1. Announcements Exam #1 will be handed back Wednesday or Friday.
First homework assignment due next Monday.
No class Wednesday before Thanksgiving.

2. Cloud drops grow rapidly into raindrops because of _________________:
The Bergeron process.
The fact they have a curved surface.
They collide and coalesce with each other.
They form on hygroscopic cloud condensation nuclei.

3. Summary of Lecture 14
Reviewed the basic concepts of pressure from earlier in the course.
Ideal gas low relates pressure to density and temperature. Breaking this down (Boyles law, Charles law, etc.) we find:
Pressure is proportional to density
Pressure is proportional to temperature
Temperature is inversely proportional to density
Heating (cooling) a column of air expands (contracts) it and decreases (increases) density. The pressure gradient will force air to go from high to low pressure.
The example of an Arctic high was used to illustrate these concepts. At the surface, high pressure is associated with very cold temperatures.
Upper air charts show the height of a pressure surface above the ground. In the Arctic high example, because the air is cold it had a relatively low height at 300-mb.
Station model sea-level pressure must be adjusted for altitude.
Air pressure can be measured using a mercury barometer and aneroid barometer.

4. Why does the wind blow? Part I
NATS 101 Section 4: Lecture 15
Why does the wind blow?
Part I

5. To begin the answer to this question we first have to revisit Sir Isaac Newton

6. Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will remain at a constant velocity if the net force exerted on it is zero.
Constant velocity = same speed, same direction.
An external force is required to change either the direction or speed of an object (or air in the case of the atmosphere)
Sir Isaac Newton

7. Newton’s Second Law of Motion
The net force exerted on an object is equal to it’s mass times acceleration, or change in velocity over time.
FORCE = MASS X ACCELERATION
F = ma
SI Units: Newton (kg m s-2)
Sir Isaac Newton
Velocity is a vector property of the object’s speed AND its direction, so to change it and cause acceleration either:
Change the speed of the object
Change the direction of the object.

8. Two causes of acceleration
Change in speed (magnitude) over time (t).
Change in direction over time (t) with no change in speed V1 V1 V2 V2
INITIAL VELOCITY
INITIAL VELOCITY
FINAL
VELOCITY
FINAL VELOCITY V1 V2 V2 V1
ACCELERATION
ACCELERATION

9. Centripetal Force CENTRIFUGAL FORCE CENTRIPETAL FORCE
You experience acceleration without a change in speed, for example, on a tilt-a-whirl carnival ride.
The force is directed toward the center of the wheel.
An equal an opposite (fictitious) centrifugal force is exerted by the inertia of your body on the wheel—so you stay put and don’t fall off even when upside down.
Important when considering curved flows, as well see later…
CENTRIFUGAL FORCE
CENTRIPETAL
FORCE

10. Newton’s second law can be used to derive a governing equation for atmospheric motion
The simplified form in the horizontal we’ll consider has four terms. By understanding how each of these terms works, we’ll be able to explain
why the wind blows.

11. Simplified equation of horizontal atmospheric motion
(1)
(2)
(3)
(4)
Term
Force
Cause 1
Pressure gradient force
Spatial differences in pressure 2
Coriolis force
Rotation of the Earth 3
Centripetal force
Curvature of the flow 4
Friction force
Acts against direction of motion due to interaction with surface
FOCUS ON FIRST TWO TODAY…

12. Force Balance Geostrophic Balance
What we’re looking for in the equation of motion is the condition where the forces exactly balance—or the sum of the forces is equal to zero.
When this happens, there is no net acceleration and the wind speed is constant, by Newton’s first law.
0 = Pressure gradient force + Coriolis force + Centripetal Force + Friction
Geostrophic Balance
0 = Pressure gradient force + Coriolis force

13. Pressure gradient force
Definition: Force to a the difference in pressure (Δp) over a distance (d). (In the equation ρ is the density of air)
The pressure gradient force is directed perpendicular to lines of constant pressure (isobars).

14. Strength of the pressure gradient force
How strong the pressure gradient force is depends on the distance between the areas of high and low pressure, or how close the lines of constant pressure are.
Strong pressure gradient: Isobars close together
Weak pressure gradient: Isobars far apart.
STRONG
PRESSURE
GRADIENT
WEAK
PRESSURE
GRADIENT

15. ALOFT
Cold column  relatively less air above. LOW PRESSURE.
Warm column  relatively more above. HIGH PRESSURE
ALOFT
Result: Air moves from warm column to cold column, changing the total amount of mass of air in each.
SURFACE H L
SURFACE
Cold column  more mass above. HIGH PRESSURE
Warm column  less mass above. LOW PRESSURE.

16. The pressure gradient force is why the wind blows, but you need the other terms to complete the picture…

17. Upper Level Chart for Surface Arctic High Example (300-mb)

18. PRESSURE GRADIENT AT DENVER
Observations for upper level winds:
Wind DOES NOT follow the pressure gradient.
Wind runs parallel to the lines of constant height (i.e. isobars).
Strength of the wind IS related to the closeness, or packing, of the isobars.
For example, compare the wind speed at Denver (105 knots) to some of the surrounding upper air observations, like Albuquerque.
NEED AT LEAST ONE OF THE OTHER THREE FACTORS TO ACCOUNT FOR WIND MOTION
DENVER
105 knots
LOW
HIGH
PRESSURE GRADIENT AT DENVER
ALBUQUERQUE
90 knots

19. Coriolis Force
Definition: Apparent force due to rotation of the Earth (Ω). Depends on the speed (V) and the latitude (Φ).
Gaspard Coriolis
Causes apparent deflection in reference from of an observer at a fixed point on Earth

20. Coriolis force on a merry-go-round
From perspective of person NOT on merry-go-round, path of ball is straight.
From perspective of person on merry-go-round, path of ball deflects. This is an apparent (or fictitious force).

