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  "Winds do Blow from High to Low"  
     
  Wind can be defined simply as air in motion. Wind develops as a result of spatial differences in atmospheric pressure. Generally, these differences occur because of uneven absorption of solar radiation at the Earth's surface.  
 

 
  Source: http://www.physicalgeography.net/fundamentals/7n.html  
     
  The Pressure Gradient Force:  
  Differences in pressure are shown on maps by isobars, which are lines joining places of equal air pressure. The closer together the isobars the greater the difference in pressure - the pressure gradient - and the stronger the wind. Wind is nature's way of balancing out differences in pressure as well as temperature and humidity.  
 

 
  Association between wind speed and distance between isobars. In the illustration above thicker arrows represent relatively faster winds.
Source: http://www.physicalgeography.net/fundamentals/7n.html 		 
 

 

  
  Two basic pressure systems:  
 

 
  Source: Waugh  
     
  The Coriolis Force:  
  If the Earth did not rotate and if it was composed entirely of land or sea there would be one large convection cell in each hemisphere. Surface winds would blow parallel to the pressure gradients and would blow directly from high to low  pressure areas.  
 

 
  Source: Waugh  
  But...  
  the Earth does rotate and the distribution of land and sea is uneven. The rotation causes an effect which makes it appear that moving air is deflected to the right of its path in the northern hemisphere. This is a result of the Coriolis force.  
 

 
  Source: Waugh  
     
  Animation of Coriolis force - click on pic!  
 

 
  Source:  http://www.s-cool.co.uk/a-level/geography/introduction-to-weather-and-climate/revise-it/atmospheric-circulation-and-motion  
     
  Geostrophic Winds: Air under the influence of both the pressure gradient force and Coriolis force tends to move parallel to isobars in conditions where friction is low (1000 meters above the surface of the Earth) and isobars are straight. Winds of this type are usually called geostrophic winds. Geostrophic winds come about because pressure gradient force and Coriolis force come into balance after the air begins to move  
 

 
  Source: http://www.physicalgeography.net/fundamentals/7n.html  
     
  A geostrophic wind flows parallel to the isobars. In this model of wind flow in the Northern Hemisphere, wind begins as a flow of air perpendicular to the isobars under the primary influence of the pressure gradient force (PGF). As the movement begins, the Coriolis force (CF) begins to influence the moving air causing it to deflect to the right of its path. This deflection continues until the pressure gradient force and Coriolis force are opposite and in balance with each other.  
     
  Friction:  
  Friction caused by the Earth's surface tends to upset the balance between the pressure gradient force and the Coriolis force - by reducing the Coriolis effect. Surface winds tend to blow at a slant to the isobars in towards the low pressure centre.  
 

 
  Source: Waugh  
     
  So that is why the surface winds here are blowing obliquely away from the high pressure and in towards the centre of the low pressure.  
 

 
     
 

 

  
 

Student Voice: Click on Pic

 
  In the northern hemisphere high pressure light winds tend to rotate clockwise away from the centre of the high and low pressure winds tend to move anticlockwise in towards the centre of the low pressure. The opposite trends occur in the southern hemisphere.  
 

 
  Source: http://www.physicalgeography.net/fundamentals/7n.html  
     
  Research Task:  
  What is Buy Ballot's Law? Use diagrams and notes to explain why the 'law' is correct.