Weather Patterns and
Severe Storms
Air Masses and Weather
 An air mass is an
immense body of air that is
characterized by similar
temperatures and amounts
of moisture at any given
time.
 Carries temp and moisture
conditions with it when it
moves, but can change
(modify) over time
Classifying Air Masses
 Named according to
their source region
–
–
–
–
–
Continental (land)
Maritime (water)
Polar (high latitudes)
Tropical (low latitudes)
Describe both moisture
and temperature
Weather in North America
 cP - cold & dry in winter
and cool and dry in
summer; “lake-effect”
snow
 mT - warm, moist and
unstable
 mP - North Pacific and
Atlantic; rain and snow
 cT - least influence for NA;
very hot and dry; droughtlike conditions
Front Formation
 When two air masses meet, they form a front,
which is a boundary that separates two air
masses.
 Often associated with some form of precipitation.
 When collide, pressure differences influence how
the air masses will behave (one moves faster).
 Classified according to the temperature of the
advancing front.
Warm Front
 Forms when warm air
moves into an area
formerly covered by cooler
air
 Warm air rises (less
dense) on top of colder air
(more dense)
 Gradual slope, so light-tomoderate precipitation
 Gradual increase in
temperature when passes
Cold Front
 Forms when cold, dense
air moves into a region
occupied by warmer air.
 Steep slope as cold air
wedges and forces warm
air aloft; moves faster than
warm front
 Can lead to heavy
downpours and severe
weather
 Temperatures drop when a
cold front passes
 Stationary front forms
when air masses run
parallel to each other
Occluded Front
 When an active cold front
overtakes a warm front
 Complex weather patterns
 Much warm front
precipitation, yet
advancing cold air
produces its own
precipitation pattern
Middle-Latitude Cyclone
 Main weather producer
 Large centers of low
pressure that generally
travel from west to east
and cause stormy weather
 Counter-clockwise air
movement
 Most have a cold front and
often a warm front
 Forceful lifting so much
precipitation
Thunderstorms
 A storm that generates
lightning and thunder.
 Frequently produce gusty,
winds, heavy rain, and
hail.
 May be produced by a
single cumulonimbus cloud
or clusters of along a cold
front
 About 45,000/day
 Most frequent in tropics
Thunderstorm Development
 Form when warm, humid
air rises in an unstable
environment
– Cumulus stage: strong
updrafts provide moisture
– Mature stage: heavy
precipitation (winds,
lightening, hail)
– Dissipating stage: falling
precipitation and descending
cold air from above causes
storm to die down
Tornadoes
 Violent windstorms that
take the form of a rotating
column of air called a
vortex which extends from
a cumulonimbus cloud.
 770/year in U.S.
 Greatest frequency April
through June (mT meets
cP)
 “Tornado Alley”
Tornado Development
 Typically form with severe
t-storms
 Often begin as a
mesocyclone (“rolling” air
at surface pushed up from
rising air)
 Air pressure 10% lower
than surrounding air, so air
near ground rushes in and
spirals upward
 Fujita scale measures
intensity
Hurricanes
 Whirling tropical cyclones
that produce winds of at
least 199 kilometers per
hour
 Generate high waves at
sea and strong winds and
flooding inland
 Most form near equator
(warmer ocean waters)
 Most powerful storms on
Earth
Hurricane Development
 Fueled by energy (latent
heat) when large quantities
of water condense
 Develop most often in late
summer when water
temperatures very warm
 Begin as tropical
disturbance (t-storms)
 As warm air rushes in to
core, air turns upward and
rises creating doughnutshaped wall (eye wall)
where greatest winds and
rain are found
 Weakens over land and
colder water (no “fuel”)
The bottom line…
 Severe weather occurs
when unstable air is forced
up
 Severe weather is
seasonal
– Frontal wedging between cP
and mT (Spring)
– Warm ocean waters
(Summer and Fall)
– Isolated T-Storms (localized
convective lifting in summer)
– Lake-effect snow (cP moves
over water)