Section 6: Atmospheric and Oceanic Circulations

SLOs: air pressure and wind, force of gravity/pressure gradient force/Coriolis force/force of friction, land-sea breeze and mountain-valley conditions.


Lesson 1:

Instruction

There are three levels of atmospheric circulation: primary (worldwide), secondary (high-low pressure systems), and tertiary (local weather and winds).  Atmospheric circulation is defined by a number of different factors, one of which is differences in air pressure.  Air pressure is produced by air mass temperature and density.  This pressure exerts force on all surfaces it comes in contact with and is measured by a barometer instrument.  Winds are a product of differences in horizontal air pressure from location to location.  Wind has two main properties: speed and direction.  Speed is measured by a anemometer in miles per hour (mph), kilometers per hour (kmph), meters per second (mps), or knots (nautical miles per hour).  Direction is measured by a wind vane.  Note that winds are named from the direction in which they originate.                         

Photos and Images

 (winds from directly west)     (winds from the north-east)

Quiz

In Ventura County, wind speeds and directions vary.  Two of the more notable directions are onshore and offshore winds (the origin of winds in relation to the shoreline).  At shorelines that run north-south along the western United States, onshore winds are westerly in origin, while offshore winds are easterly in origin.  Hollywood-by-the-Sea and Hueneme Beach are examples of just such shorelines in Ventura County.  Look at the pictures above from the aforementioned locations and determine which is an example of onshore winds and which is an example of offshore winds.      

 


Lesson 2:

Instruction

Other forces that drive atmospheric circulation and winds include the force of gravity, the pressure gradient force, the Coriolis force, and the force of friction.  The force of gravity is simply the uniform compression of the atmosphere worldwide.  The pressure gradient force is the force that drives high and low pressure air masses, causing winds.  The Coriolis force is the force exerted as the world rotates on its axis, deflecting winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.  The friction force is the force of wind dragging on the surface of the Earth, causing local changes in wind speeds and directions.  For more on global atmospheric-oceanic circulations, read chapter 6 in the Christopherson text.       

Photos and Images

 
 (predominant air masses from high to low pressure per hemisphere) (wind direction given pressure gradient, Coriolis, and friction forces per hemisphere)

Quiz

In Ventura County, name the wind direction if: the pressure gradient is from the west, the Coriolis force is from the northeast, and the friction force is from the southeast.  Note: this is an example of onshore winds for much of Ventura County.

 


Lesson 3:

Instruction

Local winds form based on local terrain.  In the case of Ventura County, which is adjacent the Pacific Ocean, land-sea breezes play a dominant role in determining winds.  These land-sea breezes usually change with the hours of the days and nights.  Therefore, daytime sea-breeze conditions (which occur when the land is warmer and the ocean is cooler) and nighttime land-breeze conditions (which occur when the land is cooler and the ocean is warmer) are very common.  In the Ventura County mountains and valleys, mountain-valley breezes are common.  The mountain-valley breeze conditions are as follows: during the day, warm air in the valleys move upslope; during the nights, cool air from the mountains move downslope.  However, there are exceptions to these rules (such as the Santa Ana offshore winds that occur in the fall months at all hours of the day and night).           

Photos and Images
    (daytime sea-breeze conditions)          (nighttime land-breeze conditions) 
(daytime valley-breeze conditions)          (nighttime mountain-breeze conditions) 

Quiz

Where do you live in Ventura County? Does your home experience land-sea breeze conditions or mountain-valley breeze conditions?  Why?   


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2006 Jessica Douglas