I. Air Masses, Fronts, Midlatitude
Cyclones, Tornadoes, & Hurricanes
A. Air Masses
air mass (Defn)- a huge volume of
air (hundreds of square miles in horizontal size) - uniform in its temperature
and humidity characteristics
air mass advection (Defn) - movement of
an air mass (and thus its properties of temp & moisture) in the horizontal
from one location to another
Air Masses Affecting North America
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continental arctic (cA) - very cold & dry
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continental polar (cP) - cold & dry
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continental tropical (cT) - hot & dry
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maritime polar (mP) - cool & moist
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maritime tropical (mT) - warm & moist
Air Masses
B. Atmospheric Lifting Mechanisms
1) Orographic Lifting - air is lifted upslope as it is pushed
against a mountain; the airs cools adiabatically as it is lifted
-
the "windward" slope receives more precipitation than the "leeward"
(downwind) slope of the mountain
Orographic Lifting
2) Convection (convectional lifting) - a cooler (& moist)
air mass moves over a warmer land area -heating from the land causes the
air to rise - if the atmosphere is unstable it continues to rise (e.g.,
occur at: tropical islands, Florida, ITCZ)
Average Annual # of T'Storm Days
3) Fronts - also provide a means of atmospheric lifting
C. Fronts
Front (defn) - a transition zone between two air
masses having different densities (temperatures)
-
fronts typically separate air masses with different temperatures and humidities
-
since air masses have both a vertical and horizontal extent, a front also
extends upward in the vertical - the "frontal surface" (frontal
zone)
Fronts, Air Masses, Pressure Systems
Mid-Latitude Cyclone
Types of Fronts
(1) Cold Front
-
cold, dry, stable polar air is advancing into warm, moist, unstable tropical
air
-
a cold front is represented by a solid blue line with triangles pointing
in the direction that the front is moving
Criteria Used to Locate Fronts:
(a) sharp temperature changes in the horizontal
(b) sharp moisture (Td) changes in the horizontal
(c) wind direction shifts
(d) clouds and precipitation
Cold Front
(2). Warm Front
-
advancing warm, moist, tropical air replaces colder air
-
is depicted by a solid red line with half-circles pointing in the direction
of movement (movement is toward the colder air)
-
warm fronts move more slowly than cold fronts (~1/2 the speed of cold fronts)
-
the rising of warmer, less-dense air over the colder more-dense air is
referred to as "overrunning"
-
overrunning produces clouds and precipitation well in advance of the front's
surface boundary
-
warm fronts have more inclined slopes than do cold fronts
Warm Front
(3). Stationary Fronts
-
there is essentially no movement of the front
-
is depicted by alternating red semicircles and blue triangles (the blue
triangles point into the warmer air; while the red semicircles point into
the colder air
(4). Occluded Fronts
-
occur when a cold front catches up with and overtakes a warm front, and
all the warm air is forced aloft
Occluded Front
D. Midlatitude Cyclones & Cyclogenisis:
Satellite Image of a Mid-Latitude Cyclone
-
the frontal systems that we've been examining are part of a larger storm
system referred to as a "middle latitude (wave) cyclone"
-
midlatitude (wave) cyclones - are migrating centers of low
pressure - the air flows inward and CCW in the N. Hemisphere (inward &
CW in the S. Hemisphere) - there is rising motion in the center of
the low/cyclone > clouds & precip.
-
Norwegian meteorologists (just after W.W. I) developed a model explaining
the life cycle of the mid-latitiude cyclone called the "polar front
theory"
-
according to this theory/model the cyclone begins its development along
the polar front
-
cyclogenisis (defn) - the development or strengthening
of a mid-latitude cyclone
-
in addition to the polar front, there are other preferred areas of cyclogenisis:
eastern slopes of the Rockies ("lee-side-lows"), the Gulf of Mexico,
and the Atlantic Ocean east of the Carolinas.
-
Lifecycle of a midlatitude cyclone:
-
"birth", "maturity", & "death"
Cyclogenisis
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why do some waves grow into big storms, while others never really develop?
-
we need to look to the upper level winds for the answer
!!
