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Fronts
Chapter 21 Section 2
Front: boundary between air masses of
different densities.
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Fronts can be hundreds to thousands of kilometers long.
Cooler air is denser and often does not mix with
warmer air so a front forms.
Cold Front:
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Cold air mass moves into a warm air mass.
It pushes underneath the warm air & lifts it.
Lots of cold air arrives at once.
Warm air cooling makes clouds & rain.
Fast moving cold fronts
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often form cumulonimbus
clouds…can result in
short lived violent storms.
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Squall line: line of heavy
thunderstorms that form
ahead of a fast cold front.
Cold front: shown as a blue line with
triangles pointing toward warmer air.
Cold front moves in: sudden short
rainstorm, then temperature drops.
Warm Front:
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Warm air moves over a cold air mass.
Warm air rises as a gentle slope.
Clouds can form way beyond the base of the front so
warm fronts cause precipitation over large areas.
Warm fronts form clouds and rain over a
long distance.
Warm front: shown as a red line with
semicircles pointing toward colder air.
Warm front moves in: lots of rain, then
warmer weather.
Stationary Front:
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Forms when two air
masses move very slowly
or not at all
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Air masses may move
parallel to each other.
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Rain where fronts meet.
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Symbol combines cold &
warm front symbols.
Occluded Front:
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Fast moving cold front completely overtakes a warm
front and lifts it off the ground.
Polar Front
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In the main area of the US,
between 60o and 30o N,
weather is mainly caused
by the polar front
between two large wind
belts.
Mid-latitude Cyclones
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Large, rotating storms often occur in the middle latitudes
where the polar front meets warmer air.
Mid-latitude cyclone vs hurricane
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Mid-latitude
cyclones are
similar to
hurricanes, but
much larger and
less intense.
Anticyclones
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Anticyclones blow outward from a high pressure area,
the opposite of a cyclone.
Thunderstorms have three stages:
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Cumulus – warm, moist air rises (updraft) & forms cloud.
Mature – cumulonimbus forms; air pulled downward by
rain. Anvil appears.Torrential rain and hailstones.
Dissipating – no more warm air rising, only downdraft.
Water vapor decreases.
Lightning – go to NOAA for animations
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http://www.lightningsafety.noaa.gov/science/scienceoverview.shtml
Lightning
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an electrical spark in the
atmosphere, or between
the atmosphere and the
ground.
Electricity is a stream of
moving electrons.
Air is an insulator that
keeps electricity from
flowing easily. So the
charge builds up until it
can zap across.
Lightning: A charge is formed
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Particles of ice and hail
bounce around in the
cloud.
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Ice crystals become
positively charged and rise
to the top of the cloud.
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Hailstones become
negative and drop to the
bottom.
Lightning
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The charge on the bottom of the cloud attracts positive
charges on the ground.
Lightning: between clouds, between
bottom of cloud and ground.
Positive lightning can occur between the top
of the cloud and the ground.
Positive lightning has farther to go; it is rare
and stronger than negative lightning.
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Positive lightning
strikes cause
forest fires.
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Negative lightning
doesn’t.
Forestry departments have machines
and watchtowers to record lightning
data and find fires early.
Hurricanes – most destructive storms
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Develop over tropical oceans.
Winds greater than 120 km/hr, spiralling in towards the
low pressure center.
Hurricanes cause huge storm surges.
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Most hurricane deaths are
caused by drowning.
Hurricane Formation:
Warm moist ocean air rises rapidly &
condenses. Large amounts of latent
heat are released, which strengthens
the rising air.
Hurricane structure
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Thick cumulonimbus clouds spiral upward around a very
low pressure center and cause the eyewall winds to
increase speeds. The center (eye) is clear, sinking dry air.
Hurricane Ratings
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How do we rate them?
The Saffir-Simpson scale
(uses several factors:
pressure, wind speed,
storm surge)
Categories are from 1-5
http://www.nhc.noaa.gov/a
boutsshws.php
Alerts
Hurricane Watch
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Hurricane conditions
possible within 48 hours.
Hurricane Warning
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Hurricane conditions
expected within 36 hours.
Hurricane History
Hurricane Andrew, 1992
Hurricane Wilma, 2005
Tornadoes – highest speed winds
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destructive, rotating column of air with VERY high wind
speeds - up to 400 km/h.
If it touches the ground, it follows a haphazard
(wandering) path.
Forms when a thunderstorm meets high
horizontal winds.
Most common in Tornado Alley.
Most deaths are from being trapped in
collapsing buildings and flying debris.
Alerts
Tornado Watch
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Weather conditions are
favorable for severe
thunderstorms that can
produce tornadoes.
Tornado Warning
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Severe thunderstorms
with tornadoes are
imminent or occurring.
Tornado In Coconut Creek, Broward
County, FL Jan 27, 2016