1. Atmospheric Forces
Wind Relationships

2. Atmospheric Forces Wind Relationships Pressure Forces Apparent Forces
Friction
Confluence and Difluence
Gradient winds
Super and Subgradient Winds
Geostrophic Winds

3. Forces of Nature Pressure Gradient (PG) -
The rate of pressure change with
distance. Always measured perpendicular to the isobars, or
PG=Dp / Dn (Change in pressure over distance)
Pressure Gradient Force (PGF) - A force acting on a parcel of air,
resulting from the difference in pressure on either side of
the parcel.
1. What force initiates
the wind?
Natural
Coordinate
System
Pressure Gradient Force
2. If the gradient force is
higher, what happens to
the wind?
It Increases
Pressure Gradient Pressure Gradient Force
Measured on the surface of the Earth.

4. Forces of Nature
How do we find highs and lows in the upper atmosphere?
Use a constant pressure surface chart
Contour Gradient
500mb Surface (Avg 18 kft)

5. Forces of Nature Contour Gradient (CG) -
The rate of height change (Dz) with
distance (Dn) on a constant pressure surface.
CG=Dz / Dn (Change in height over distance)
Contour Gradient Force (CGF) - The force that represents PGF
on a constant pressure chart.
Differences in virtual temperature
and quantity of mass over a given
point in the atmosphere results in
differences in thickness between
layers. These variations in height
result in the CG.
500mb A B
Contour Gradient Contour Gradient Force
500mb Surface (Avg 18 kft)

6. Forces of Nature
Coriolis Force (CoF) - An apparent force exerted on moving
objects caused by the rotation of the Earth and it’s coordinate
system.
Zero at the Equator and maximum at the poles
Velocity dependent. With constant latitude, as a parcel’s
velocity increases, then CoF increases.
CoF acts 90 degrees to the wind direction (toward the right of
motion in the northern hemisphere)
The closer you get to the poles,
the stronger the CoF, if the wind
speed remains the same.

7. What are the relationships of velocity and curvature on the
Forces of Nature
Centrifugal Force (CeF) - The apparent force that deflects
particles and therefore winds away from the center of rotation.
What are the relationships of
velocity and curvature on the
strength of CeF?
Strength of force is dependent on
the velocity of the parcel, and the
rate of curvature:
Centrifugal Force is always directed
outward from the axis of rotation and
perpendicular to rotation.
CeF helps weaken the gradient
in a High and tighten the gradient
in a Low.
Constant curvature Constant Velocity
as v CeF as r Cef ? ?

8. Forces of Nature
Frictional Force (Fr) - The force that opposes motion.
Acts opposite to the direction of motion.
No direct effect on PGF or CGF, but does affect CoF and CeF.
Increasing Fr results in decreasing wind velocity.
Fr causes both CoF and CeF to decrease, therefore…
Winds tend to back
towards the stronger
force (PGF or CGF).
Wind without the effects of friction,
and PGF/CGF and CoF in balance, is
called a … ?
Geostrophic Wind

9. Forces of Nature In Conclusion…. Pop Quiz!
Force Effect Wind Speed… Effect Wind Direction…
PGF/CGF
CeF
CoF Fr
Yes (intiates wind)
Yes (high to low) No
Yes (curved flow) No
Yes (90 deg to RT of flow)
Yes (slows)
Yes (backs)

10. Wind Relationships
Geostrophic Wind: The wind that would result if there were a
balance between the Coriolis Force and Pressure Gradient Force.
Assumes: Straight line flow - no curvature.
Assumes: No friction. Most often possible above the Earth’s surface.
Forces are in balance, so wind speed is constant.
Can there ever truly be a Geostrophic wind in our atmosphere?
Maybe? How about very short periods of time?
Straight Line Flow (No Friction)
Ageostrophic flow (friction added-straight line)

11. Wind Relationships
Gradient Winds - In Curved Flow with no change in gradient -
Anticyclonic Gradient Wind:
Coriolis force must balance both Pressure/Contour
Gradient Force and Centrifugal Force. H
CeF
CGF
CoF Va
* Va will be stronger than the expected
geostrophic wind at the same latitude and
PGF or CGF (supergeostrophic).
** Remember - with latitude constant, an
increase in CoF will increase wind speed.
Max Speed is limited due
to finite CoF.

12. Wind Relationships
Gradient Winds - In Curved Flow with no change in gradient -
Cyclonic Gradient Wind:
Coriolis Force and Centrifugal Force act
together to balance Pressure Gradient Force. L
CeF
CoF
CGF Vc
** Vc will be weaker than the expected
geostrophic wind at the same latitude and
PGF or CGF (subgeostrophic).
Max speed is unlimited.

13. Wind Relationships Gradient Level Winds What is the Gradient Level?
The top of the boundary layer, or the height where friction,
wind shear, and/or surface stability end.
Terrain - Rougher the terrain, the greater the mixing.
Wind - Greater wind means more mechanical mixing
Stability - The less stable
the atmosphere, the more
mixing (stronger upward
vertical motion)
Sfc Heating Normal Sfc Cooling
Gradient Level

14. Wind Relationships Confluence and Difluence What is Confluence?
The directional merging of wind flow.
What is Difluence?
The directional spreading apart of wind flow.

15. Wind Relationships Supergradient and Subgradient Winds
Supergradient Winds - A wind adjusting to the rapid decrease
in Contour Gradient Force downstream.
540
CGF
CoF Vg
552
546
Winds turn “Uphill”
towards higher values

16. Wind Relationships Supergradient and Subgradient Winds
Subgradient Winds - A wind adjusting to a rapid increase in
Contour gradient force downstream.
CGF
CoF Vg
546
546
552
CGF
CoF Vg
552
CGF Vg
558
558
564
CoF
Winds turn “downhill”
towards lower values
564