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NATS 101-05 Lecture 10 Air Pressure

This lecture reviews the concepts of air pressure and the processes involved in precipitation, including the Environmental Lapse Rate, Supercooled Water Droplets, and the Ice Crystal Process. It also explains the Ideal Gas Law and its relationship to temperature, density, and pressure.

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NATS 101-05 Lecture 10 Air Pressure

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  1. NATS 101-05Lecture 10Air Pressure

  2. Review • ELR-Environmental Lapse Rate Temp change w/height measured by a thermometer hanging from a balloon DAR and MAR are Temp change w/height for an air parcel (i.e. the air inside balloon) • Why Do Supercooled Water Droplets Exist? Freezing needs embryo ice crystal First one, in pure water, is difficult to make

  3. Review • Updraft velocity and raindrop size Modulates time a raindrop suspended in cloud • Ice Crystal Process SVP over ice is less than over SC water droplets • Accretion-Splintering-Aggregation Accretion-supercooled droplets freeze on contact with ice crystals Splintering-big ice crystals fragment into many smaller ones Aggregation-ice crystals adhere on snowflakes, which upon melting, become raindrops!

  4. As cloud droplet ascends, it grows larger by collision-coalescence Cloud droplet reaches the height where the updraft speed equals terminal fall speed As drop falls, it grows by collision-coalescence to size of a large raindrop Warm Cloud Precipitation Terminal Fall Speed (5 m/s) Updraft (5 m/s) Ahrens, Fig. 5.16

  5. Ice Crystal Process Since SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice Ice crystals grow at the expense of water drops, which freeze on contact As the ice crystals grow, they begin to fall Effect maximized around -15oC Ahrens, Fig. 5.19

  6. Accretion-Aggregation Process Small ice particles will adhere to ice crystals Supercooled water droplets will freeze on contact with ice snowflake ice crystal Ahrens, Fig. 5.17 Accretion (Riming) Splintering Aggregation Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process

  7. What is Air Pressure? Pressure = Force/Area What is a Force? It’s like a push/shove In an air filled container, pressure is due to molecules pushing the sides outward by recoiling off them Recoil Force

  8. Recoil Force Air Pressure Concept applies to an “air parcel” surrounded by more air parcels, but molecules create pressure through rebounding off air molecules in other neighboring parcels

  9. Recoil Force Air Pressure At any point, pressure is the same in all directions But pressure can vary from one point to another point

  10. Higher density at the same temperature creates higher pressure by more collisions among molecules of average same speed Higher temperatures at the same density creates higher pressure by collisions amongst faster moving molecules

  11. Ideal Gas Law • Relation between pressure, temperature and density is quantified by the Ideal Gas Law P(mb) = constant  (kg/m3)  T(K) • Where P is pressure in millibars • Where  is density in kilograms/(meter)3 • Where T is temperature in Kelvin

  12. Ideal Gas Law • Ideal Gas Lawdescribes relation between 3 variables: temperature, density and pressure P(mb) = constant  (kg/m3)  T(K) P(mb) = 2.87  (kg/m3)  T(K) • If you change one variable, the other two will change. It is easiest to understand the concept if one variable is held constant while varying the other two

  13. Ideal Gas Law P = constant    T (constant) With T constant, Ideal Gas Law reduces to  P varies with  Denser air has a higher pressure than less dense air at the same temperature Why? You give the physical reason!

  14. Ideal Gas Law P = constant   (constant)  T With  constant, Ideal Gas Law reduces to  P varies with T Warmer air has a higher pressure than colder air at the same density Why? You should be able to answer the underlying physics!

  15. Ideal Gas Law P (constant) = constant    T With P constant, Ideal Gas Law reduces to T varies with 1/ Colder air is more dense ( big, 1/ small) than warmer air at the same pressure Why? Again, you reason the mechanism!

  16. Summary • Ideal Gas Law Relates Temperature-Density-Pressure

  17. Pressure-Temperature-Density Pressure Decreases with height at same rate in air of same temperature Isobaric Surfaces Slopes are horizontal 300 mb 400 mb 500 mb 9.0 km 9.0 km 600 mb 700 mb 800 mb 900 mb 1000 mb Minneapolis Houston

  18. Pressure-Temperature-Density WARM Pressure (vertical scale highly distorted) Decreases more rapidly with height in cold air than in warm air Isobaric surfaces will slope downward toward cold air Slope increases with height to tropopause, near 300 mb in winter 300 mb COLD 400 mb 500 mb 9.5 km 600 mb 700 mb 8.5 km 800 mb 900 mb 1000 mb Minneapolis Houston

  19. Pressure-Temperature-Density WARM Pressure Higher along horizontal red line in warm air than in cold air Pressure difference is a non-zero force Pressure Gradient Force or PGF (red arrow) Air will accelerate from column 2 towards 1 Pressure falls at bottom of column 2, rises at 1 Animation 300 mb COLD 400 mb 500 mb H L PGF 9.5 km 600 mb 700 mb 8.5 km 800 mb 900 mb 1000 mb H L PGF Minneapolis Houston SFC pressure rises SFC pressure falls

  20. Summary • Ideal Gas Law Implies Pressure decreases more rapidly with height in cold air than in warm air. • Consequently….. Horizontal temperature differences lead to horizontal pressure differences! And horizontal pressure differences lead to air motion…or the wind!

