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NATS 101-05 Lecture 10 Air Pressure 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 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! Warm Cloud Precipitation Terminal Fall Speed (5 m/s) Updraft (5 m/s) Ahrens, Fig. 5.16 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 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 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 What is Air Pressure? Recoil Force 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 Air Pressure Recoil Force Concept applies to an “air parcel” surrounded by more air parcels, but molecules create pressure through rebounding off air molecules in other neighboring parcels Air Pressure Recoil Force At any point, pressure is the same in all directions But pressure can vary from one point to another point 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 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 Ideal Gas Law • Ideal Gas Law describes 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 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! 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! 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! Summary • Ideal Gas Law Relates Temperature-Density-Pressure Pressure-Temperature-Density 300 mb 400 mb 9.0 km 9.0 km 500 mb 600 mb 700 mb 800 mb 900 mb Minneapolis Houston Pressure Decreases with height at same rate in air of same temperature Isobaric Surfaces Slopes are horizontal Pressure-Temperature-Density WARM 8.5 km 9.5 km COLD Minneapolis Houston 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 Pressure-Temperature-Density WARM L H 8.5 km PGF H PGF Minneapolis SFC pressure rises 9.5 km COLD L Houston SFC pressure falls 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 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! 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 Consequently………. 1,000 m 899 mb 955 mb Vertical pressure changes from 500 m 1013 mb differences in station elevation 0 m 1 mb per 10 m height dominate horizontal changes 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 km Adjust station pressures to one standard level: Mean Sea Level Reduction to Sea-Level-Pressure Ahrens, Fig. 6.7 Station pressures are adjusted to Sea Level Pressure Make altitude correction of 1 mb per 10 m elevation 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 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 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? Sea Level Pressure Values 882 mb Hurricane Wilma October 2005 Ahrens, Fig. 6.3 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 PGF Ahrens, Fig. 6.7 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 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! Isobaric Maps (Constant height) 496 mb 504 mb Some generalities: 1) The 2) 3) Warm/Cold High/Low PGF on heights temps an isobaric on onan ansurface isobar isobaric surface correspond corresponds to the downhill to Warm/Cold High/Low direction temps pressures on a constant on aheight constant surface height surface Ahrens, Fig. 2, p141 Contour Maps Display undulations of 3D surface on 2D map A familiar example is a USGS Topographic Map It’s a useful way to display atmospheric quantities such as temperatures, dew points, pressures, wind speeds, etc. Gedlezman, p15 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.” Contour Maps “To successfully isopleth the 50degree isotherm, imagine that you're a competitor in a rollerblading 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.” https://www.e-education.psu.edu/gened/meteo101/Examples/Section2p02.html Click “interactive exercise” From 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” 570 dam contour 576 dam contour 570 and 576 dam contours All contours at 6 dam spacing All contours at 6 dam spacing -20 C and –15 C Temp contours -20 C, –15 C, -10 C Temp contours All contours at 5o C spacing Height contours Temp shading PGF Wind 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 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 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