1. ESCI 106 – Weather and Climate Lecture 6
Jennifer D. Small 

2. Weather Fact of the Day: September 8
1994: A Nor’Easter wreaked havoc on costal MD.
50 mph winds (gusts to 79 mph) destroyed 100s of tents/vending areas at the end-of-summer Sunfest in Ocean City.
Windblown fires burned several shops along the boardwalk
9 foot waves flooded other areas.
Damage up to $5 million!!

3. National Watches and Warnings

4. “ Chapter 6- Air Pressure and Winds”

5. Understanding Pressure
AIR PRESSURE is the pressure exerted by the weight of the air above.
Is DEFINED as: the FORCE exerted against a surface by the continuous collision of gas molecules

6. Measuring Air Pressure
Unit: Newton (N)
At Sea Level one “atmosphere” exerts
14.7 pounds per square inch
101,325 N per square m (N/m2)
Meteorologist use millibars (mb)
1 mb = 100 N/m2
Standard Sea Level Pressure
~ mb*
* This is a number you MUST memorize!!!!

7. Understanding Pressure
Example: Why aren’t we crushed by the weight of the air above us?
1) We developed under this pressure.
2) Pressure force of air is exerted in all directions
3) If you lower the pressure drastically the cells of our bodies would burst!!
Balloon SHRINKS in all directions and dimensions equally!!

8. Understanding Pressure
Example: Why aren’t we crushed by the weight of the air above us?
1) We developed under this pressure.
2) Pressure force of air is exerted in all directions
3) If you lower the pressure drastically the cells of our bodies would burst!!
Force is only in one direction.
Just the weight of an aquarium on top, not equally in all dimensions
POP!!!!

9. Measuring Air Pressure
Besides mb you may also have heard “inches of mercury” or in of Hg.
Refers to Mercury Barometers
Barometer = instrument to measure pressure.

10. Comparison of Pressures

11. Pressure and Weather - Intro
Aneroid Barometers
Often found in homes
No Mercury (safer!!)
Typically you find the following relationships:
LOW Pressure = “rain”
HIGH Pressure = “fair weather”
Not ALWAYS true
NO LIQUID!! An air chamber changes shape as pressure changes.

12. Pressure and Weather - Intro
CHANGE in pressure is a better predictor of the weather
Decreasing Pressure
Increasing cloudiness
Increasing Pressure
Clearing conditions

13. Pressure Changes with Altitude
FACT: The pressure at any given altitude in the atmosphere is equal to the weight of the air directly above that point!!!
Air becomes less dense because the weight of the air above it decreases.
Why air is “thin” higher in the atmosphere
Pressure reduces by ½ for each 5 kilometers

14. Pressure Changes with Altitude
Upper Atmosphere
(Mesosphere)
Middle Atmosphere
(Stratosphere)
Sea Level
(Troposphere)
Canister of air fitted with a movable piston
Weight is added…. Pressure increases
More weight is added…. Pressure increases further

15. Horizontal Variations in Air Pressure
Adjustments need to be made for elevation
Everything is converted to SEA-LEVEL equivalents
A) = 1008
B) = 1014
C) = 1020

16. Influence of Temp and Water Vapor
(A) Warm Air
Fast moving molecules
Typically less dense
LOW PRESSURE
(B) Cold Air
Slow moving molecules
Typically more dense
HIGH PRESSURE
**Factors other then Temp can affect Pressure… you can have “warm” high pressure

17. Influence of Temp and Water Vapor
The addition of water vapor actually makes air LIGHTER (less Dense)!!!!
Molecular weights of N2 (14) and O2 (16) are greater than H2O (10)
If you “substitute” some of the N2 and O2 with H20 the overall weight of air will be less!
N2: 4 * 14 = 56
O2: 2 * 16 =32
H2O: 5 * 10 = 50
Total = 138
N2: 7 * 14 = 98
O2: 3 * 16 =48
Total = 146

