ESCI 106 – Weather and Climate Lecture 6

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ESCI 106 – Weather and Climate Lecture 6
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 ~ 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
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
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
Adjustments need to be made for elevation Everything is converted to SEA-LEVEL equivalents A) = 1008 B) = 1014 C) = 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 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 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!!! Pressure Gradient Force Coriolis Force Friction

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

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

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

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 ΔP T = 20 T = 30 T = 20 T = 30 LARGE DISTANCE SMALL DISTANCE

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!!!

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
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

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!

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
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

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 PGF Fc Fc Fc WIND 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 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

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 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 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
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

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

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

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 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)

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

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.

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
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 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 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.

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