WEATHER SYSTEMS

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WEATHER SYSTEMS
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Figure 7. Approximate locations of air masses d
eveloped over the Northern Hemisphere (left) and
Southern Hemisphere (right) in July. cA -
continental Arctic/Antarctic cP - continental
polar cT - continental tropical mP - maritime
polar mT - maritime tropical. Original globes
courtesy of NOAA's National Geophysical Data
Center.
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Air Masses

• Air masses are large regions of the lower
  atmosphere with uniform characteristics that are
  originally defined by a source area.
  
• Air masses are identified by temperature (polar
  vs. tropical) and the nature of the source area
  (continental vs. maritime).
  
• North American weather patterns are dominated by
  continental polar and maritime tropical air
  masses.
  
• Air masses are modified as they move over areas
  with different temperatures or topography than
  the source area.
  

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Modification of Air Masses
The initial characteristics of air masses change
as the mass of air moves out of its source area
and passes over regions with contrasting
attributes. Modification are caused by the tem
perature and topography of the underlying
surface. Air masses will be heated or cooled from
below. Heating (cP air moving south) will lead to
instability as air near the ground surface rises,
mixing the air column. Cooling (mT air moving no
rth) has the opposite effect because cold air
cannot rise but remains in a stable configuration
near the land surface. Orographic lifting forces
maritime air upward over mountain ranges in the
western U.S., leading to condensation and
precipitation that converts the formerly humid
air to a much dryer air mass.
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Frontal Systems
Frontal systems represent the meteorological
battle that ensues when air masses of contrasting
properties clash along their boundaries. As air
masses move across Earth's surface they
inevitably interact to create relatively narrow,
curvilinear zones that mark a front, a transition
from one air mass to another.
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Warm and Cold Fronts
Figure 11. Weather conditions associated with c
ross section A-B on Figure 10. Warm air (mT) lies
between the cold front and warm front. The cold
front advances more rapidly than the warm front,
forcing warm air to rise, forming thunderclouds
and heavy rains. Warm air is forced to rise above
the more gently sloping warm front, resulting in
the formation of a series of low to high
clouds.
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Squall line
Figure 12. A squall line highlighted by intense
thunderstorms associated with a rapidly advancing
cold front, Gulf of Mexico. Image courtesy of
NASA's Johnson Space Center Image Services
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Mid-Latitude Cyclone
Much of the weather experienced over the eastern
U.S. is the result of the west-to-east migration
of regional-scale low pressure systems known as
mid-latitude cyclones (Fig. 15). Mid-latitude
cyclones develop where continental polar and
maritime tropical air masses collide over the
U.S. along the polar front. The position of the
collision zone migrates south during winter and
moves north with summer.
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Mid-Latitude Cyclone
The boundary between air masses is initially a
stationary front, with airflow in opposite
directions on either side.
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Cyclones and Anticyclones
The combined effect of the controls on wind
direction is that airflow spirals inward
(converges) into areas of low-pressure,
generating a feature known as a cyclone (Fig.
27). In contrast, winds diverge from
high-pressure systems (anticyclones Fig. 27).
Circulation in low pressure systems is therefore
counterclockwise, whereas flow is clockwise in
anticyclones.
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Cyclones and Anticyclones
Low-pressure systems would be rapidly dissipated
by converging air unless the in rushing air was
balanced by a rising air column. Rising air
becomes cooler and may reach saturation,
resulting in clouds and rain.
In contrast, air descends in high-pressure zones,
warming as it approaches Earths surface. As the
air becomes warmer its relative humidity
decreases resulting in dryer air. Low-pressure
systems are often associated with rainfall and
cloud formation whereas high-pressure systems
result in clear skies and dry weather.
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Thunderstorms

• Thunderstorms form where warm, humid air is
  forced upward at cold fronts or as a result of
  differential heating at Earth's surface.
  
• Latent heat, released during condensation,
  generates updrafts that maintain upward movement.
  
