Weather Patterns

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Weather Patterns and Severe Storms Ch. 20
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air mass

• a large body of air that has the same temperature
  and humidity throughout
• classified according to where they originate
• as it moves, the characteristics of an air mass
  change and so does the weather in the area over
  which the air mass moves.

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Air Masses Are Classified by Region
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Originates over the ocean? humid air mass (m)
(maritime)
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Originates over the land? dry air mass (c)
(continental)
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Originates in cold air? cool air mass (P)
(Polar)
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Originates in warm air? warm air mass (T)
(Tropical)
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Which one affects us?

• Maritime Tropical Atlantic (mT)
• moves northward across eastern US
• brings mild, cloudy winters and hot humid summers
  with hurricanes and thunderstorms

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View satellite movies of air masses moving
acrossNorth America.
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radiosonde
small balloon carried observatory that carries a
radio transmitter that sends out signals about
air pressure, temperature, and humidity
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fronts boundary between two air masses having
different temperatures and humidity
Types of Fronts
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warm front

• forms when warm air moves into an area formerly
  covered by cooler air
• brings hot, humid days and precipitation over a
  large area

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cold front

• forms when cold, dense air moves into a region
  occupied by warmer air
• brings strong storms (squall) with clear days
  following

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Compare and contrast warm and cold fronts.
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occluded front

• two cold masses sandwich a warm mass
• brings strong winds and heavy rains

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stationary front
Air masses move parallel to one another the
surface position of the front does not move
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Thunderstorms

• small-area storms formed by the strong upward
  movement of warm, moist, unstable air
• always accompanied by lightning, thunder, rain
  and sometimes hail
• formed from cumulonimbus clouds

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Observe an animation of a thunderstorm.
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Two types of thunderstorms

• Air-mass thunderstorm
• form in warm, moist air mass
• occur in spring and summer- last less than 1 hour
• single, widely scattered

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• Frontal thunderstorm
• occur in lines along a frontal surface
• stronger and may last several hours
• can produce heavy rain and flooding

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All thunderstorms produce lightning!

• Lightning
• a discharge of electricity from a thundercloud to
  the ground, or cloud to cloud, or ground to cloud

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• temperature inside lightning flash can reach
  28,000C
• at this temperature, air expands
  explosively-sudden expansion makes thunder!

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Tornado
a narrow, funnel-shaped column of spiral winds
that extends downward from the cloud base and
touches the ground
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• strongest winds between 360 and 500 km/hr
• funnel less than 500 m across
• always travel with a parent thunderstorm at
  speeds ranging from 40 to 65 km/hr

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funnel is a mixture of clouds and dust
pressure gets lower in center
air flows toward the funnel and cools to dew
point- drops form
lowering of condensation level due to low
pressure causes cloud to funnel
Extremely low pressure-when it touches ground,
acts like a giant vacuum
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Waterspout tornados over the water- weaker than
tornados
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Tornados usually occur during spring and summer
and most likely occur in late afternoon
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Fujita Scale scale used for categorizing
tornados
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Hurricane
an intense tropical low-pressure area with
sustained winds starting at 74 mph
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parts of a hurricane
storm surge currents formed when hurricanes pile
water up along the shore and blow it inland most
damaging part of a hurricane
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eye central area of sinking air 15 to 20 km wide
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Eye of hurricane Floyd
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• Winds most violent just outside the eye

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Aug. 28, 2005 Katrina's eyewall was seen from a
hurricane-hunter plane. Photo NOAA
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Check out this website!
http//www.weather.com/web/newscenter/specialrepor
ts/hurricanes/inside/elements.html
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Tropical depression wind speeds up to 38 mph
some circular rotation at surface
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Tropical storm wind speeds from 39-74 mph can
be named, shows drop in pressure, distinct
rotation
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Hurricane wind speeds greater than 74 mph
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Movement of Air in a Hurricane
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There are sometimes gaps in between these bands
where no rain is found. In fact, if one were to
travel between the outer edge of the hurricane to
its center, one would normally progress from
light rain to dry back to slightly more intense
rain again over and over with each period of
rainfall being more intense and lasting longer
until reaching the eye. Upon exiting the eye and
moving towards the edge of the hurricane, one
would see the same events as they did going in,
but in opposite order.
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A schematic of this banding feature can be seen
in the diagram above. The thunderstorms are now
organized into regions of rising and sinking air.
Most of the air is rising, but there is a small
amount found in between the thunderstorms that is
sinking.
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Satellite image of Hurricane Katrina on Aug. 28,
2005, when the storm was a Category Five
hurricane. Hurricanes don't get much stronger
than this. Photo NOAA
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The coasts of Louisiana, Mississippi, Alabama and
part of Florida, seen from space on October 15,
2001. Original photo from NASA
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A gigantic, dangerous storm, Hurricane Katrina
takes aim at New Orleans and the Mississippi
coast. Photo NASA
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Meteorologists interpret weather information from
satellites
commercial aircraft
weather balloons
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Eastern U.S.-Infrared Satellite
Ophelia
1145 am 9/14/05
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Eastern U.S.- Visible Satellite
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weather stations around the world
Radar electronic device that transmits radio
waves in the form of a beam
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• Data is collected and put into a central computer
  at the National Weather Service
• data includes winds, temperature, pressure,
  humidity, clouds, precipitation
• Makes a computer model (copy of the atmosphere in
  computer

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• Maps are made and forecasts are reported to local
  stations across the country
• Weather forecasts are issued by the Weather
  Service at 10 am, 4 pm, 9pm, and 4 am. Forecasts
  are updated more often during severe weather.

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Interactive Weather Page
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watch threat of storm conditions within 24-36
hours warning due to strike within 24 hours