Upper-Level Mid-Latitude Westerlies
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the air-flow aloft is comprised of a series of waves, both long and short
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longwaves (Rosby Waves): wavelengths of thousands of miles;
4-to-6 around the globe; move very slowly
-
shortwaves: move more quickly through the longwaves; deepen/weaken
when they approach a trough/ridge
-
these deep troughs are important for the development of surface lows because
they are associated with divergence and convergence
-
convergence - the piling-up of air above a region - aloft it leads
to the formation & maintenance of surface anticyclones (highs)
-
divergence - the spreading-out of air above a region - aloft
it leads to the formation & maintenance of surface cyclones
Divergence and Convergence in Upper-Level Gradient Winds
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for a surface low (cyclone) to be maintained/deepened, divergence aloft
must exceed surface convergence
-
for a surface high (anticyclone) to be maintained/intensified, convergence
aloft must exceed surface divergence
-
convergence aloft occurs on the west side of the trough; divergence aloft
occurs on the east side of the trough
Convergence, Divergence and Vertical Motions Associated
With Surface Highs and Lows
E. Thunderstorms
-
defn: a storm containing lightning & thunder; it may produce
gusty winds with heavy rain & hail; severe T'storms may spawn tornadoes
-
the storm itself may be a single cumulonimbus cloud, a cluster of them,
or even a line of clouds stretching for more than 100 km (62 miles)
-
T' storms develop: within an airmass, at/ahead of fronts
(particularly cold fronts), along the ITCZ, and at mountainous locations
-
the birth of a T'storm occurs when warm humid air rises in an unstable
environment
-
in North America most t'storms occur in areas dominated by mT air masses
Life Cycle of an Air-Mass T'Storm
Lightning: flashes of light resulting from electrical
discharges within the cloud (the majority of strikes), between the cloud
and the ground (~20% of strikes), from one cloud to another, or from a
cloud to the surrounding air
-
lightning is created when there is a buildup of electrical energy between
areas within the cloud, or between the cloud and the ground
-
for lightning to occur, separate regions containing opposite electrical
charges must exist in a cumulonimbus cloud
"Thunder" - results from the expansion of suddenly heated
air, which sends out shock waves (the air is heated by the lightning -
15,000-30,0000C
"Hail" - forms within cumulonimbus clouds when raindrops
are repeatedly carried above and below the freezing level - layers of ice
are added until it becomes too heavy and can no longer be supported by
the updrafts
Synoptic Situation Most Favorable for Development
of Severe T'Storms
F. Tornadoes
-
are rapidly rotating winds that blow around a small area of intense low
pressure
-
are spawned from severe T'storms
-
"funnel cloud" - the visible swirling circulation extending
from the parent cumulonimbus cloud
-
"tornado" - a funnel cloud that extends all the way to the
ground
-
tornado diameter: a few meters to a few hundred meters
-
typical forward speed: ~20 to 40 knots
-
most last only a few minutes & have an average path length of ~ 7 km
(4 mi)
-
horizontal wind speeds in tornadoes: can exceed 300 mph -
most
have lesser wind speeds
"Fujita Scale" - based on wind speed and property damage;
F0-F5
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65% of tornadoes are weak (wind speed < 112 mph)
-
33% of tornadoes are strong (wind speed:113-206 mph)
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2% of tornadoes are violent (wind speed >207mph)
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all 50 states have experienced tornadoes
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North America is by far the biggest recipient of Tornadoes (avg. 800 annually)
-
in particular, the greatest number occur in the "tornado belt"
(alley) of the Central Plains - from Texas to Nebraska - because
of its unique geographical location with regard to contrasting air masses
Average Annual Tornado Incidence (per 10,000 square
miles)
-
3/4 of tornadoes occur from March- July; May has greatest #; most frequent
in late afternoon (4-6 pm)
Average # of Tornadoes and Tornado Days Each Month
(27-year period)
Tornado Formation:
-
in order for a t'storm to spawn a tornado, the updraft (& the t'storm
itself) must rotate
-
this rising, spinning column of air (5-10 km across) is called a "mesocyclone"
-
inside the mesocyclone a spinning vortex (tornado) may appear near the
mid-level of the cloud and gradually extend downward to the cloud base;
and then perhaps down to the ground
G. Hurricanes (Tropical Cyclones):
-
Defn: an intense storm of tropical origin, with sustained
winds exceeding 64 kts (74 mph)
-
tropical cyclones have different names in different regions of the world
-
the Saffir-Simpson Scale is used to categorize hurricanes
on the basis of pressure, wind speed, storm surge, and damage
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form within the homogeneous air of the tropics (23.5 S-23.5N)
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Form over the warm (790F) tropical waters where winds are light - from
summer-fall in the tropical N. Atlantic & N. Pacific oceans
-
(Hurricane Season: June-November)
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for a hurricane to develop from an unorganized mass of T'storms, the surface
winds must converge
-
surface convergence takes place on the eastern side of an "easterly
wave" (a weak westerly moving area of low pressure within the trades)
Easterly Wave and Surface Convergence
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hurricanes will not form on the equator - no coriolis force for convergence
-
it is the release of latent heat that drives & fuels the hurricane
-
the heat released aloft warms the upper troposphere, creating higher pressre
aloft and diverging air - surface pressures then fall
-
as long as outflow aloft exceeds surface inflow, the hurricane will intensify
Hurricane Structure
-
when hurricanes move over land (or colder water) they lose their source
of heat and fuel & die out
-
hurricane destruction results from storm surge (which is responsible
for 90% of all hurricane-caused deaths, and is responsible for a large
share of property loses), wind damge, and inland
freshwater flooding
-
"storm surge" is a dome of water 65 to 80 km wide
that sweeps across the coast near the point where the eye of the hurricane
makes landfall; the storm surge is the height of the water (ignoring wave
activity) above normal tide level - it commonly adds 2-3 meters to normal
tide heights