Atmospheric pressure

• Atmospheric pressure
  
  • Force exerted by the weight of the air above
    
  • Decreases with increasing altitude
    
• Units of measurement
  
  • Millibar (mb) – standard sea level pressure is 1013.2 mb
    
  • Inches of mercury – standard sea level pressure is 29.92 inches of mercury
    
• Instruments for measuring
  
  • Barometer
    
  • Mercury barometer
    
  • Aneroid barometer
    
  • Barograph (continuously records the air pressure)
    

Wind

• Horizontal movement of air
  
  • Out of areas of high pressure
    
  • Into areas of low pressure
    
• Controls of wind
  
  • Pressure gradient force
    
    • Isobars – lines of equal air pressure
      
    • Pressure gradient – pressure change over distance
      
    • A weather map showing isobars and wind speed/direction
      
  • Coriolis effect
    
  • Friction
    
• Upper air winds
  
  • Generally blow parallel to isobars – called geostrophic winds
    
  • Jet stream
    
    • "River" of air
      
    • High altitude
      
    • High velocity (120-240) kilometers per hour
      
• The geostrophic wind
  
• Comparison between upper-level winds and surface winds

Rotating Air Bodies

• Low Pressure Zone Formation
  
  • Warm air rises
    
  • Creates a low pressure zone
    
  • At the Earth’s surface, air “feeds” the low pressure zone, moves counterclockwise
• High Pressure Zone Formation
  
  • Cool air sinks
    
  • Creates a high pressure zone
    
  • At the Earth’s surface, winds blow clockwise
• Rotating Air Bodies
  
  • Bends in the polar jet create troughs and ridges
    
  • Forms cyclones and anticyclones
    
  • Cyclones
    
    • Low pressure zone in polar jet trough
      
    • Winds at surface flow counterclockwise towards the core
      
    • Air is updrafted and cooled
      
    • Forms clouds, rain and upper level outflow of air
      
  • Anticyclones
    
    • High pressure zone at ridge of polar jet
      
    • Air converges in upper atmosphere
      
    • Descends towards the ground
      
    • Flows outward at surface
      
    • Dry, windy conditions
      
  • Cyclonic and anticyclonic winds in the Northern Hemisphere
    

General atmospheric circulation

• Underlying cause is unequal surface heating
  
• On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat
  
• Idealized global circulation
  
  • Equatorial low pressure zone
    
    • Rising air
      
    • Abundant precipitation
  • Subtropical high pressure zone
    
    • Subsiding, stable, dry air
      
    • Location of great deserts
      
    • Air traveling equatorward from the subtropical high produces the trade winds
      
    • Air traveling poleward from the subtropical high produces the westerly winds
  • Subpolar low pressure zone
    
    • Warm and cool winds interact
      
    • Polar front – an area of storms
  • Polar high pressure zone
    
    • Cold, subsiding air
      
    • Air spreads equatorward and produces polar easterly winds
      
    • Polar easterlies collide with the westerlies along the polar front
      
• Influence of continents
  
  • Seasonal temperature differences disrupt the
    
    • Global pressure patterns
      
    • Global wind patterns
      
  • Influence is most obvious in the Northern Hemisphere
    
  • Average surface pressure and associated winds for January
    
  • Average surface pressure and associated winds for July
    
• Monsoon
  
  • Seasonal change in wind direction
    
  • Occur over continents
    
  • During warm months
    
    • Air flows onto land
      
    • Warm, moist air from the ocean
      
  • Winter months
    
    • Air flows off the land
      
    • Dry, continental air
      
• Circulation in the mid-latitudes
  
  • The zone of the westerlies
    
  • Complex
    
  • Air flow is interrupted by cyclones
    
    • Cells move west to east in the Northern Hemisphere
      
    • Create anticyclonic and cyclonic flow
      
    • Paths of the cyclones and anticyclones are associated with the upper-level airflow

Atmospheric pressure | Wind | Rotating Air Bodies | General atmospheric circulation | Local winds | El Niño and La Niña | Global distribution of precipitation | Links | top | Classes Home

Local winds

• Local winds
  
  • Produced from temperature differences
    
  • Small scale winds
    
  • Types
    
    • Land and sea breezes
      
    • Mountain and valley breezes
      
    • Chinook and Santa Ana winds
      
  • Illustration of a sea breeze and a land breeze
    
  • The Santa Ana Winds
    
• Wind measurement
  
  • Two basic measurements
    
    • Direction
      
      • Winds are labeled from where they originate
        
      • Direction indicated by either
        
      • Compass points (N, NE, etc.)
        
      • Scale of 0º to 360º
        
      • Prevailing wind comes more often from one direction
        
    • Speed
      
      • often measured with a cup anemometer
        
  • Changes in wind direction
    
    • Associated with locations of
      
      • Cyclones
        
      • Anticyclones
        
    • Often bring changes in
      
      • Temperature
        
      • Moisture conditions
        

El Niño and La Niña

• What is El Niño?
  
  • Basically, it's a giant puddle (or pod) of heated water that sloshes across the Pacific Ocean
    
  • Similar to an iceberg
    
  • Bulge on the surface
    
  • Most of “pod” beneath the surface
    
  • Due to difference in density
    
  • National Geographic’s Model
    
• ENSO - El Niño-Southern Oscillation
  
  • Typically lasts 1 year
    
  • May last up to 3
    
  • In multi-year events, first year not as affected
    
  • Affects both hemispheres
    
• Recognizing an El Niño
  
  • Sea Surface Temperatures (SST)
    
    • Normal: 6-8° C warmer in the western tropical Pacific than in the eastern tropical Pacific
      
    • Check SST to see if in “normal” range
• La Niña
  
  • Return to “normal” conditions from an El Niño strong
    
  • Produces:
    
    • Strong currents
      
    • Powerful upwelling
      
    • Chilly and stormy conditions along S. American coast
      
  • Eastern Pacific cools rapidly, Western Pacific warms rapidly
    Renewed Trade Wind activity spreads the cooler eastern Pacific waters westward
    

Global distribution of precipitation

• Relatively complex pattern
  
• Related to global wind and pressure patterns
  
  • High pressure regions
    
    • Subsiding air
      
    • Divergent winds
      
    • Dry conditions
      
    • e.g., Sahara and Kalahari deserts
      
  • Low pressure regions
    
    • Ascending air
      
    • Converging winds
      
    • Ample precipitation
      
    • e.g., Amazon and Congo basins
      
  • Average annual precipitation in millimeters
    
• Related to distribution of land and water
  
  • Large landmasses in the middle latitudes often have less precipitation toward their centers
    
  • Mountain barriers also alter precipitation patterns
    
  • Windward slopes receive abundant rainfall from orographic lifting
    
  • Leeward slopes are usually deficient in moisture