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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.

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esci 106 weather and climate lecture 6

ESCI 106 – Weather and ClimateLecture 6

9-22-2011

Jennifer D. Small 

weather fact of the day september 8
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!!
understanding pressure
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
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 pressure7
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 pressure8
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 pressure9
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.
pressure and weather intro
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 intro12
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 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

pressure changes with altitude14
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
horizontal variations in air pressure
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
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 vapor17
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

influence of temp and water vapor18
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
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
What is Wind?
  • Windis the result of horizontal differences in air pressure!
  • Air flows from areas of HIGH pressure to areas of LOW pressure

HIGH

LOW

what is wind21
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
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
basic rules for winds
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
basic rules for winds24
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
Pressure Gradient Force
  • Horizontal Pressure Differences (HPD)
  • Winds flow from High pressure to Low pressure if only affected by HPD

Lower P

Higher P

500 mb

500 mb

700 mb

700 mb

Sea Breeze

1000 mb

1000 mb

WARM Nighttime

COOL

isobars
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)
pgf change over horizontal difference
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

isobars pgf

100 m

500 m

300 m

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

The STEAPER the SLOPE the FASTER the ball will roll!!!

isobars pgf more examples
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

isobars pgf more examles
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

pressure gradient force summary
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
vertical pressure gradient
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
coriolis force
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
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
Coriolis Force – Earth’s Rotation

Rotation is Clockwise in SH

Rotation is Counter Clockwise in NH

coriolis force summary
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

friction
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

1000 mb

1004 mb

1008mb

1012 mb

1016 mb

1020 mb

PGF

Fc

Fc

Fc

Fc

Winds Aloft and Geostrophic Flow
  • Where friction doesn’t play a role!!
  • When only the PGF and Coriolis Forces (Fc) affect an air parcel

WIND

Direction of MOTION!

winds aloft and geostrophic flow43

PGF

Direction of MOTION!

Coriolis, Fc

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

900 mb

904 mb

908 mb

912 mb

curved flow and gradient wind
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

PGF

Direction of MOTION!

Friction

Coriolis, Fc

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
adding in friction to coriolis and pgf46
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
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.
factors that promote vertical airflow
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 airflow51
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.
large and small scale winds
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
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
Land and Sea Breezes
  • Most intense ones form along tropical coastlines adjacent to cool ocean currents.
chinook foehn winds
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
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
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
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
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
Single-Cell Model
  • The equator is heated
  • Rises
  • Travels toward cold Poles
  • Air cools and sinks
  • Travels back to the equator
three cell model hadley cell
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
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.
three cell model polar cell
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
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 winds68
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 winds69
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 winds70
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
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