  
• Thunderstorms are most frequent over the
  southeastern U.S.
  
• The three stages (cumulus, mature, dissipating)
  in the life cycle of a thunderstorm occur over
  approximately two hours.
  

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Tornadoes

• Tornadoes are narrow, funnel-shaped spirals of
  wind that rotate at speeds of up to 500 km/hr
  because of extreme pressure gradients.
  
• Tornadoes are ranked from F0 (weakest) to F5
  (strongest) using the Fujita Intensity scale.
  
• Most tornadoes move to the east or northeast at
  an average speed of approximately 50 km/hr.
  
• Tornadoes are associated with thunderstorms and
  develop in association with mesocyclones within
  the thunderstorm cell.
  
• The U.S. experiences more tornadoes than any
  other nation and most occur in tornado alley
  (Texas, Oklahoma, Kansas, Nebraska, Iowa,
  Illinois, and Indiana).

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Fujita Intensity Scale
Scale Wind Speed km/hr (miles/hr) Damage
Class of U.S.

Tornadoes Time on Ground F0 Weak 69 F1 116-180 (72-112) Moderate
F2 181-253 (113-157 Considerable Strong 29 20
minutes F3 254-332 (158-206) Severe F4 333-
419 (207-260) Devastating Violent 2 1
hour
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Stages in Development
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US Fatalities have declined as forcasting improves
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Hurricanes

• Hurricanes are rotating storm systems hundreds of
  kilometers across that strike the U.S. during
  summer and early fall.
  
• Hurricanes grow from tropical depressions in
  regions of convergent winds and warm oceans, and
  are sustained by divergent airflow in the upper
  troposphere.
• Destruction from high winds, heavy rains, and
  coastal flooding occurs when hurricanes make
  landfall.
  
• Hurricanes are divided into five categories by
  wind speed using the Saffir-Simpson hurricane
  intensity scale.
  

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Hurricanes
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Hurricanes

• The initial stage in the development of a
  hurricane is the formation of a tropical
  depression (low pressure system) where the trade
  winds converge near the equator.
• Water temperatures must be at least 27oC and
  should extend downward for 50 to 65 meters
  (165-215 feet) to ensure that colder water won't
  be drawn to the surface by the developing storm.
  Warm surface waters typically straddle the
  equator but are absent south of the equator in
  the eastern Pacific and Atlantic

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Recipe for a Tropical Depression

• Earth's rotation as reflected in the Coriolis
  effect imparts a clockwise (Southern Hemisphere)
  or counterclockwise (Northern Hemisphere)
  rotation to the growing storm (Fig. 33). The
  magnitude of the Coriolis effect increases with
  increasing latitude and is zero at the equator.
  Consequently, the necessary rotation is not
  imparted on storms within 5 degrees either side
  of the equator.
• The inflow of air into the developing low
  pressure system must be matched with an outflow
  of air in the upper troposphere to maintain the
  pressure gradient in the developing hurricane. If
  not, the pressure contrast decreases and wind
  speed declines.

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Hurricane Landfall
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Hurricane Landfall
Figure 35. Number of hurricane landfalls by state
1900-1996. Blue - all hurricanes red - major
(category 3, 4, and 5) storms. OT (other)
includes Delaware, Maine, Maryland,
Massachusetts, New Hampshire, and New Jersey
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Saffir- Simpson Hurricane Intensity Scale
Category Wind Speed km/hr (miles/hr) Pressure
(mb) Storm Surge Meters (feet) Damage 1
119-154 (74-95) 980 1.2-1.5 (4-5) Minimal
2 155-178 (96-110) 965-979 1.6-2.4
(6-8) Moderate 3 179-210 (111-130) 945-964 2.5-3
.6 (9-12) Extensive 4 211-250 (131-155) 920-944
3.7-5.4 (13-18) Extreme 5 250 (155) 5
.4 (18) Catastrophic