Wind, Air Masses, and Fronts

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Wind, Air Masses, and Fronts

• Chapter 18

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Pressure Gradient

• Wind
• Horizontal movement of air across Earth's surface
• Movement from high to low pressure
• Vector force-both direction and velocity
• Also known as pressure gradient force
• Pressure gradient
• Change in air pressure over specific distance

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Planetary Winds

• Large-scale wind patterns flowing across Earth
• Result of unequal distribution of insolation
• Caused by large pressure differences near
  surfaces
• Result of unequal distribution of heat
• Equator-90-degree angle causes rapid heating
• Warm air rises because of lower density

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Planetary Winds (continued)

• Also known as area of convergence
• Causes areas of low pressure near equator
• Air begins to cool as it expands
• Air becomes more dense, sinks to Earth
• Cool, dense sinking air returns to Earth
• At 30 degrees North and South latitude
• Begins to form high pressure at surface

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Planetary Winds (continued)

• Called areas of divergence
• Cool dense air pushing forward
• Causes area to spread apart or diverge
• Causes large-scale convection cell-Hadley cell
• Low pressure at equator
• High pressure 30 degrees North and South
• Large-scale planetary winds develop

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Planetary Winds (continued)

• If one maps pressure centers on a map
• Plot wind direction and air flow
• Flow directly from north or from south
• Occurrence of easterly and westerly winds
• Result of Earth's rotation
• Called the Coriolis Effect
• Cause right or left movements, depending on
  hemisphere

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Pressure Systems

• Mass of air with well-defined pressure
• center
• Pressure center
• Area low/high pressure in air mass
• Wind from high pressure center travels
• outward
• Clockwise pattern in the North Hemisphere
• Coriolis effect causes deflection to the right
• Called an anticyclone

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Pressure Systems (continued)

• Wind from low pressure area spirals inward
• Counterclockwise pattern in the Northern
  Hemisphere
• Known as a cyclone

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Mesoscale Winds

• Also called regional winds
• Smaller scale interactions of changing pressure
• Example-land and sea breeze
• Insolation received on a hot day
• Land next to large body of water
• Heats rapidly-hot air forms near surface
• Rises and becomes dense

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Mesoscale Winds (continued)

• Cooler air over water next to land
• Cooler, more dense air forms high pressure
• Result
• High pressure over cool water
• Low pressure over warm land
• Also called a sea breeze
• Blows cool air toward the coast

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Mesoscale Winds (continued)

• As sun goes down surface cools rapidly
• Cool air forms over land
• Temperature of water stays same
• Air over water remains warm
• Temperature of the land cools
• High pressure over land, low over water
• Causes a land breezes

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The Jet Stream

• Forms high in the atmosphere in tropopause
• As cool air descends
• Belt of high winds develops
• Called jet stream
• 6 to 9 miles up
• Over subtropical and polar highs
• 200 miles wide and less than 1 mile thick

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The Jet Stream (continued)

• Over United States
• Travels west to east
• Causes weather to move westward to eastward
• Subtropical jet stream
• Over lower latitudes
• Moves warm tropical air and hurricanes
• Along east coast of United States

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Air Mass

• Large body of moving air in the troposphere
• Similar temperature, pressure, and moisture
• Derives characteristics from source regions
• Geographical areas give an air mass its qualities
• Higher altitudes-cooler temperature
• Near equator-warmer temperature

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Air Mass (continued)

• Atmospheric moisture is related to source region
• Over the ocean-higher atmospheric moisture
• Over continents-considered dry

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Source Regions and Classification of Air Masses

• Five categories
• Continental polar air mass
• Over land near poles
• Cool, dry air
• Maritime polar air mass
• Near poles over ocean
• Cool, moist air-cold winter rains

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Source Regions and Classification of Air Masses
(continued)

• Continental tropical air masses
• Over land near equator
• Warm, dry air
• Maritime tropical air masses
• Over the ocean near equator
• Warm, humid air
• Often associated with hurricanes

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Source Regions and Classification of Air Masses
(continued)

• Arctic air mass
• Extremely cold and dry air
• North of 60 degrees north latitude
• Ice fields of Siberia, Greenland, and Arctic
  Ocean
• Can bring cold and dry weather
• Called an arctic express

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Fronts

• One air mass contacts with another
• Each has different characteristics of
• Temperature
• Barometric pressure
• Moisture
• Causes characteristic weather unique to each front

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Front (continued)

• Cold front
• Advancing cooler front contacts slow warm front
• Causes adiabatic cooling
• Cooling of rising air by expansion
• Air temperature meets dew point
• Clouds form-cumulonimbus
• Heavy precipitation and thunderstorms

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Front (continued)

• Fast moving cold fronts cause intense
  thunderstorms
• Called a squall line
• Slower moving cold front causes showers
• Shift in wind direction caused by cold front
• From southwest to northwest
• Change in pressure also occurs
• High pressure often occurs

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Front (continued)

• Warm front
• Warm air mass contacts slower cold air mass
• Form high, then mid-level, then low clouds
• Advancing warm front- clouds gradually thicken
• Cause light precipitation
• If snow, accumulations can be heavy
• Fog may also form along the boundary

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Front (continued)

• Wind shifts from southeast to southwest
• Less dense warm air
• Replaced by cooler, denser air
• Brings a change in barometric pressure- drops
• Occluded front
• Rapid cold front
• Moves under slower warm front- causes uplift

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Front (continued)

• Results in widespread precipitation
• Can be intense or sustained
• Winds shift from southeast to northwest
• Some fronts are stationary
• Cold and warm next to each other
• Create clear weather and few clouds
• Eventually become a cold or warm front

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Mid-latitude Cyclones

• Form in Northern Hemisphere around low-pressure
  systems
• Counterclockwise inward rotation
• When cold air mass
• Moves behind slower warm air masses
• Both systems centered around spiraling low
  pressure
• Wide band of precipitation result

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Mid-latitude Cyclones (continued)

• As system intensifies
• A comma cloud forms
• Names comes from its shape
• Cold front overtakes the warm front
• Develops an occluded front
• Pressure weakens and cyclone breaks apartmoves
  out over the Atlantic Ocean