Download CH 20: Air Masses and Fronts

CH 20 Patterns & Severe Weather
• AIR MASSES—are immense bodies of air.
• Air Masses—are characterized by 2 things:
1. Temperature
2. Moisture Content
They can be 1600 km (1000 miles) across!
4 Air mass letter symbols:
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(P) = polar, cool or cold
(T) = tropical, warm or hot
(c) = continental, land = dry
(m) = maritime, ocean = moist
• Choose 1 from P or T & 1 from c or m.
• What air mass usually brings us precip.?
• What 2nd air mass affects Asheville?
• What air mass affects the northwest?
• What air mass causes lake effect snow?
• What air mass causes Indian Summers?
`20.2 FRONTS = are boundaries
between 2 air masses!
• Fronts are much narrower than air
masses—usually 15-200 km wide.
• 4 TYPES OF FRONTS:
1.
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WARM FRONTS
Warmer air moves into area of cooler air
Symbol red line with semi-circles
Gently sloped boundary & move slow
Steady (long-term) light to moderate
precipitation & warmer temperatures.
• Cloud sequence—cirrus, cirrostratus, alto
stratus, & then nimbostratus.
`2. COLD FRONTS
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Colder air moves into area of warmer air
Symbol blue line with triangles
Steeper sloped & move fast
Heavy (short-term) precipitation, even
severe thunderstorms or tornadoes
• Cloud sequence—cumulus begin to
vertically develop into cumulonimbus =
anvil head clouds!
3. STATIONARY FRONTS
• Movement is parallel to the line of the
front. (no forward movement)
• Surface position does not move
• Symbol is blue triangle on one side and
red semi-circles on the other side
• Weather is similar to a warm front but
usually last even longer!
`4. OCCLUDED FRONTS
• Occur when an active (fast) cold front
overtakes a warm front.
• Symbol is either purple triangles &
semi-circles or just purple semi-circles
on the same side of the line
• Weather is (at first) similar to a cold
front and then ends up lasting longer
with a steady rain.
Mid-Latitude Cyclones!
• M-L C. are the main stormy weather
producers for the United States.
• M-L C. are large low pressure centers.
• 6 Formation Steps: (page 569)
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• 1. Formation of a stationary front.
• 2. Wave shape (indentation) forms.
• 3. Counterclockwise flow (separation of
front into 2 fronts (cold & warm).
• 4. Cold front catches up to warm front
& an occluded front forms.
• 5. You now have 3 fronts.
• 6. The low or cyclone weakens.
20.3 Severe Weather!!!
Thunderstorms
• Static electricity forms in cumulonimbus
clouds.
• High winds, hail, heavy rains, L & T.
• Lightning bolt starts from ground goes
up.
• 5-6 times hotter than sun.
• One inch in diameter.
Tornadoes
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Season is in spring & summer.
Tornado alley—cP & mT clash.
Unstable air—forms mesocyclone
Doppler Radar- or visual.
Fujita scale F0-F5
Small intense storms, usually last for
short time.
Hurricanes
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Season is in late summer & fall.
Occur where water is warm.
Form from tropical storms.
Alternate male & female names.
Some names (Katrina) retired.
Saffir-Simpson scale Category 1-5
Storm surge, winds, & flooding.
Sandy Oct. 2012
Katrina August 2005
Over 1,800 people dead
Hurricane Floyd September 1999
• 52 deaths and 8,000 homes destroyed