18. Influence of Temp and Water Vapor
HIGH PRESSURE
SUMMARY
Cold, dry air masses produce High Surface Pressures
Cold, humid air masses are less “high” than cold, dry
Warm, dry air masses are less “low” than warm, humid
Warm, humid air masses produce Low Surface Pressures
LOW PRESSURE

19. Airflow and Pressure Movement of air can cause variations in pressure
Net flow of air into a region = CONVERGENCE
Net flow of air out of a region = DIVERGENCE

20. What is Wind?
Wind is the result of horizontal differences in air pressure!
Air flows from areas of HIGH pressure to areas of LOW pressure
HIGH
LOW

21. What is Wind?
Wind is nature’s attempt at balancing inequalities in pressure
FACT: Unequal heating of the Earth’s surface generates these inequalities.
FACT: Solar radiation is the ultimate source of energy for Wind

22. Factors Affecting Wind
If the Earth did NOT rotate and if there was NO friction wind would flow in a straight line from High to Low pressure
Three main forces that affect wind
YOU NEED TO MEMORIZE THESE!!!
Pressure Gradient Force
Coriolis Force
Friction

23. Basic Rules for Winds: Horizontal differences in pressure causes winds
Horizontal differences in pressure are caused by differences in heating
Winds flow from regions of high pressure to regions of low pressure
Horizontal differences in P lead to the PRESSURE GRADIENT FORCE

24. NO TEMPERATURE DIFFERENCE TEMPERATURE DIFFERENCE
Basic Rules for Winds:
NO TEMPERATURE DIFFERENCE
TEMPERATURE DIFFERENCE
WIND
NO WIND
600 mb
700 mb
1000 mb
T = 20
T = 20
T = 20
T = 30

25. Pressure Gradient Force
Horizontal Pressure Differences (HPD)
Winds flow from High pressure to Low pressure if only affected by HPD
Higher P
Lower P
500 mb
500 mb
700 mb
700 mb
Sea Breeze
1000 mb
1000 mb
WARM Nighttime
COOL

26. ISOBARS Isobars or contours (lines or curves) of constant Pressure
Just like your isotherms for temperature
They are corrected for altitude to equivalent Sea Level Pressure (SLP)

27. ISOBARS – Let’s do an example!

28. PGF – Change over Horizontal Difference
STRONGER when isobars are closer together
Same CHANGE in Pressure (ΔP)
When given Pressure Heights, the PGF points from regions of High Pressure to regions of Low Pressure ΔP ΔP
T = 20
T = 30
T = 20
T = 30
LARGE DISTANCE
SMALL DISTANCE

29. The STEAPER the SLOPE the FASTER the ball will roll!!!
ISOBARS & PGF
If all we had was the PGF wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope!
100 m
500 m
200 m
300 m
400 m
400 m
500 m
300 m
200 m
100 m
500 m
300 m
100 m
The STEAPER the SLOPE the FASTER the ball will roll!!!

30. ISOBARS & PGF - More Examples
1020 mb
1000 mb
1016 mb
1004 mb
1008mb
1012 mb
1012 mb
1008 mb
1016 mb
1004 mb
1020 mb
1000 mb
For a conical hill, the PGF points in all direction
PGF
PGF
PGF, perfectly down hill at right angles to the isobars

31. ISOBARS & PGF - More Examles
Winds if we ONLY knew the PGF.
WIND IS
SLOW
WIND IS
FAST
If the isobars are further or closer together…
1004 mb
992 mb
996 mb
1008 mb
1000 mb
1012 mb
1004 mb
1008 mb
1016 mb
1012 mb
1016 mb
1020 mb
1020 mb
PGF
PGF
Change in P over large distance:
SMALL PGF
Change in P over small distance:
LARGE PGF

32. Pressure Gradient Force Summary:
Change in P over large distance = small PGF
Change in P over small distance = large PGF
PGF is at right angles to isobars
Causes wind to START MOVING
However… two forces cause wind speed and direction to be different than predicted by the PGF
Coriolis (rotation of the Earth)
Friction

