Download Severe Weather

Severe Weather
1. Thunderstorms
2. Tornadoes
3. Hurricanes
Thunderstorm Formation
Three Conditions:
1. Abundant source of moisture.
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As moisture condenses heat is released.
So cloud is warmer than air around it. How is it
moving?
2. Mechanism to lift the air.
3. Atmosphere must be unstable.
•
Air must continue to cool with altitude so that the
cloud stays warmer than the surrounding air.
Limits to Growth
1. Rising air meets stable air that it cannot
overcome.
2. Rate of condensation is not producing
enough heat.
Air-Mass Thunderstorms
• Air rises because of
unequal heating of
Earth’s surface within
one air mass
– Max >> during midafternoon
• TWO TYPES:
1.
2.
Mountain: air mass moving
up side of mountain
Sea-breeze: temperature
differences between air
over land and air over
water.
Frontal Thunderstorms
• Produced by advancing cold fronts and
sometimes warm fronts
Cold Fronts:
- Air rises rapidly
- Lines of thunderstorms
- Get lift from push of
cold air
- Can persist long into
night
Warm Fronts:
- Air rises gradually
- Relatively mild
thunderstorms
Stages of Thunderstorms
•
•
•
•
Classified by direction of air movement
Cumulus: air rises vertically; clouds
Mature: downdrafts & updrafts; rain cools air
Dissipation: smaller downdrafts; no more
energy
Stages of Development
1. Cumulus Stage:
– Air rises vertically
upwards
– Creates updrafts =
moisture goes to the
upper parts of the cloud
2. Mature Stage
-- Strong updrafts
and downdrafts
What do you think causes downdrafts?
--As precipitation falls, it cools the air
around it. This newly cooled air sinks
rapidly = downdraft 
Stages of Development
3. Dissipation Stage:
-- Storm loses energy
-- lingering downdrafts
• The convection cell can only exist if there is a steady
supply of warm, moist air.
– Knowing this, What do you think is the ultimate cause of
a thunderstorm losing energy?
• DOWNDRAFTS! – They spread in all directions when they
reach the surface and cool the surface air.
Severe Thunderstorms
• Continuous supply of surface
moisture
– Cold fronts moving into warm
areas
• Instability of air
– Increases strength of updrafts
and downdrafts
• Supercells: intense, rotating
updrafts
• Only about 10% severe in US
Thunderstorm Hazards
1. Lightning:
 Electricity caused by rapid rush of air in a
cumulonimbus cloud. (friction)
 Heats air to 30,000 °C
 What is thunder?!
 Sound from heated air expanding and
contracting
Lightning
Thunderstorm Hazards
2. Floods:
 Rain falls faster than it can be transported or absorbed
3. Hail:
 Water droplets encounter ice pellets and freeze
 Ice pellets can get caught in the up and downdrafts
Hail Storm Video
• http://video.nationalgeographic.com/video/n
ews/samaras-hail-storm?source=relatedvideo
Thunderstorm Hazards
4. Wind:
 Downbursts: concentrated in one area
 Macrobursts and microbursts
TORNADOES
http://www.discovery.com/tv-shows/stormchasers/videos/tornado-near-miss/
Tornadoes
• Form when wind speed and
direction change suddenly
with height
– Wind Shear
• Funnel cloud: a tornado
before it reaches the ground
• Why can we see tornadoes?
– Dust and debris that is drawn
into the swirling air
Tornado Formation
• Wind Shear can cause
horizontal rotations
near Earth’s surface.
• Updrafts can tilt this
column of wind vertical
and speed up their
rotations.
• As rotation speed
increases, wind is
removed from the
center = low air
pressure
• Pressure gradient
produces violent winds
https://www.youtube.com/watch?v=0LXExpb3pY
Tornadoes
• F-scale: path of destruction, wind
speed, and duration
– F0-F5
• Only 1% reach F4-F5; Most are F1
• When do most tornadoes occur?
– Spring. Why?
– Large temp. contrasts between polar
and tropical air
• Where do most tornadoes occur?
– Central United States. Why? Think air
masses!
– Cold cP air collides with warm mT air
from the gulf
April 27, 2011 Tornadoes
• https://www.youtube.com/watch?v=5ohIVzIZL
uQ
• https://www.youtube.com/watch?v=vhKjv9Gu
ARQ
Tornado Alley
Fujita Scale
SAFETY!
Warning signs:
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Dark, greenish skies
Hail
Towering wall of clouds
Roaring noise
SEEK SHELTER!
http://ngm.nationalgeographic.com/2013/11/
biggest-storm/draper-text
TROPICAL CYCLONES
 Large, rotating, low-pressure storms
 Hurricanes = strongest
 Coriolis effect causes rotation
TROPICAL CYCLONE FORMATION
CONDITIONS:
 Abundant supply of warm ocean water
 Disturbance to lift warm air and keep it rising
 What oceans would cyclones NOT form in?
 South Atlantic and South Pacific oceans. Why?
 Cooler waters
Stages of Development
1. DISTURBANCE:

Tropical disturbance causes air to rise
2. DEPRESSION:

Cyclonic circulation at a low pressure center
3. TROPICAL STORM:

Winds exceed 65 km/hr (40 mph)
4. HURRICANE:

Air pressure falls; winds can reach 120 km/hr (75 mph)
Classifying Hurricanes
Hurricane Map
Hurricane Paths
HURRICANE FORMATION
 Water evaporates, latent heat is stored
 Heat is released as air rises and clouds form
 Disturbances produce more precipitation; more
energy is released
 Rising air creates low pressure at ocean’s surface
 Coriolis effect causes moving air to turn
counterclockwise in the northern hemisphere
 Cyclonic rotation of a tropical cyclone
HURRICANE HAZARDS
• Storm surge: mound of ocean water towards
coast
• Flooding: caused by vast amounts of rain
• http://www.youtube.com/watch?v=s76Qn7bp
CsQ&safety_mode=true&persist_safety_mode
=1
Floods and Droughts
• Droughts are extended periods of well-belownormal rainfall
• Floods can occur when storms and hurricanes
unleash enough rain and remain over an area
for an extended time
• Both are considered recurring weather.
Heat and Cold Waves
• Heat waves usually come with drought and
involve extended periods of above-normal
temperatures
• Cold Waves are extended periods of belownormal temperatures
• Both are brought on by high-pressure systems
except cold waves are brought on by continental
polar or arctic air masses.