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Chapter 20
Weather Patterns & Severe Storms
Standards: Concept 2 PO: 11,12,13
Air mass
• Large section of air with the same temperature
and moisture throughout
• Can stay in the same spot for days and weeks
Movement of Air Masses
• As an air mass moves away
from where it formed it
takes the temperature and
moisture conditions with it
• As an air mass moves the
characteristics change and
so does the weather in the
area over which the air
mass moves.
Classifying Air Masses
• Air masses are named according to their source
region
Classifying Air masses
Temperature
Humidity
• Polar (P)
▫ Formed near the poles at
high latitudes
▫ Cold air
• Tropical (T)
▫ Formed near the equator at
low latitudes
▫ Warm air
• Continental ( c )
▫ Formed over land
▫ Dry air
• Maritime (m)
▫ Formed over ocean
▫ Wet air
*Temperature is always written first with a capital letter. Humidity is written
second with a lower case letter
Air Masses
Formation of Fronts
• When two air masses meet they form a front
• Associated with some form of precipitation
Warm Front
• Forms when warm air moves into an area
formerly covered by cooler air
Cold Front
• A cold front forms when cold, dense air moves
into a region occupied by warmer air
• Brings more violent weather, moves quickly
Stationary Front
• The air flow is neither toward or away from the
air masses it moves parallel to the front
• Brings gentle to moderate rain
Occluded Front
• When an active cold front overtakes a warm
front.
• Produces complex weather systems
Weather Map Symbols
Thunderstorms
• Generates lightning and thunder
• Produce gusty winds heavy rain and hail
• From when warm, humid air rises in an unstable
environment
• At any given time there are about 2,000
thunderstorms in progress
Stages of a Thunderstorm
Fun Facts
• Earth as a whole is struck by an average of more
than a hundred lightning bolts every second.
• The odds of becoming a lightning victim in the
U.S. in any one year is 1 in 700,000. The odds of
being struck in your lifetime is 1 in 3,000.
More Fun Facts
• Lightning is not confined to thunderstorms. It's
been seen in volcanic eruptions, extremely
intense forest fires, surface nuclear detonations,
heavy snowstorms, and in large hurricanes.
• Use the 30-30 rule. When you see lightning,
count the time until you hear thunder. If that
time is 30 seconds or less, the thunderstorm is
within six miles of you and is dangerous. Seek
shelter immediately.
Thunderstorm Arizona
Thunderstorm Phoenix
Tornados
• Violent windstorms that take the form of a
rotating column of air called a vortex. The vortex
extends downward from a cumulonimbus cloud.
• In the US occur most often from Nov. – June
Tornado Development
Tornado Intensity
Tornado Safety
Watches
Warnings
• Alert people to the possibility
of tornadoes in a specified area
for a particular time period
• Issued when a tornado has
actually been sighted in an
area or is indicated by weather
radar
Joplin, MO Tornado 2011
Tornado Damage
Tornado Damage Joplin, MO
Tornado Damage Joplin, MO
Hurricanes
(typhoons, cyclones or tropical cyclones)
• Whirling tropical cyclones that produce winds of
at least 119km per hour
• Most powerful storms on Earth
• Develop in that late summer when water
temperatures are warm enough to provide the
necessary heat and moisture in the air
• Eye Wall – greatest wind speed
• Eye – center of storm, no rain, calm
• Storm Surge – most dangerous part water comes
ashore
Hurricane Katrina
Eye
Eye
Wall
Storm Surge
Hurricane Irene 2011
Hurricane Fun Facts
• Names are alphabetically and alternate between
male and female names
• Deadliest hurricane was the Great Galveston
Hurricane in Texas (1900) category 4 storm with
8,000 deaths.
Hurricane Intensity