33. ISOBARS – Add in the PGF!

34. Vertical Pressure Gradient
In general higher pressures closer to the surface.
Hydrostatic Equilibrium
The balance maintained between the force of gravity and the vertical pressure gradient that does not allow air to escape to space.
If we combine the effects of vertical and horizontal pressure gradients we get circulation.
SEA BREEZE is a great example

35. Example: Sea Breeze

36. Coriolis Force Results from the rotation of the Earth
Causes the PGF to cross isobars NOT at right angles.
Winds curve to the RIGHT in the Northern Hemisphere
Winds curve to the LEFT in the Southern Hemisphere

37. Coriolis Force - Example
On a non-rotating Earth, the rocket would travel straight to it’s target.
Earth rotates 15 deg per hour….
Even though the rock travels in STRAIGHT line, when we plot it’s path on the surface it follows a path that CURVES to the RIGHT!

38. Coriolis Force – Earth’s Rotation
Rotation is Clockwise in SH
Rotation is Counter Clockwise in NH

39. Coriolis Force – Summary
Always Deflects a moving body (wind) to the right
Only affect wind direction, not speed
Is affected by wind speed (the stronger the wind, the greater the deflecting force)
Is strongest at the poles and nonexistent at the equator… latitude dependent
These two determine the MAGNITUDE of the Coriolis Force

40. ISOBARS – Add in PGF + Coriolis!

41. Friction Applied to wind within ~1.5 km of the surface
Friction ALWAYS acts in the direction OPPOSITE the direction of motion!!!!
Friction affect air at the surface more than air aloft.

42. Winds Aloft and Geostrophic Flow
Where friction doesn’t play a role!!
When only the PGF and Coriolis Forces (Fc) affect an air parcel
1020 mb
1016 mb
1012 mb
1008mb
1004 mb
1000 mb
PGF Fc Fc Fc
WIND Fc
Direction of MOTION!

43. Winds Aloft and Geostrophic Flow
An air parcel is at equilibrium only if PGF acts in the opposite direction to the Coriolis force (no net force).
Therefore in Geostrophic Flow, winds run parallel to isobars in a straight path
Direction of MOTION!
Coriolis, Fc
PGF
WIND
900 mb
904 mb
908 mb
912 mb

44. Curved Flow and Gradient Wind
Gradient Wind – winds that follow curved paths around high and low pressure cells.
Speed of the wind depends on how close the isobars are L H
PGF
Coriolis
Wind

45. Adding in Friction to Coriolis and PGF
Geostrophic Flow and Friction
Causes parcel to slow down
Coriolis decreases in strength
Friction cases wind to lean towards the direction of the PGF
Direction of MOTION!
Coriolis, Fc
PGF
Friction

46. Adding in Friction to Coriolis and PGF
The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres.
Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.

47. Surface Winds - Friction + Coriolis + PGF
The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres.
Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.

48. ISOBARS – PGF + Coriolis + Friction!

49. How Winds Generate Vertical Air Motion

50. Factors that Promote Vertical Airflow
Friction – can cause convergence and divergence
When air moved from the smooth ocean to the “rough” land, the wind slows down
Results convergence as air “pile up” upstream (like on a highway with construction).
When air goes from land to ocean you see divergence and subsidence

51. Factors that Promote Vertical Airflow
Mountains – hinder the flow of air
As air passes over it is compressed vertically, causing divergence aloft
After going over, onto the lee side, air experiences vertical expansion… causing horizontal convergence.

52. “ Chapter 7- Circulation of the Atmosphere”

53. Scales of Atmospheric Motion
Time Scale
Distance Scale
Examples
Macroscale
Planetary
Weeks or longer km
Westerlies, trade winds
Synoptic
Days to weeks km
Mid-latitude cyclones, anticyclones, hurricanes
Mesoscale
Minutes to hours
1-100 km
Thunderstorms, tornadoes, and land-sea breeze
Microscale
Seconds to minutes
<1 km
Turbulence, dust devils and gusts

54. Large and Small Scale Winds
Macroscale Winds
Planetary: Westerlies, trade winds
Synoptic: Cyclones and anti-cyclones, Hurricanes (weather map size)
Mesoscale Winds
Thunder storms, tornadoes, etc
Part of larger macroscale wind systems.
Microscale Winds
Chatoic motions including gusts and dust devils

