ESCI 106 – Weather and Climate
Lecture 6
9-22-2011
Jennifer D. Small 
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!!
National Watches and Warnings
“ Chapter 6- Air Pressure and Winds”
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
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
 ~ 1013.25 mb*
* This is a number
you MUST
memorize!!!!
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!!
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!!!!
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.
Comparison of Pressures
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.
Pressure and Weather - Intro
CHANGE in pressure is
a better predictor of the
weather
Decreasing Pressure
Increasing cloudiness
Increasing Pressure
Clearing conditions
Pressure Changes with Altitude
Pressure reduces by ½
for each 5 kilometers
 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 Changes with Altitude
a)
b)
c)
Upper Atmosphere
Middle Atmosphere
Sea Level
(Mesosphere)
(Stratosphere)
(Troposphere)
Canister of air fitted with a movable piston
Weight is added…. Pressure increases
More weight is added…. Pressure increases further
Horizontal Variations in Air Pressure
 Adjustments need to be made for elevation
 Everything is converted to SEA-LEVEL equivalents
A) 1008 + 0 = 1008
B) 915 + 99 = 1014
C) 840 + 180 = 1020
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
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
N2: 7 * 14 = 98
O2: 2 * 16 =32
O2: 3 * 16 =48
H2O: 5 * 10 = 50
Total = 146
Total = 138
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
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
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
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
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!!!
1. Pressure Gradient Force
2. Coriolis Force
3. Friction
Basic Rules for Winds:
1.
Horizontal differences in pressure causes winds
2.
Horizontal differences in pressure are caused by
differences in heating
3.
Winds flow from regions of high pressure to regions
of low pressure
4.
Horizontal differences in P lead to the PRESSURE
GRADIENT FORCE
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
Pressure Gradient Force
 Horizontal Pressure Differences (HPD)
 Winds flow from High pressure to Low pressure
if only affected by HPD
500 mb
Lower P
Higher P
700 mb
500 mb
700 mb
Sea Breeze
1000 mb
1000 mb
COOL
WARM
Nighttime
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)
ISOBARS – Let’s do an example!
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
T = 20
T = 30
SMALL DISTANCE
ΔP
T = 20
T = 30
LARGE DISTANCE
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
500 m
400 m
300 m
200 m
100 m
The STEAPER the
SLOPE the FASTER
the ball will roll!!!
100 m
300 m
500 m
ISOBARS & PGF - More Examples
1000 mb
1020 mb
1004 mb
1016 mb
1008mb
1012 mb
1012 mb
1008 mb
1016 mb
1004 mb
1020 mb
1000 mb
PGF
PGF
PGF, perfectly down hill at right angles
to the isobars
For a conical
hill, the PGF
points in all
direction
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
1000 mb
1008 mb
1012 mb
1004 mb
1008 mb
1016 mb
1012 mb
1020 mb
1016 mb
1020 mb
PGF
PGF
Change in P over
large distance:
Change in P over
small distance:
SMALL PGF
LARGE PGF
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
ISOBARS – Add in the PGF!
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
Example: Sea Breeze
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
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!
Coriolis Force – Earth’s Rotation
Rotation is Clockwise
in SH
Rotation is Counter
Clockwise in NH
Coriolis Force – Summary
1. Always Deflects a moving body (wind) to the
right
2. Only affect wind direction, not speed
3. Is affected by wind speed (the stronger the
wind, the greater the deflecting force)
4. Is strongest at the poles and nonexistent at the
equator… latitude dependent
These two determine the MAGNITUDE of the Coriolis Force
ISOBARS – Add in PGF + Coriolis!
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.
Winds Aloft and Geostrophic Flow
Where friction doesn’t play a role!!
When only the PGF and Coriolis Forces
(Fc) affect an air parcel
1000 mb
Fc
WIND
1004 mb
1008mb
1012 mb
Fc
1016 mb
1020 mb
Direction of MOTION!
PGF
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
WIND
PGF
900 mb
904 mb
Direction of
MOTION! 908 mb
Coriolis, Fc
912 mb
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
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
PGF
Friction
Direction of
MOTION!
Coriolis, Fc
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.
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.
ISOBARS – PGF + Coriolis + Friction!
How Winds Generate Vertical Air Motion
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
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.
“ Chapter 7- Circulation of the
Atmosphere”
Scales of Atmospheric Motion
Scale
Time Scale
Distance
Scale
Examples
1000-40000km
Westerlies, trade
winds
Days to weeks
100-5000 km
Mid-latitude cyclones,
anticyclones,
hurricanes
Mesoscale
Minutes to
hours
1-100 km
Thunderstorms,
tornadoes, and landsea breeze
Microscale
Seconds to
minutes
<1 km
Turbulence, dust
devils and gusts
Macroscale
Planetary Weeks or longer
Synoptic
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
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
Land and Sea Breezes
 Most intense ones form along tropical
coastlines adjacent to cool ocean currents.
Mountain and Valley Breezes
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
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.
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.
Country Breezes
 Associated with large urban areas
 Light wind blowing in from the countryside
 Clear, calm nights
 City is warmer (urban heat island)
Global Circulation
 Single-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)
Single-Cell Model
1. The equator is heated
2. Rises
3. Travels toward cold
Poles
4. Air cools and sinks
5. Travels back to the
equator
Three-Cell Model – Hadley Cell
 Air rises at the equator
 Air travels north and
subsides between 25-30
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.
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 anticyclones disrupts the general
westerly flow.
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.
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)
Observed distribution of Pressure and Winds
 Subtropical Highs
 At about 25-30 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.
Observed distribution of Pressure and Winds
 Subpolar Low
Another low-pressure region between 50-60
corresponding to the polar front
Responsible for much of the stormy weather in the
mid-latitudes
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.
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
H
WINTER MONSOON
L
COLD H
L
H
L
H
Down sloping air
= No clouds
L
Hot Indian Continent
Warm Ocean
Warm Ocean