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ESCI 106 – Weather and Climate Lecture 6

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

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  1. ESCI 106 – Weather and ClimateLecture 6 9-22-2011 Jennifer D. Small 

  2. 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!!

  3. National Watches and Warnings

  4. “ Chapter 6- Air Pressure and Winds”

  5. 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

  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/m2) • Meteorologist use millibars (mb) • 1 mb = 100 N/m2 • Standard Sea Level Pressure • ~ 1013.25 mb* * This is a number you MUST memorize!!!!

  7. 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!!

  8. 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!!!!

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

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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  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!!! • Pressure Gradient Force • Coriolis Force • Friction

  23. 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

  24. 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

  25. 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

  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 – Let’s do an example!

  28. 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

  29. 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!!!

  30. 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

  31. 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

  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 • 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 • 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. 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!

  38. Coriolis Force – Earth’s Rotation Rotation is Clockwise in SH Rotation is Counter Clockwise in NH

  39. 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

  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. 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!

  43. 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

  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 L H PGF Coriolis Wind

  45. 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

  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

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