  21. Review: Pressure-Height Remember • Pressure falls very rapidly with height near sea-level 3,000 m 701 mb 2,500 m 747 mb 2,000 m 795 mb 1,500 m 846 mb 1,000 m 899 mb 500 m 955 mb 0 m 1013 mb 1 mb per 10 m height Consequently……….Vertical pressure changes from differences in station elevation dominate horizontal changes

  22. Station Pressure Ahrens, Fig. 6.7 Pressure is recorded at stations with different altitudes Station pressure differences reflect altitude differences Wind is forced by horizontal pressure differences Horizontal pressure variations are 1 mb per 100 kmAdjust station pressures to one standard level: Mean Sea Level

  23. Reduction to Sea-Level-Pressure Ahrens, Fig. 6.7 Station pressures are adjusted toSea Level PressureMake altitude correction of 1 mb per 10 m elevation

  24. Correction for Tucson Elevation of Tucson AZ is ~800 m Station pressure at Tucson runs ~930 mb So SLP for Tucson would be SLP = 930 mb + (1 mb / 10 m)800 m SLP = 930 mb + 80 mb = 1010 mb

  25. Correction for Denver Elevation of Denver CO is ~1600 m Station pressure at Denver runs ~850 mb So SLP for Denver would be SLP = 850 mb + (1 mb / 10 m)1600 m SLP = 850 mb + 160 mb = 1010 mb Actual pressure corrections take into account temperature and pressure-height variations, but 1 mb / 10 m is a good approximation

  26. You Try at Home for Phoenix Elevation of Phoenix AZ is ~340 m Assume the station pressure at Phoenix was ~977 mb at 3pm yesterday So SLP for Phoenix would be?

  27. Sea Level Pressure Values 882 mb Hurricane Wilma October 2005 Ahrens, Fig. 6.3

  28. Summary • Because horizontal pressure differences are the force that drives the wind Station pressures are adjusted to one standard level…Mean Sea Level…to remove the dominating impact of different elevations on pressure change

  29. PGF Ahrens, Fig. 6.7

  30. Key Points for Today • Air Pressure Force / Area (Recorded with Barometer) • Ideal Gas Law Relates Temperature, Density and Pressure • Pressure Changes with Height Decreases more rapidly in cold air than warm • Station Pressure Reduced to Sea Level Pressure

  31. Isobaric Maps • Weather maps at upper levels are analyzed on isobaric (constant pressure) surfaces. (Isobaric surfaces are used for mathematical reasons that are too complex to explain in this course!) • Isobaric maps provide the same information as constant height maps, such as: • Low heights on isobaric surfaces correspond to low pressures on constant height surfaces! • Cold temps on isobaric surfaces correspond to cold temperatures on constant height surfaces!

  32. (Constant height) Downhill 496 mb PGF 504 mb Ahrens, Fig. 2, p141 Isobaric Maps Some generalities: 1) High/Low heights on an isobar surface correspond to High/Low pressures on a constant height surface 2) Warm/Cold temps on an isobaric surface correspond to Warm/Cold temps on a constant height surface 3) The PGF on an isobaric surface corresponds to the downhill direction

  33. Contour Maps Display undulations of 3D surface on 2D map A familiar example is aUSGS Topographic Map It’s a useful way to display atmospheric quantities such astemperatures, dew points, pressures, wind speeds, etc. Gedlezman, p15

  34. Rules of Contouring(Gedzelman, p15-16) “Every point on a given contour line has the same value of height above sea level.” “Every contour line separates regions with greater values than on the line itself from regions with smaller values than on the line itself.” “The closer the contour lines, the steeper the slope or larger the gradient.” “The shape of the contours indicates the shape of the map features.”

  35. https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p02.htmlhttps://www.e-education.psu.edu/gened/meteo101/Examples/Section2p02.html Click “interactive exercise” https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p03.html https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p04.html Click “interactive isotherm map” From Contour Maps “To successfully isopleth the 50-degree isotherm, imagine that you're a competitor in a roller-blading contest and that you're wearing number "50". You can win the contest only if you roller-blade through gates marked by a flag numbered slightly less than than 50 and a flag numbered slightly greater than 50.”

  36. 570 dam contour

  37. 576 dam contour

  38. 570 and 576 dam contours

  39. All contours at 6 dam spacing

  40. All contours at 6 dam spacing

  41. -20 C and –15 C Temp contours

  42. -20 C, –15 C, -10 C Temp contours

  43. All contours at 5o C spacing

  44. Height contours Temp shading

  45. PGF Wind

  46. Key Concepts for Today • Station Pressure and Surface Analyses Reduced to Mean Sea Level Pressure (SLP) PGF Corresponds to Pressure Differences • Upper-Air Maps On Isobaric (Constant Pressure) Surfaces PGF Corresponds to Height Sloping Downhill • Contour Analysis Surface Maps-Analyze Isobars of SLP Upper Air Maps-Analyze Height Contours

  47. Key Concepts for Today • Wind Direction and PGF Winds more than 1 to 2 km above the ground are perpendicular to PGF! Analogous a marble rolling not downhill, but at a constant elevation with lower altitudes to the left of the marble’s direction

  48. Assignment Reading - Ahrens pg 148-149 include Focus on Special Topic: Isobaric Maps Problems - 6.9, 6.10 Topic – Newton’s Laws Reading -Ahrens pg 150-157 Problems -6.12, 6.13, 6.17, 6.19, 6.22

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