| NATS 101 Lecture 14 Air Pressure |
| What is Air Pressure? |
| Pressure = Force/Area | |
| What is a Force? ItŐs 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 Ňair 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 is complex | |
| 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 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 24 |
| 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 |
| Assignment |
| Reading - Ahrens pg 148-149 | |
| include Focus on Special Topic: Isobaric Maps | |
| Problems - 6.9, 6.10 |