55. Local Winds (mesoscale)
True local winds are caused by topographic effects or variations in local surface composition
Land and Sea Breezes
Mountain and Valley Breezes
Chinook (Foehn Winds)
Katabatic (Fall Winds)
Country Breezes

56. Land and Sea Breezes
Most intense ones form along tropical coastlines adjacent to cool ocean currents.

57. Mountain and Valley Breezes

58. Lee side air is heated by compression
Chinook (Foehn Winds)
Warm Dry air moving down the east slopes of the Rockies (Chinook) or Alps (Foehn).
Lee side air is heated by compression

59. Local Chinook-like Wind
Santa Ana Winds
Hot and dry winds increase the threat of fire in Southern California.
Typically September to March but can happen at any time the desert is cooler than SoCal.

60. Katabatic (Fall) Winds
Originate when cold air, situated over a highland area (like an ice sheet) is set in motion.
Gravity carries the cold air over the rim like a waterfall.
The air is heated like a Chinook, but because it start so cold it stays cold.

61. Country Breezes Associated with large urban areas
Light wind blowing in from the countryside
Clear, calm nights
City is warmer (urban heat island)

62. Global Circulation Single-Cell Model Three-Cell Model First idea
George Hadley in 1735
Solar energy drives the winds
Doesn’t account for rotation
Three-Cell Model
Proposed in1920s
Equator and 30 N (S)
30 N (S) and 60 N (S)
60 N (S) and 90 N (S)

63. Single-Cell Model The equator is heated Rises
Travels toward cold Poles
Air cools and sinks
Travels back to the equator

64. Three-Cell Model – Hadley Cell
Air rises at the equator
Air travels north and subsides between N (S) (Horse latitudes)
From the center of the Horse Latitudes the surface flow splits
Trade Winds: equator-ward due to Coriolis
Westerlies: Go towards the poles
Where the trade winds (N and S) meet is called the Doldrums. Light winds and humid conditions.

65. Three-Cell Model – Ferrell Cell
30-60 N (S)
More complicated than the Hadley cell.
Net surface flow is toward the poles
Coriolis bends them to the west….called Westerlies!
More sporadic and less reliable than the trade winds
Migration of cyclones and anti-cyclones disrupts the general westerly flow.

66. Three-Cell Model – Polar Cell
60-90 N (S)
Relatively little is known about the circulation at high (polar) latitudes
Subsidence at the poles produces a surface flow that moves equatorward and is deflected by Coriolis into the Polar Easterlies.
As cold air moves equatorward it meets with the warmer westerly flow and clashes forming the Polar Front.

67. Observed distribution of Pressure and Winds
Equatorial Low
Near the equator the warm rising branch of the Hadley cells is associated with a low pressure zone.
Ascending moist, hot air with lots of precipitation
Also referred to as the Intertropical Convergence Zone (ITCZ)

68. Observed distribution of Pressure and Winds
Subtropical Highs
At about N(S) where westerlies and trade winds originate (subsidence from aloft)
Caused mainly by the Coriolis deflection
Generally the rate at which air accumulates in the upper troposphere exceeds the rate at which the air descends to the surface
Thus they are called semi-permanent highs.

69. Observed distribution of Pressure and Winds
Subpolar Low
Another low-pressure region between corresponding to the polar front
Responsible for much of the stormy weather in the mid-latitudes

70. Observed distribution of Pressure and Winds
Polar Highs
At the poles, where the polar easterlies originate
High pressure develops over the cold polar areas due to extreme surface cooling.
Because the air near the poles is cold and dense it exerts a higher than average pressure.

71. Down sloping air = No clouds
Monsoons
A seasonal reversal in weather patterns
An alternation between two types of weather patters
Ex: India – Wet hot summer, dry cool(ish) winter
A seasonal reversal of wind also
SUMMER MONSOON
WINTER MONSOON H L H L
Down sloping air = No clouds H
COLD L L H
Hot Indian Continent
Warm Ocean
Warm Ocean