| 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 |
| 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 |
| Accretion-Aggregation Process |
| What is Air Pressure? |
| Pressure = Force/Area | |
| What is a Force? ItÕs like a push/shove | |
| In an air filled container, pressure is due to molecules pushing the sides outward by recoiling off them |
| Air Pressure |
| Concept applies to an Òair parcelÓ surrounded by more air parcels, but molecules create pressure through rebounding off air molecules in other neighboring parcels |
| Air Pressure |
| At any point, pressure is the same in all directions | |
| But pressure can vary from one point to another point |
| "Higher density" |
| Higher density at the same temperature creates higher pressure by more collisions among molecules of average same speed |
| Ideal Gas Law |
| Relation between pressure, temperature and density is quantified by the Ideal Gas Law | |
| P(mb) = constant « r(kg/m3) « T(K) | |
| Where P is pressure in millibars | |
| Where r 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 « r(kg/m3) « T(K) | |
| P(mb) = 2.87 « r(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 « r « T (constant) | |
| With T constant, Ideal Gas Law reduces to | |
| F P varies with r E | |
| 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 « r (constant) « T | |
| With r constant, Ideal Gas Law reduces to | |
| F P varies with T E | |
| 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 « r « T | |
| With P constant, Ideal Gas Law reduces to | |
| F T varies with 1/r E | |
| Colder air is more dense (r big, 1/r 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 |
| Pressure | |
| Decreases with height at same rate in air of same temperature | |
| Isobaric Surfaces | |
| Slopes are horizontal |
| Pressure-Temperature-Density |
| 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 |
| Summary |
| Ideal Gas Law Implies | |
| Pressure decreases more rapidly with height in cold air than in warm air. | |
| ConsequentlyÉ.. | |
| Horizontal temperature differences lead to horizontal pressure differences! | |
| And horizontal pressure differences lead to air motionÉor 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 | |
| 1,000 m 899 mb | |
| 500 m 955 mb | |
| 0 m 1013 mb | |
| 1 mb per 10 m height |
| Station Pressure |
| Reduction to Sea-Level-Pressure |
| 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 |
| Summary |
| Because horizontal pressure differences are the force that drives the wind | |
| Station pressures are adjusted to one standard levelÉMean Sea LevelÉto remove the dominating impact of different elevations on pressure change |
| Slide 29 |
| 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 |
| Contour Maps |
| Display undulations of 3D surface on 2D map | |
| A familiar example is a USGS Topographic Map | |
| ItÕs a useful way to display atmospheric quantities such as temperatures, dew points, pressures, wind speeds, etc. |
| Rules of Contouring (Gedzelman, p15-16) |
| ÒEvery point on a given contour line has the same value of height above sea level.Ó | |
| ÒEvery contour line separates regions with greater values than on the line itself from regions with smaller values than on the line itself.Ó | |
| ÒThe closer the contour lines, the steeper the slope or larger the gradient.Ó | |
| ÒThe shape of the contours indicates the shape of the map features.Ó |
| Contour Maps |
| ÒTo 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.Ó |
| Slide 36 |
| Slide 37 |
| Slide 38 |
| Slide 39 |
| Slide 40 |
| Slide 41 |
| Slide 42 |
| Slide 43 |
| Slide 44 |
| Slide 45 |
| 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 marbleÕs direction | |
| Assignment |
| Reading - Ahrens pg 148-149 | |
| include Focus on Special Topic: Isobaric Maps | |
| Problems - 6.9, 6.10 | |
| Topic – NewtonÕs Laws | |
| Reading - Ahrens pg 150-157 | |
| Problems - 6.12, 6.13, 6.17, 6.19, 6.22 | |