Chapter 7 Water and Atmospheric Moisture

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Chapter 7Water and Atmospheric Moisture

• Geosystems 6e
• An Introduction to Physical Geography

Robert W. Christopherson Charles E. Thomsen
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Frontal Lifting

• Fronts named after attacking air mass
• Remember cold air is denser, heavier
• Cold Fronts
• Cold air forces warm air aloft
• 400 km wide (250 mi)
• Warm Fronts
• Warm air moves up and over cold air
• 1000 km wide (600 mi)

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Frontal Lifting

• Both cold and warm fronts cause air to be
  uplifted.
• This causes adiabatic cooling, low pressure,
  cloud development, rainfall.
• Keep in mind the properties of each type of air
  mass and their changes as the front passes.

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Frontal Lifting

• The BIG FIVE! These change depending on front
• Humidity
• Temperature
• Pressure
• Wind direction/speed
• Cloud types
• Cold front blue triangles
• Warm front red/orange half circles

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Cold Front
Figure 8.11
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Warm Front
Figure 8.13
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Midlatitude Cyclone
Figure 8.14
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Violent Weather

• Ice
• Thunderstorms  
• Tornadoes  
• Tropical Cyclones
• hurricanes
• All of these are examples of ___ pressure
• All operate on same principles, but occur on
  different spatial scales
• All can bring devastating weather

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Freezing Rain

• Supercooled water that falls to the surface as
  rain but upon impact freezes
• The weight of accumulated ice breaks power lines
  and tree limbs

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Hail

• Hail consists of ice pellets formed in roughly
  concentric layers
• Formed when water is frozen in the atmosphere.
  The ice pellet falls and encounters water, which
  freezes to the ice pellet forming a second layer
• The size of hail is determined by the strength of
  the updraft
• Hail has more water volume, snowflakes have more
  air volume

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Hailstones
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• Air mass thunderstorms (ordinary thunderstorms)
  self-extinguishing localized short lived
  phenomena limited vertical wind shear.

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Severe Thunderstorms self-propagating
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Supercell
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• Where moist air uplifting
• ? Occurs frequently in the tropics, nearly daily
  in some locations
• The U.S.s most frequent region the Gulf South
  absolute peak in Florida (land protrusion into
  warm waters)

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Thunderstorms
Figure 8.20
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Mesoscale Convective Complexes
A mesoscale convective complex over eastern South
Dakota
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Squall Line Thunderstorms
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Tornadoes
Figure 8.23
Figure 8.24
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Tornadoes

• A tornado is defined as a small, very rapid wind
  vortex with extremely low air pressure in its
  center, formed beneath dense cumulonimbus clouds
  in proximity to a passing cold front.

• Known as waterspouts when they form over oceans.
  If the circulation remains aloft, it is known as
  a funnel cloud.

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Tornadoes Characteristics Dimensions

• A wide variety of shapes and sizes.
• Diameters typical 100 yards, some 15 times as
  larger
• Duration usually a short lived phenomena, only
  a few minutes some have lasted for hours
• Movement is generally about 50km/hr (30 mph)
  over an areas about 3-4 km (2-2.5 mi) long
• Wind speeds from 40mph to 280 mph

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Tornadoes

• The most damaging aspect of a tornado is wind
  speed.

• Tornadoes are spawned by frontal boundaries,
  squall lines, MCCs, and tropical cyclones.
• The most severe tornadoes are spawned by severe
  thunderstorm cells known as supercells.

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Mesocyclone andTornado
Figure 8.23
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Nonsupercell Tornado Formation

• Related to strong convection along a convergence
  zone

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• Tornadoes predominate in North America,
  especially in south-central states of Texas,
  Oklahoma, Kansas, Nebraska, and Iowa, and
  throughout the Midwest.
• This region is commonly called tornado alley.

• A secondary concentration occurs in the
  southeastern U.S., especially Florida and
  Mississippi.

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Tornadoes
Figure 8.25
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• In the U.S. alone, we can expect to have on
  average about 800 tornadoes, so this is a common
  phenomenon.
• Most occur between 10 AM and 6 PM, and tornado
  season runs from March to July, with May seeing
  the heaviest occurrence.

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• Hurricanes Around the Globe
• Atlantic and eastern Pacific hurricanes.
• Western Pacific typhoons.
• Indian Ocean and Australia cyclones

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Hurricane Characteristics

• Hurricanes winds gt 120 km/hr (gt 74 mph)
• Compared with tornadoes lesser in intensity,
  much larger in size and longer life span ? much
  more devastating
• Size Average diameters 600 km (350 mi) and
  central pressure averages 950 mb may be as low
  as 870 mb
• Fuel from latent heat release in the cloud
  formation process
• When Where
• Where warm waters abound and during the times of
  highest SSTs
• When August and September in the Northern
  Hemisphere January to March in the Southern
  Hemisphere

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Hurricane Characteristics

• Hurricanes consist of
• A central eye surrounded by large cumulonimbus
  thunderstorms occupying the adjacent eye wall
• Pressure differences into the center of the storm
  are about twice as great as the average
  mid-latitude cyclone, resulting in strong
  sustained winds

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Eye and Eye Wall

• Eye an area of descending air and light winds
  about 25 km (15 mi) in diameter on average air
  in the eye to be warmer than elsewhere
• A shrinking eye indicates storm intensification
• Eye wall comprised of the strongest winds, the
  largest clouds, and the heaviest precipitation
  with rainfall rates as high as 2500 mm/day (100
  in.)

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Hurricane Formation

• Start Tropical disturbances
• Often begin in the eastern ocean basins as
  disorganized clusters of thunderstorms
• Some form in association with mid-latitude
  troughs migrating toward lower latitudes
• Some from ITCZ-related convection.
• Most associated with easterly wave.

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Hurricane Formation

• Tropical depression below 37 mph
• Tropical storm further intensification to wind
  speeds of 60 km/hr (37 mph)
• Hurricane when winds reach or exceed 120 km/hr
  (74 mph)
• A high percentage of depressions become tropical
  storms and an even higher percentage reach
  hurricane status

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Conditions Necessary for Hurricane Formation

• Hurricanes form only over deep water layers with
  surface temperatures in excess of 27 oC (81 oF)
• Coriolis force is an important contributor, and
  as such, hurricanes do not form equatorward of 5o
• Strong vertical shear must be absent

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Destruction by Hurricanes

• Winds
• Heavy rainfall
• Storm surge is responsible for a large
  percentage of damage along coastal regions (e.g.,
  hurricane Camille caused a storm surge of 7 m (23
  ft) along the Mississippi coast)
• High surf occurs atop the surge, increasing
  damage

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Destruction by Hurricanes

• Winds and surge are typically most intense in
  the right front quadrant of the storm. Why?
  Combination of wind speeds and the speed of the
  storms movement.

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Destruction by Hurricanes

• The right front quadrant also produces the
  greatest frequency of tornadoes within the
  hurricane.