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ATMOSPHERIC PRESSURE & WIND

A. INTRODUCTION

Important because:

• wind patterns affect temperature (atmospheric circulation)

• wind patterns drive oceanic circulation, which affects temperature & precipitation patterns

• pressure patterns drive wind

• pressure patterns related to precipitation patterns

Global Wind Speed Patterns

Image credit: NASA Earth Observatory http://earthobservatory.nasa.gov/Newsroom/NewImages/images_topic.php3?topic=atmosphere&img_id=5181

B. DENSITY AND PRESSURE

Density: number of molecules per some volume of space

Atmospheric pressure

• force exerted by the atmosphere on a surface

• millibars or hectopascals (1 mb = 1 hPa)

• density directly affects atmospheric pressure

Pressure patterns

• spatial: equatorial low, subtropical high, subpolar low, polar high

• vertical: as elevation increases, pressure decreases

• temporal: pressure varies from day to day

Pressure differences

• set the atmosphere in motion

• wind blows down the pressure gradient

• isobars: lines of equal pressure  

C. CAUSES OF PRESSURE (density) DIFFERENCES

1. Unequal heating of the atmosphere

2. Mechanical actions in the atmosphere

• molecules rising or sinking
  
  
• molecules moving into a region
  
  
• molecules moving out of a region
  
  

D. CORIOLIS

• deflection due to rotation of earth



• deflection to right in northern hemisphere when looking down the pressure gradient



• deflection to left in southern hemisphere when looking down the pressure gradient

• magnitude of deflection greatest at poles, least at equator



• result:
  
  

  • cyclones: low pressure centers  

  counter clockwise rotation	clockwise rotation
  Tropical cyclone Kujira 2003 Image credit: NASA Visible Earth http://visibleearth.nasa.gov/view_rec.php?id=5325	Tropical cyclone Ingrid 2005 Image credit: NASA Visible Earth http://visibleearth.nasa.gov/view_rec.php?id=8029

  
  

  • anticyclones: high pressure centers

  clockwise rotation

  counterclockwise rotation

E. SUMMARY  

• molecule density determines atmospheric pressure



• differences in pressure set the atmosphere in motion



• winds blow from higher to lower pressure



• differences in pressure are created thermally & mechanically



• coriolis causes winds to curve, which results in cyclones & anticyclones



• pressure is mapped using isobars every 4 mb