Download Air Masses, Fronts, and Storms Lecture

Air
Masses
Air mass - a body of air
with characteristic
properties of
temperature and
humidity.
Classified by temperature &
by surface (land or
water) over which they
form.
cP – Continental Polar
cT – Continental Tropical,
mP – Maritime Polar,
mT – Maritime Tropical,
Properties depend on where mass originated – air
takes on characteristics of area over which it has lingered.
When it moves, it takes those characteristics with it.
Continental Polar
Air Masses
• Cool/cold and dry
with little precipitation
EXCEPT after
crossing the Great
Lakes, especially late
fall early winter.
• Picks up heat &
humidity from lakes
to become unstable
and produce lake
effect snow on
leeward side of lakes.
Maritime Tropical Air Masses
• Warm, humid,
unstable
• Responsible for
most precipitation
in eastern 2/3 of
U.S.
• Cause oppressive
heat waves in
summer.
Maritime Polar Air Masses
• Cool/cold & humid
• Mostly affect Pacific
Northwest and
Canadian Maritime
provinces.
• Orographic lifting in
Oregon & Washington
produces heavy rain &
snow on windward
slopes of Cascades,
rain shadow on
leeward slopes.
Continental
Tropical Air
Masses
•
•
•
•
Least influence on weather in North America.
Warm, dry, unstable
Usually form in summer and stay in southwest.
Can bring drought to midwest & Indian Summer
to northeast.
Air Masses Change as They Move
• Over a different
area, air mass
gradually takes
on the
characteristics of
the new region.
• Accompanying
weather also
changes.
IX. How do fronts influence the
weather?
• Boundary between air masses is called a
front
• http://earth.usc.edu/~stott/Catalina/Weather
Patterns.html
Cold front - Frontal Wedging Frontal Wedging 2
• 1. Cold air mass meets warm air mass
• 2. Cold air forces warm
air up along steep front
• 3. Sudden showers or
thunderstorms result
Warm front Warm Front Summary
• 1. Warm air mass meets cold air
mass
• 2. Warm air slides up over cold air
in long wedge
• 3. Nimbostratus clouds form and
may cause precipitation over long
period
Stationary front
• Cool air & warm air
battle to a standstill.
• Prolongs bad weather.
• Stationary Front
http://www.usatoday.com/weather/t
g/wsfront/wsfront.htm
Occluded
Front
A cold front catches up to a warm front. The two
cool air masses trap & lift warm air between them,
causing high winds and varied precipitation
• Occluded Front 1 Occluded Front 2
Formation of Middle-Latitude
Cyclones
• Stationary front develops w/ 2 air masses moving
in opposite, parallel directions.
• Wave develops w/ warmer air pushing into colder
air toward poles, cold air sweeping toward equator.
Formation of Middle-Latitude
Cyclones
• Low pressure develops at center of flow.
• Air movement forms warm & cold fronts.
• Cold front moves faster, overtakes warm front,
forming occlusion. Pressure decreases, winds &
precipitation increase. ML Cyclone 1
Airflow Aloft
• Airflow aloft feeds cyclones & anticyclones.
• Cyclones & Anticyclones feed each other.
Severe Storms
• Thunderstorms form when unstable warm, humid
air rises in thermals or along a cold front & builds
into cumulus, then cumulonimbus clouds.
• May occur individually or in a line ahead of a cold
front. TS2 TS3
• Produce
lightning &
often gusty
winds, heavy
rains, hail.
• Lightning
Hail - forms in
cumulonimbus clouds
• Ice pellets grow by
colliding with
supercooled water
droplets
• Strong updrafts carry
them back up to repeat
the cycle
• Develop one “growth
ring” for each cycle
through cloud before
falling.
Precipitation
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1805/es1805page01.cfm?chapter_no=visualizatio
n
• Vortex – rotating
column of air
extending from
cumulonimbus cloud.
• Strong winds aloft
cause lower winds to
roll horizontally.
• Strong updrafts in
thunderstorm tilt
vertically –
mesocyclone, which
may strengthen into a
tornado, w/ extremely
low pressure at center.
Tornado
Formation
TF2 TF3
Hurricane Formation
• A.k.a. tropical cyclones or typhoons, winds of 74+ mph
• Starts at tropical (between 5-20 degrees N & S latitude)
depression (low pressure system) over water.
• Heat engine - energy released from condensation feeds
storm. Form in late summer. Severe Storms Hurricane
• Inward rush of
warm air turns
upward, forms
ring of
cumulonimus
clouds, the eye
wall, where winds
are strongest &
precipitation
heaviest.
• Storm surge –
dome of water
pushed ahead of
hurricane.
Hurricanes,
Typhoons,
Tropical
Cyclones
X. How is weather
portrayed
on weather maps?
Local collecting
stations contribute
data in form of
station models
http://teach.fcps.net/imm
ex/preview/Weather_Stat
ion/case3/lib_weather_st
ation_model.htm
Calculating Relative Humidity
from Temperature and Dew Point
Precipitation
Cold cloud precipitation depends on supercooled
•
•
•
•
water (< 0oC, pure water in clouds won’t freeze solid
until -40oC) and supersaturated air (saturated air, 100%
relative humidity, is supersaturated with respect to ice
crystals).
Excess water vapor becomes ice, which
lowers relative humidity around cloud droplets, so
they evaporate, providing more water vapor for growth
of ice crystals, (a.k.a. snowflakes).
http://www.atmosedu.com/meteor/Animations/IceCryst
alProcess.mov
Precipitation
Cold Cloud Precipitation
Warm Cloud
Precipitation
Collision-coalescence
process – Larger
cloud droplets move
fast through cloud,
colliding and joining
together with smaller
droplets.
Precipitation
Rain – often begins
high in troposphere
as snow, melts as it
falls through
warmer air.
Snow – in low temp.,
low humidity air,
snow is light and
fluffy. In temps. >5oC, flakes clump
together to make
heavy, wet snow.
Precipitation
Virga –
precipitation
evaporates
before it
reaches the
ground.
Precipitation
Precipitation
Glaze (a.k.a.
Freezing Rain) –
raindrops become
supercooled as
they fall through
cold air and
impact cold
surfaces.
Sleet – rain
falls through
cold air and
partially
freezes into
clear ice
pellets.
Precipitation