Weather Patterns and Severe
Storms
20.1 Air Masses
Air Masses
• Weather patterns result from a movement of
a large body of air.
• Characterized by a similar temperatures and
amounts of moisture at any given altitude.
• Could take several days for an air mass to
move across an area = constant weather
Movement of Air Mass
• When the air mass
moves out of the region
it formed in ---- it takes
the temp and moisture
with it.
• The air mass changes as
it moves as well.
Classifying Air Mass
• Source Region – the area that an air mass gets
it characteristics properties of temp and
moisture.
• Polar (P) and Tropical (T) – describe
temperatures
• Continental (c) and Maritime (m) – where they
form and moisture characteristics
Weather in N. America
• Majority of weather is influenced by the cP
and mT air masses.
– Form in Alaska/N. Canada, and the Gulf of Mexico
• cP (continental polar) – uniformly cold and dry
in winter, can cool and dry in summer.
– Lake – effect snow = when the temp of the lake
and land are extremely different causing heave
and unstable air, leads to heavy snow
(precipitation)
• m(T) – warm, loaded with moisture, usually
unstable
– Responsible for most of the precipitation in the
eastern US
– In summers brings high temps and humidity
• m(P) – originate off the coast of eastern
Canada
– Can bring with it heavy snow/rain (from Siberia to
western coast of US)
– Nor’easter = snow and cold temperatures as low
pressure center passes, going counterclockwise
• c(T) – least influence on the weather in NA
– Hot dry air masses begin in the southwestern US
and Mexico during summer
– Cause extremely hot, droughtlike conditions in the
Great Plains in the summer
– Indian Summer – mild weather in the Great Lake
region (unseasonably warm and mild)
20.2 Fronts
Formation of Fronts
• When two air masses
meet, from a front, a
boundary that separates
two air masses.
• Fronts are narrow,
associated with
precipitation, usually acts
as a barrier that travels
with an air mass,
classified according to the
temp of the advancing
front.
Warm Front
• When warm air moves into an area formerly
covered by cooler air.
– Gradual slope as warm air rises it produces clouds
leads to precipitation
– Slow moving front
– Light to moderate precipitation
• Shown by red lines and semicircles on a weather
map that point towards the cooler air.
Cold Front
• Forms when cold dense air moves into a
region occupied by warmer air.
– Becomes a steep slope, advance rapidly
– Violent weather (heavy downpours, gusty wind)
– Behind a cold front weather is dominated by a
cold air mass
• Shown by a blue line with triangles that point
toward the warmer air mass
Stationary Front
• When the surface position of the front does
not move
– The flow of air is neither toward the cold air mass
or the warm air mass
– Gentle to moderate precipitation can occur
• Shown by blue triangles on one side and red
semicircles on the other of the front.
Occluded Front
• An active cold front overtakes a warm front
– The cold air mass wedges the warm front upward
– Complex weather (due to warm air going up)
– Can make its own light precipitation
• Shown on a weather map by blue triangles
and red semicircles on the same side of the
front line.
Middle Latitude Cyclones
• Main weather producers in the country
– Centers of low pressure that generally travel from
west to east, cause stormy weather
– Move in a counterclockwise direction (toward the
center of the low)
– Most have a cold front – lifting causes the
formation of clouds and abundant precipitation
• Low pressure systems shown on a map by a
“L”
How do they form?
1.) Development of a front (two air masses with
different temps move in opposite directions)
2.) Front takes on a wave shape
3.) Warm air moves towards Earth’s poles, cold air
moves toward the equator
4.) The change in airflow near the surface is
associated with a change in pressure (resulting in a
counterclockwise direction)
5.) Occluded front forms – storm gets stronger
6.) When all the warm air is gone the friction of the
air and the surface slows the airflow and it “dies”
The Role of Airflow Aloft
• More often then not, air high up in the
atmosphere fuels a middle latitude cyclone.
• Usually can find anticyclones and cyclones
next to each other.
20.3 Severe Storms
Thunderstorms
• Vertical movement of
warm unstable air can
lead to a thunderstorm.
• Generates lighting and
thunder, produce gusty
winds, heavy rain, and
sometimes hail
• Could be a single
cumulonimbus cloud or a
large stretch along a cold
front.
• Greatest number of thunderstorms occur in
the tropics – due to warm, moisture rich, and
unstable air
• Development – when warm, humid air rises in
an unstable environment (lifespan about 2hrs)
– 1.) Strong upward movements of warm moist air
– 2.) Mature Stage – amount and size of
precipitation due to the updraft is to large to be
supported (most active stage)
– 3.) Dissipating Stage – when the downdrafts
dominate, cooling effect of the falling
precipitation causes the storm to die
Tornadoes
• Violent windstorms that
take the form of a
rotation of air called a
vortex that extends
downward.
• Tornado Season = April
through June
(December and January
less frequent)
• Form with severe
thunderstorms
• Mesocyclone –
important to form a
tornado, a vertical
cylinder of rotating air
that develops in the
updraft
Tornado Intensity and Safety
• Wind speed is measured by the Fujita Scale –
winds cannot be measured directly so
determined by the amount of damage done.
• Tornado Watch – the possibility of a tornado
(favorable conditions)
• Tornado Warning – tornado has actually been
see in the area or on the weather radar
Hurricanes/Typhoons/Cyclones
• Tropical cyclones that
produce winds of at least
119 km/hr
• Most powerful storm on
Earth
• Can bring to shore strong
winds, storm surge, and
flooding
• A higher threat now that
more people live and
work near the coastline
Development of a Hurricane
• A heat engine that is fueled by the energy
given off when huge quantities of water vapor
condense
• Develop most often in the late summer when
water temps are warm enough to provide
heat and moisture to the air
• Tropical disturbance to a category 5 hurricane
– Eye – wall – is formed and usually helps to sustain
the hurricane (strongest wind gusts are here)
Hurricane Safety
• Intensity measured by the Saffir – Simpson
Scale
• Storm Surge – water that precedes the
hurricane and sweeps onto the shore/land.
• Hurricanes weaken as they move over colder
water (adequate heat and moisture) or land
(due to friction and lack of moisture)
21.3 Climate Change
Climate Change
• Climate is always changing (short-term or
long-term)
• Natural Process that change climate
– Volcanic Eruptions
– Ocean Circulation
– Solar Activity
– Earth Motions
Volcanic Eruptions
• Emits large amounts of
ash and dust into the
atmosphere
• Aerosols in the air
increase solar radiation
reflected back into the
space – lowering the
atmosphere
temperature (Volcanic
winter)
Ocean Circulation
• El Nino is the change in ocean circulation
• Short term climate fluctuations
Ex.) An area that usually has a lot of rain could
experience a drought during an El Nino
Solar Activity
• Changes in the output of the solar energy
• Sunspots = warm periods in Europe and North
America
• Short term Effects
• No long term variations due to solar activity exist
(yet)
Earth Motions
• Most are long-term changes
• Tectonic plate movement – change in position
on land and ocean
• Shape of orbit and tilt of the Earth on the axis
– Smaller the tilt the smaller the temperature
difference between summer and winter
Human Impact on Climate Change
• Greenhouse Effect – a natural warming of
both Earth’s lower atmosphere and surface
– Major gasses involved – water vapor and CO2
– Makes life possible on Earth
– Human impact has increased CO2 levels in the
atmosphere (burning of fossil fuels and clearing of
forest)
• Global Warming – a result of the increases in
CO2 and other gases ---- global temperatures
have increases
• Sea level rising, flooding, heat waves,
droughts, more water vapor in the air,
temperature increase across the globe.