21. Merry-go-round example

22. Rotation of the Earth (from the polar perspective)
NORTHERN HEMISPHERE
SOUTHERN HEMISPHERE
(Getzelman)
COUNTERCLOCKWISE ROTATION
Deflection to the right
CLOCKWISE ROTATION
Deflection to the left
SAME IDEA AS THE MERRY-GO-ROUND!

23. Coriolis Effect: An Apparent Force
Cannonball follows a straight path to an observer in space
Earth rotates counter-clockwise underneath cannonball
(in Northern Hemisphere)
Cannonball appears to deflect to the right to an observer on earth

24. Coriolis Force and Latitude
All three airplanes travel in a straight line with respect to an outside observer (from space).
The largest deviation, or deflection to the right, with respect to an observer on Earth occurs for the one traveling closest to the pole.
The higher the latitude, the greater the Coriolis force. Accounted for by the sine term in the mathematical expression.
Zero at equator (sin 0° = 0)
Maximum at poles (sin 90° = 1)

25. Coriolis force and speed
The Coriolis force is proportional to the wind speed.
The faster the speed (or velocity), the greater the amount of Coriolis force.
Note also the dependence on latitude here.

26. Coriolis Force vs. Wind Direction
NORTHERN HEMISPHERE
SOUTHERN HEMISPHERE
WIND
CORIOLIS FORCE
(TO LEFT)
WIND
CORIOLIS FORCE
(TO RIGHT)
Coriolis force acts perpendicular (at a right angle) to the wind direction, to the right or left depending on which hemisphere.

27. Geostrophic Wind
PARCEL RELEASED
Positions 1 and 2:
Pressure gradient force accelerates the parcel towards the low pressure.
Coriolis force acts to the right of the velocity of the parcel, making it curve to the right.

28. Geostrophic Wind Positions 3 and 4:
Pressure gradient force continues to accelerate the parcel towards the low pressure.
As the velocity of the parcel increases, the Coriolis force increases, making the parcel continue to curve to the right.

29. Geostrophic Wind Position 5: FINAL STATE
Pressure gradient force is balanced by the Coriolis force.
Velocity of the parcel is constant (no acceleration). Direction is parallel to the isobars.
FINAL STATE is called geostrophic balance.

30. Geostrophic Wind
PRESSURE GRADIENT
FORCE
Isobar 2
WIND
Isobar 1
CORIOLIS FORCE
Pressure gradient force is equally balanced by the Coriolis force, so net force is zero.
Wind speed and direction (velocity) is constant (no acceleration).
Direction of wind is parallel to the isobars, or lines of constant pressure.

31. WEAK GEOSTROPHIC WIND Isobars far apart STRONG GEOSTROPHIC WIND
PRESSURE GRADIENT
FORCE
WEAK
GEOSTROPHIC WIND
Isobars far apart
Isobar 2
WIND
Isobar 1
CORIOLIS FORCE
PRESSURE GRADIENT
FORCE
STRONG
GEOSTROPHIC WIND
Isobars close
together
Isobar 2
WIND
Isobar 1
CORIOLIS FORCE

32. Geostrophic Wind and Upper Level Charts
Winds at upper levels are pretty close to being geostrophic:
Wind is parallel to isobars
Wind strength dependent on how close together isobars are
PRESSURE GRADIENT FORCE
CORIOLIS FORCE
GEOSTROPHIC
WIND

33. Simplified equation of horizontal atmospheric motionX
GEOSTROPHY:
No centripetal force or friction
(1)
(2)
(3)
(4)
Term
Force
Cause 1
Pressure gradient force
Spatial differences in pressure 2
Coriolis force
Rotation of the Earth 3
Centripetal force
Curvature of the flow 4
Friction force
Acts against direction of motion due to interaction with surface

34. Summary of Lecture 15
Newton’s first law of motion: an object will remain at rest and an object in motion will maintain a constant velocity if the net force is zero.
Newton’s second law of motion: F = ma. Change acceleration by a change in speed or direction.
The simplified equation of horizontal atmospheric motion has four force terms: pressure gradient force, Coriolis force, centripetal force, and friction.
The pressure gradient force is due to the difference in pressure over a distance.
The Coriolis force is an apparent force due to the rotation of the Earth, and depends on speed (of the wind) and latitude. It causes deflection from the reference point of an observer in a rotating frame.
Coriolis force deflects the wind to the right or left depending on which hemisphere.
Geostrophic wind occurs when the pressure gradient force balances the Coriolis force and the wind is parallel to the isobars. A good approximation for upper level winds.

35. Reading Assignment and Review Questions
Reading: remainder of Chapter 8.
Chapter 8 Questions
Questions for Review (8th ed.): 9,10,11,12,13
Questions for Review (9th ed.): 10,11,12,13,14
Questions for Thought: 8