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

How wind directions are named Class #11 Monday, February 22

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 23

Both the direction and strength of forces influence the net force Class #11 Monday, February 24

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. 5

Applying Newton’s law is complex with multiple forces Class #11 Monday, February 26

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 7

Class #11 Monday, February 2 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 8

Class #11 Monday, February 2 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 9

Hydrostatic and other force balances Class #11 Monday, February 210

Class #11 Monday, February 2 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 11

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 212

The HPGF always acts from high to low pressure Class #11 Monday, February 213

A surface pressure map Winds don’t blow along the PGF Class #11 Monday, February 214

Class #11 Monday, February 2 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 15