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Class #11: Monday, February 2

Class #11: Monday, February 2. Chapter 6 Forces and winds. How wind directions are named. Forces cause the wind to blow. Forces that act on air create horizontal wind A force acting through a distance does work Work is equivalent to energy

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Class #11: Monday, February 2

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  1. Class #11: Monday, February 2 Chapter 6 Forces and winds Class #11 Monday, February 2

  2. How wind directions are named Class #11 Monday, February 2

  3. Forces cause the wind to blow • Forces that act on air create horizontal wind • A force acting through a distance does work • Work is equivalent to energy • Ultimately, the sun provides the energy that allows the winds to blow • Radiation causes temperature imbalances, which lead to pressure imbalances and force Class #11 Monday, February 2

  4. Both the direction and strength of forces influence the net force Class #11 Monday, February 2

  5. Newton’s second law of motion • Says that • the sum of the forces = mass x acceleration • Or that acceleration = sum of forces/mass • Helps scientist forecast changes in the wind direction and speed, or its acceleration • Requires that we specify which forces are acting and how strong they are. Class #11 Monday, February 2

  6. Applying Newton’s law is complex with multiple forces Class #11 Monday, February 2

  7. Gravity, the strongest force • Does not act horizontally, so does not influence the horizontal winds. • Does influence vertical air motions • Is directed downward toward the center of Earth • Is a very strong force • Keeps our atmosphere from escaping Class #11 Monday, February 2

  8. The pressure gradient force • Is always directed from high to low pressure • Is stronger when the isobars are more closely spaced • Is stronger when the difference in pressure is greater over a particular distance • Determines the way air moves only if no other forces are acting Class #11 Monday, February 2

  9. The vertical pressure gradient force • Is directed upward, from high to low pressure • Is nearly balanced by the downward force of gravity; This balance is called hydrostatic balance. • Does not influence the horizontal wind • Buoyancy is a slight imbalance between the vertical PGF and gravity, in cumulus Class #11 Monday, February 2

  10. Hydrostatic and other force balances Class #11 Monday, February 2

  11. Free convection • Is a slight imbalance between the vertical pressure gradient (upward) and gravity (downward) • In upward motion has a stronger vertical pressure gradient force • Is called buoyancy, and is a cloud-formation mechanism for Cu and cumulonimbus • Causes updrafts in cumulus clouds Class #11 Monday, February 2

  12. The horizontal pressure gradient force • Is always directed from high to low pressure • Is stronger where the density is less—higher in the troposphere • When stronger, causes stronger winds • Is always important in horizontal winds • Is not generally in the same direction the wind blows, because other forces can act Class #11 Monday, February 2

  13. The HPGF always acts from high to low pressure Class #11 Monday, February 2

  14. A surface pressure mapWinds don’t blow along the PGF Class #11 Monday, February 2

  15. The Coriolis Force • Is an apparent (not a true) force • Is due to Earth’s rotation • Is always directed to the right of the wind in the Northern Hemisphere • Is always directed to the left of the wind in the Southern Hemisphere • Is strongest at the poles and is zero at the equator • Is stronger when winds are stronger; zero for calm Class #11 Monday, February 2

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