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ESCI 106 – Weather and Climate Lecture 6 9-22-2011 Jennifer D. Small Jennifer D. Small.

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Presentation on theme: "ESCI 106 – Weather and Climate Lecture 6 9-22-2011 Jennifer D. Small Jennifer D. Small."— Presentation transcript:

1 ESCI 106 – Weather and Climate Lecture Jennifer D. Small Jennifer D. Small

2 Weather Fact of the Day: September : A NorEaster 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!!

3 National Watches and Warnings

4 Chapter 6- Air Pressure and Winds Chapter 6- Air Pressure and Winds

5 Understanding Pressure AIR PRESSURE is the pressure exerted by the weight of the air above. FORCE Is DEFINED as: the FORCE exerted against a surface by the continuous collision of gas molecules

6 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/m 2 ) Meteorologist use millibars (mb) 1 mb = 100 N/m 2 Standard Sea Level Pressure ~ mb* * This is a number you MUST memorize!!!!

7 Understanding Pressure Example: Why arent 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!!

8 Understanding Pressure Example: Why arent 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!!!!

9 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.

10 Comparison of Pressures

11 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 ALWAYS Not ALWAYS true NO LIQUID!! An air chamber changes shape as pressure changes.

12 Pressure and Weather - Intro CHANGE CHANGE in pressure is a better predictor of the weather Decreasing Pressure Increasing cloudiness Increasing Pressure Clearing conditions

13 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

14 Pressure Changes with Altitude a)Canister of air fitted with a movable piston b)Weight is added…. Pressure increases c)More weight is added…. Pressure increases further Sea Level (Troposphere) Middle Atmosphere (Stratosphere) Upper Atmosphere (Mesosphere)

15 Horizontal Variations in Air Pressure Adjustments need to be made for elevation Everything is converted to SEA-LEVEL equivalents A) = 1008 B) = 1014C) = 1020

16 Influence of Temp and Water Vapor (A) Warm Air (A) Warm Air Fast moving molecules Typically less dense LOW PRESSURE LOW PRESSURE (B) Cold Air (B) Cold Air Slow moving molecules Typically more dense HIGH PRESSURE HIGH PRESSURE **Factors other then Temp can affect Pressure… you can have warm high pressure

17 Influence of Temp and Water Vapor The addition of water vapor actually makes air LIGHTER (less Dense)!!!! The addition of water vapor actually makes air LIGHTER (less Dense)!!!! Molecular weights of N 2 (14) and O 2 (16) are greater than H 2 O (10) If you substitute some of the N 2 and O 2 with H 2 0 the overall weight of air will be less! N 2 : 7 * 14 = 98 O 2 : 3 * 16 =48 Total = 146 N 2 : 4 * 14 = 56 O 2 : 2 * 16 =32 H 2 O: 5 * 10 = 50 Total = 138

18 Influence of Temp and Water Vapor SUMMARY 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 HIGH PRESSURE

19 Airflow and Pressure Movement of air can cause variations in pressure CONVERGENCE Net flow of air into a region = CONVERGENCE DIVERGENCE Net flow of air out of a region = DIVERGENCE

20 What is Wind? is the result of horizontal differences in air pressure! Wind is the result of horizontal differences in air pressure! Air flows from areas of HIGH pressure to areas of LOW pressure HIGHLOW

21 What is Wind? Wind is natures attempt at balancing inequalities in pressure FACT: Unequal heating of the Earths surface generates these inequalities. FACT: Solar radiation is the ultimate source of energy for Wind

22 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

23 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 PRESSURE GRADIENT FORCE 4.Horizontal differences in P lead to the PRESSURE GRADIENT FORCE

24 Basic Rules for Winds: T = 20 T = mb 700 mb 600 mb NO TEMPERATURE DIFFERENCE TEMPERATURE DIFFERENCE NO WIND WIND

25 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

26 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)

27 ISOBARS – Lets do an example!

28 PGF – Change over Horizontal Difference T = 20T = 30 SMALL DISTANCE LARGE DISTANCE STRONGER 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 T = 20T = 30 ΔPΔP ΔPΔP

29 ISOBARS & PGF 500 m 400 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 m 300 m 200 m 100 m 500 m 300 m 100 m The STEAPER the SLOPE the FASTER the ball will roll!!!

30 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 PGF 1016 mb 1012 mb 1008 mb 1004 mb 1000 mb PGF PGF, perfectly down hill at right angles to the isobars

31 ISOBARS & PGF - More Examles 1020 mb 1016 mb 1012 mb 1008 mb 1004 mb PGF Change in P over large distance: SMALL PGF 1020 mb 1016 mb 1012 mb 1000 mb 996 mb 992 mb PGF 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 WINDISSLOW WINDISFAST

32 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 START MOVING 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

33 ISOBARS – Add in the PGF!

34 Vertical Pressure Gradient In general higher pressures closer to the surface. Hydrostatic Equilibrium 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

35 Example: Sea Breeze

36 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

37 On a non-rotating Earth, the rocket would travel straight to its target. Earth rotates 15 deg per hour…. Even though the rock travels in STRAIGHT line, when we plot its path on the surface it follows a path that CURVES to the RIGHT! Coriolis Force - Example

38 Coriolis Force – Earths Rotation Rotation is Clockwise in SH Rotation is Counter Clockwise in NH

39 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

40 ISOBARS – Add in PGF + Coriolis!

41 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.

42 Winds Aloft and Geostrophic Flow Where friction doesnt play a role!! When only the PGF and Coriolis Forces (F c ) affect an air parcel 1020 mb 1016 mb 1012 mb 1008mb 1004 mb 1000 mb PGF WIND FcFc FcFc FcFc FcFc Direction of MOTION!

43 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). Geostrophic Flow Therefore in Geostrophic Flow, winds run parallel to isobars in a straight path Direction of MOTION! Coriolis, F c PGF 900 mb 904 mb 908 mb 912 mb WIND

44 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 H L PGF Coriolis Wind

45 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, F c PGF Friction

46 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.

47 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.

48 ISOBARS – PGF + Coriolis + Friction!

49 How Winds Generate Vertical Air Motion

50 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

51 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.

52 Chapter 7- Circulation of the Atmosphere Chapter 7- Circulation of the Atmosphere

53 Scales of Atmospheric Motion ScaleTime ScaleDistance Scale Examples Macroscale PlanetaryWeeks or longer km Westerlies, trade winds SynopticDays to weeks km Mid-latitude cyclones, anticyclones, hurricanes Mesoscale Minutes to hours km Thunderstorms, tornadoes, and land- sea breeze Microscale Seconds to minutes <1 km Turbulence, dust devils and gusts

54 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

55 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

56 Land and Sea Breezes Most intense ones form along tropical coastlines adjacent to cool ocean currents.

57 Mountain and Valley Breezes

58 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

59 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.

60 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.

61 Country Breezes Associated with large urban areas Light wind blowing in from the countryside Clear, calm nights City is warmer (urban heat island)

62 Global Circulation Single-Cell Model First idea George Hadley in 1735 Solar energy drives the winds Doesnt 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)

63 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

64 Three-Cell Model – Hadley Cell 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.

65 Three-Cell Model – Ferrell Cell 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.

66 Three-Cell Model – Polar Cell 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.

67 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 Intertropical Convergence Zone (ITCZ) Also referred to as the Intertropical Convergence Zone (ITCZ)

68 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.

69 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

70 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.

71 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 LH LH L H H L Warm Ocean Hot Indian Continent COLD Down sloping air = No clouds


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