esci 106 – weather and climate lecture 6 9-22-2011 jennifer d. small jennifer d. small
TRANSCRIPT
ESCI 106 – Weather and ESCI 106 – Weather and ClimateClimate
Lecture 6Lecture 6
9-22-20119-22-2011
Jennifer D. Small 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”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 FORCEFORCE exerted against a surface by the continuous collision of gas molecules
Measuring Air Pressure
Unit: Newton (N)
At Sea Level one “atmosphere” exerts14.7 pounds per square inch101,325 N per square m (N/m2)
Meteorologist use millibars (mb) 1 mb = 100 N/m2
Standard Sea Level PressureStandard Sea Level Pressure ~ 1013.25 mb*~ 1013.25 mb*
* This is a * This is a number you number you
MUST MUST memorize!!!!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 directions3) 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 directions3) 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!!!!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 BarometersOften found in homesNo Mercury (safer!!)
Typically you find the following relationships:LOW Pressure = “rain”HIGH Pressure = “fair
weather”Not ALWAYSALWAYS true
NO LIQUID!! An air chamber changes shape as pressure
changes.
Pressure and Weather - Intro
CHANGECHANGE in pressure is a better predictor of the weatherDecreasing Pressure
Increasing cloudiness
Increasing PressureClearing conditions
Pressure Changes with Altitude
FACT: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
Pressure Changes with Altitude
a) Canister of air fitted with a movable pistonb) Weight is added…. Pressure increasesc) More weight is added…. Pressure increases further
Sea Level
(Troposphere)
Middle Atmosphere
(Stratosphere)
Upper Atmosphere
(Mesosphere)
Horizontal Variations in Air Pressure
Adjustments need to be made for elevationEverything 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(A) Warm AirFast moving moleculesTypically less denseLOW PRESSURELOW PRESSURE
(B) Cold Air(B) Cold AirSlow moving moleculesTypically more denseHIGH PRESSUREHIGH 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 The addition of water vapor actually makes air LIGHTER (less Dense)!!!!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: 7 * 14 = 98
O2: 3 * 16 =48
Total = 146Total = 146
N2: 4 * 14 = 56
O2: 2 * 16 =32
H2O: 5 * 10 = 50
Total = 138Total = 138
Influence of Temp and Water Vapor
SUMMARYSUMMARY 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 PressuresLOW
PRESSURE
HIGH PRESSUR
E
Airflow and Pressure
Movement of air can cause variations in pressureNet flow of air into a region = CONVERGENCECONVERGENCENet flow of air out of a region = DIVERGENCEDIVERGENCE
What is Wind?
Wind is the result of horizontal differences is the result of horizontal differences in air pressure!in air pressure!
Air flows from areas of HIGH pressure to areas of LOW pressure
HIGHHIGH LOWLOW
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 Force2. Coriolis Force3. 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 PRESSURE GRADIENT FORCEGRADIENT FORCE
Basic Rules for Winds:
T = 20 T = 20 T = 20 T = 301000 mb
700 mb
600 mb
NO TEMPERATURE DIFFERENCE
TEMPERATURE DIFFERENCE
NO WIND WIND
Pressure Gradient Force
Horizontal Pressure Differences (HPD)Winds flow from High pressure to Low pressure
if only affected by HPD
1000 mb
700 mb
500 mb
1000 mb
700 mb
500 mb
COOL WARM Nighttime
Higher P
Lower P
Sea Breeze
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
T = 20 T = 30
SMALL DISTANCE LARGE DISTANCE
STRONGERSTRONGER 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
T = 20 T = 30
ΔP ΔP
ISOBARS & PGF
500 m400 m
300 m
200 m
100 m
If all we had was the PGF wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope!If all we had was the PGF wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope!
500 m
400 m300 m200 m100 m
500 m300 m100 m
The STEAPER the SLOPE the FASTER the ball will roll!!!
ISOBARS & PGF - More Examples
1020 mb For a conical hill, the PGF points in all
direction
1016 mb
1012 mb
1008mb
1004 mb
1000 mb 1020 mb
PGFPGF
1016 mb
1012 mb
1008 mb
1004 mb
1000 mb
PGFPGF
PGF, perfectly down hill at right angles to the isobars
ISOBARS & PGF - More Examles
1020 mb
1016 mb
1012 mb
1008 mb
1004 mb
PGFPGF
Change in P over large distance:
SMALL PGF
1020 mb1016 mb
1012 mb
1000 mb996 mb992 mb
PGFPGF
1004 mb
1008 mb
Winds if we ONLY knew the PGF.
If the isobars are further or closer together…
Change in P over small distance:
LARGE PGF
WINDWINDISIS
SLOWSLOW
WINDWINDISIS
FASTFAST
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 MOVINGSTART 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 EquilibriumHydrostatic 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
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 - Example
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
1020 mb
1016 mb
1012 mb
1008mb
1004 mb
1000 mb
PGFPGFW
IND
WIN
D
Fc
Fc
Fc
Fc
Direction of MOTION!
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 FlowGeostrophic Flow, winds run parallel to isobars in a straight path
Direction of MOTION!
Coriolis, Fc
PGF900 mb
904 mb
908 mb
912 mb
WIND
Curved Flow and Gradient WindGradient Wind – winds that follow curved paths
around high and low pressure cells.
Speed of the wind depends on how close the isobars are
H L
PGFCoriolis
Wind
Adding in Friction to Coriolis and PGF
Geostrophic Flow and FrictionCauses parcel to slow downCoriolis decreases in strength
Friction cases wind to lean towards the direction of the PGF
Direction of MOTION!
Coriolis, Fc
PGF
Friction
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 divergenceWhen 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 airAs 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 Chapter 7- Circulation of the Atmosphere”Atmosphere”
Scales of Atmospheric Motion
Scale Time Scale Distance Scale
Examples
Macroscale
Planetary Weeks or longer 1000-40000km Westerlies, trade winds
Synoptic Days to weeks 100-5000 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
Large and Small Scale Winds
Macroscale WindsPlanetary: Westerlies, trade windsSynoptic: Cyclones and anti-cyclones, Hurricanes
(weather map size)
Mesoscale WindsThunder storms, tornadoes, etcPart of larger macroscale wind systems.
Microscale WindsChatoic motions including gusts and dust devils
Local Winds (mesoscale)
True local winds are caused by topographic effects or variations in local surface compositionLand and Sea BreezesMountain and Valley BreezesChinook (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 WindsHot 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 CellAir 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 splitsTrade 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 Cell30-60 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.
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 LowNear the equator the warm rising branch of the Hadley
cells is associated with a low pressure zone.Ascending moist, hot air with lots of precipitationAlso referred to as the Intertropical Convergence Intertropical Convergence
Zone (ITCZ)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 LowAnother low-pressure region between 50-60
corresponding to the polar frontResponsible for much of the stormy weather in the
mid-latitudes
Observed distribution of Pressure and Winds
Polar HighsAt the poles, where the polar easterlies originateHigh 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.
MonsoonsA seasonal reversal in weather patternsAn 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
L H
LHL
H
H
L
Warm Ocean Warm OceanHot Indian Continent
COLD
Down sloping air = No clouds