Download WEATHER SYSTEM AND AIR MASSES CH. 6 Weather is the day

WEATHER SYSTEM AND AIR MASSES CH. 6
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Weather is the day - to - day state of the atmosphere. In this chapter we'll
examine weather systems at a variety of geographic scales that affect our
daily lives.
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The weather of the humid tropics is very similar throughout the year as a
constant flow of energy keeps temperatures uniformly high.
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However, the daily weather is quite variable in the mid latitudes.
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Here, huge air masses collide to create powerful storm systems.
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An air mass is a vast pool of air having similar temperature and moisture
characteristics over its horizontal extent.
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An air mass occupies thousands of square miles of the Earth's surface.
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Air masses are born in a source region where they take on their characteristic
temperature and moisture content.
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Source regions are often regions of low relief and calm wind that prevent
turbulent mixing and allow the air to take on the conditions of the surface over
which it forms.
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Areas dominated by high pressure serve as good source regions. High pressure
also enables the air to move outward from the source region.
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Air Masses are identified by letter code. m for maritime, (originate over ocean
and moist), and c, for continental, originating over land and therefore dry.
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The second letter is always a capital, help to locate the latitude of the source
region
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continental Arctic air (cA) is typically described as extremely cold and dry.
continental Antarctic (cAA) air mass. This too is an exceedingly cold air mass
and is drier than its arctic counterpart as the source region is the continent of
Antarctica.
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continental Polar (cP) air is considered a cold and dry air mass that is warmer
than the arctic air mass located to the north.
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continental Tropical (cT) Major source regions are the great deserts of the
Earth such as the Sahara, Arabian, and Australian. The extremely low humidity is
due to the lack of available water for evaporation.
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maritime Polar (mP) air is cool and moist air that brings mild weather to coastal
locations.
maraitime Tropical (mT) air masses are warm and moist air masses that are
responsible for much of the precipitation east of the Rocky Mountains in the
United States.
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maritime Equatorial (mE) air masses form near the equator and are
exceedingly warm and humid Convection and convergence of this air mass in the
Intertropical Convergence Zone is one for the reason for the heavy rainfall
experienced in the rain forests of this region.
SEVERE WEATHER
Tornadoes are the most powerful weather phenomenon known.
A violently rotating column of air extending from a thunderstorm to the ground
A tornado is an intense system of low pressure with violent updrafts and converging
winds.
Though tornadoes have been intensely studied for years, the mechanism that actually
creates them still eludes us.
Tornadoes have been documented in most all the regions of the Earth, though they are
most prevalent in the United States.
Tornadoes are most common during the Spring when greatly contrasting air masses
collide to produce severe storm systems.
Wind shear within the severe thunderstorm causes rotation of air about a horizontal
axis.
The rotating circulation is tilted into the vertical by the updrafts in the thunderstorm.
As the rotating air increases in height and shrinks in size a mesocyclone is formed.
For whatever reason, a tornado funnel is spawned within the mesocyclone
Today we use observations, numerical forecast models, Doppler Radar, weather
balloons and Satellites to predict our weather.
Tornado Alley is an area in the great plain states that runs from Texas to Nebraska+.
Between the Rocky mountains and the Appalachian mountains. On both sides of the
Mississippi river.
-On average 800 Tornadoes occur every year. -Tornadoes can be on the ground for an
instant to several hours, but the average time is 5 minutes.
In 1971 Theodore Fujita devised a six category scale to classify Tornadoes, known as
the Fujita tornado Scale.
Conditions for Tornado formation: Cool, dry air colliding with warm, moist air.
http://www.youtube.com/watch?v=GM-abtIT6Is&feature=related
A hurricane (or typhoon) is a large rotating cyclonic system born in the tropics. They are the
largest and most destructive storms on Earth. Most associate high winds with the devastation that
these massive storms create, yet dangerous flooding, tornadoes, lightning often accompany or are
spawned by a hurricane.
Hurricane Katrina struck the Gulf coast of the United States wreaking havoc in Louisiana,
Mississippi and Alabama.
Courtesy NASA
Hurricane Formation
A hurricane develops from a tropical disturbance once it reaches sustained winds in excess of 74
mph (64.3 knots) . Most hurricanes form poleward of 10o latitude as the Coriolis effect is too
weak closer to the equator. Hurricanes form in a uniform mass of warm air over tropical oceans
with temperatures of 80o F (26.5o C) through a depth of 200 feet (60 meters).
The "fuel" for a hurricane comes from the enormous amount of latent heat released from the
warm ocean water.
(Source: NOAA)
A hurricane is a warm-core low pressure
system that weakens rapidly with altitude
to be replaced by anticyclonic airflow
above the hurricane. The center or eye of
the hurricane is an area of nearly
cloudless skies, subsiding air, and light
winds. The eye ranges from 12 to 40
miles across (20 - 65 kilometers). At the
periphery of the eye is a ring of
cumulonimbus clouds that produce torrential rains and extremely strong winds. Surrounding the
core of storms are the typical spiraling rain bands.
As a hurricane moves over a colder surface or land, it loses its source of energy and dissipates.
However, the system can remain an organized storm for several days as it moves inland,
inundating the interior with rainfall causing severe flooding. Destructive tornadoes often
accompany hurricanes as the move ashore.
http://www.youtube.com/watch?v=75qAgSuMbzA&eurl=http%3A%2F%2Fwww.uwsp.edu%2F
geo%2Ffaculty%2Fritter%2Fgeog101%2Ftextbook%2Fweather_systems%2Fhurricanes.html&f
eature=player_embedded
WHAT DO HURRICANES AND TORNADOES HAVE IN COMMON?
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Low Pressure
Latent Heat of Condensation
Strong Winds
Hazardous to Life and Property
HOW DO THEY DIFFER?
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Location
Size
Duration
Season of Occurrence
Distinctive Hazards
WHEN DO THEY OCCUR?
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Hurricane season?
Late Summer and Fall
Tornado season?
Spring and Early Summer
LOCATION - WHERE DO STORMS FORM?
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Hurricanes form over warm (80 °F) tropical seas
Latitude 7-15 degrees North and South of the equator
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Tornadoes form over land in midlatitudes
SIZE AND DURATION OF HURRICANES
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Hurricanes 100-300 miles wide
Last 10-14 days
Rate of travel 5 mph
Travel with Trade Winds
HURRICANE STRUCTURE
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Intense low pressure system
Steep pressure gradient - over 74 mph winds
Strong convectional lifting
Eye with calm, clear conditions
HURRICANE DAMAGE
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High winds
75-125 mph
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Torrential rains
Flash floods
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Storm surges as sea level rises
LOCATION OF TORNADOES
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Most common in the United States
Plains States, Midwest, and Florida
In advance of a cold front
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Embedded in hurricanes
SIZE AND DURATION OF TORNADOES
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Narrow intense storm
1/4 mile wide
Path of 6 miles SW to NE
A few minutes to three hours
STRUCTURE OF A TORNADO
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Cumulonimbus cloud
A narrow funnel
A dust cloud
TORNADO HAZARDS
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Wind Speed 150-450 kph
Strong updraft in the funnel
Explosions due to pressure contrast