associated with thunderstorms?
15) What is a tornado and how does it form?
16) What area of the world experiences the most tornadoes?

17) Which is the most dangerous: regular tornado, multiple-vortex tornado, or squall line
tornado?
18) How do we measure the intensity of a tornado? What are some issues with getting data?
19) How do multiple-vortex tornadoes form?
20) Where can non-supercell tornadoes form?
21) What are the best ways to protect yourself if caught in a tornado?