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