Chapter 9, Part 2

Forces that Influence Winds

Newton’s Laws

An object at rest will remain at rest and an object in motion will remain in motion as long as no force is exerted on the object.

The force exerted on an object equals its mass times the acceleration.
(Acceleration is speeding up, slowing down, or changing direction.)

Forces that Influence Wind

Pressure gradient force

Coriolis force

Centripetal force

Friction

Pressure Gradient Force

Pressure is force per unit area.

The high pressure region exerts more force so the net force is from high (H) to low (L).

Pressure Gradient

The pressure gradient is the change in pressure divided by the distance over which that change occurs.

A large pressure gradient will create a strong wind. (O’Dome example)

Coriolis Force

The Coriolis force describes the apparent force due to the rotation of the earth. (demonstration)

Direction of the Coriolis Force

The Coriolis force causes the wind to deflect to the right in the Northern Hemisphere.

Dependence on Latitude

Except at the equator a free-moving object heading east or west will appear to deviate from its path.

There is more deviation at higher latitudes.

Dependence on Speed

The apparent deflection of the Coriolis force increases with speed.

Effect of Coriolis Force on Wind

With only the pressure gradient force the wind would blow directly from high pressure to low pressure.

Straight-line Flow Aloft

500mb chart (~3 miles up): Aloft the winds blow more or less parallel to isobars (constant pressure contours).

These are called geostrophic winds (earth-turning).

Origin of Geostrophic Winds

The pressure gradient force (PGF) balances the Coriolis force (CF) so the wind flows parallel to isobars.

Wind Speed and Isobars

Closer contours imply a larger pressure gradient force.

This is balanced by a larger Coriolis force, which means higher wind speeds.

Isobar and Wind Pattern

The orientation and spacing of isobars allows one to determine the wind direction and speed.

Wind Flow around Highs and Lows

In the northern hemisphere air flows counterclockwise around lows (cyclones) and clockwise around highs (anticyclones).

Centripital Acceleration

Circular motion means the direction is constantly changing, which is a form of acceleration.

Centripital acceleration = velocity²/radius.

To create this acceleration, the pressure grad. force is larger than the Coriolis force.

Direction of Rotation

Air leaving a high curves to the right in the northern hemisphere.

The rotation about an anticyclone (H) is thus clockwise in the northern hemisphere.

For cyclones (L) the rotation is counterclockwise.

Comparison of Northern and Southern Hemispheres (L)

Wind Direction at Upper Levels

The zonal wind flow (west to east) across the US is the reason it is faster to fly from west to east.

Meridonal wind flow goes in a north-south trajectory.

Wind Direction near the Surface

Near the surface, wind direction is not parallel to the isobars, but crosses them.

Frictional Force

Friction near the surface slows down the wind and hence reduces the Coriolis force.

The pressure gradient force dominates.

The wind crosses isobars.

Relative Orientation of Wind Direction and Pressure Gradient

If we stand with our back to the wind, then turn clockwise about 30^o, lower pressure will be to our left.

Surface Friction and Cyclones

The Coriolis force is reduced and the wind crosses isobars.
(N. Hemisphere shown)

Where does the air go?

Divergence and Convergence

To keep the pressure the same at a low, the converging air at the surface must be balanced diverging air above.

If the divergence slows, the pressure in the low will rise.

If the divergence increases, the pressure in the low will decrease.

Hydrostatic Equilibrium

Since the pressure is higher on the ground than further up in the atmosphere, why doesn’t the pressure gradient force cause the air to flow uniformly up?

Gravity balances the pressure gradient force to create hydrostatic equilibrium.

Summary

The wind is influenced by four forces:

Pressure gradient force

Coriolis force

Centripetal force

Friction.

From these forces we are able to understand the speed and direction of the wind both near the ground and further aloft.


	An object at rest will remain at rest and an object in motion will remain in motion as long as no force is exerted on the object.
	The force exerted on an object equals its mass times the acceleration. (Acceleration is speeding up, slowing down, or changing direction.)


	Pressure gradient force
	Coriolis force
	Centripetal force
	Friction


	Pressure is force per unit area.
	The high pressure region exerts more force so the net force is from high (H) to low (L).


	The pressure gradient is the change in pressure divided by the distance over which that change occurs.
	A large pressure gradient will create a strong wind. (O’Dome example)


	The Coriolis force describes the apparent force due to the rotation of the earth. (demonstration)


	The Coriolis force causes the wind to deflect to the right in the Northern Hemisphere.


	Except at the equator a free-moving object heading east or west will appear to deviate from its path.
	There is more deviation at higher latitudes.


	The apparent deflection of the Coriolis force increases with speed.


	With only the pressure gradient force the wind would blow directly from high pressure to low pressure.


	500mb chart (~3 miles up): Aloft the winds blow more or less parallel to isobars (constant pressure contours).
	These are called geostrophic winds (earth-turning).


	The pressure gradient force (PGF) balances the Coriolis force (CF) so the wind flows parallel to isobars.


	Closer contours imply a larger pressure gradient force.
	This is balanced by a larger Coriolis force, which means higher wind speeds.


	The orientation and spacing of isobars allows one to determine the wind direction and speed.


	In the northern hemisphere air flows counterclockwise around lows (cyclones) and clockwise around highs (anticyclones).


	Circular motion means the direction is constantly changing, which is a form of acceleration.
	Centripital acceleration = velocity²/radius.
	To create this acceleration, the pressure grad. force is larger than the Coriolis force.


	Air leaving a high curves to the right in the northern hemisphere.
	The rotation about an anticyclone (H) is thus clockwise in the northern hemisphere.
	For cyclones (L) the rotation is counterclockwise.


	The zonal wind flow (west to east) across the US is the reason it is faster to fly from west to east.
	Meridonal wind flow goes in a north-south trajectory.


	Near the surface, wind direction is not parallel to the isobars, but crosses them.


	Friction near the surface slows down the wind and hence reduces the Coriolis force.
	The pressure gradient force dominates.
	The wind crosses isobars.


	If we stand with our back to the wind, then turn clockwise about 30^o, lower pressure will be to our left.


	The Coriolis force is reduced and the wind crosses isobars. (N. Hemisphere shown)
	Where does the air go?


	To keep the pressure the same at a low, the converging air at the surface must be balanced diverging air above.
	If the divergence slows, the pressure in the low will rise.
	If the divergence increases, the pressure in the low will decrease.


	Since the pressure is higher on the ground than further up in the atmosphere, why doesn’t the pressure gradient force cause the air to flow uniformly up?
	Gravity balances the pressure gradient force to create hydrostatic equilibrium.


	The wind is influenced by four forces:
		Pressure gradient force
		Coriolis force
		Centripetal force
		Friction.
	From these forces we are able to understand the speed and direction of the wind both near the ground and further aloft.