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ATMS 320 – Meteorological Instrumentation

ATMS 320 – Meteorological Instrumentation. Barometry objectives: Learn some of the methods used to measure the static pressure exerted by the atmosphere Understand the advantages/disadvantages to each of the methods

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ATMS 320 – Meteorological Instrumentation

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  1. ATMS 320 – Meteorological Instrumentation • Barometry objectives: • Learn some of the methods used to measure the static pressure exerted by the atmosphere • Understand the advantages/disadvantages to each of the methods • Experiment with exposure and calibration issues related to measuring atmospheric static pressure

  2. ATMS 320 – Barometry • A review on pressure Static pressure- Force/Area against any surface in the absence of air motion. Dynamic pressure- Force/Area due to air motion. http://www.godchecker.com/pantheon/greek-mythology.php?deity=ATLAS

  3. ATMS 320 – Barometry • The Earth’s atmosphere exerts a static pressure on the surface of the Earth equal to the weight of a vertical column of air of unit cross-section. . . W&H f2-1 here Since air is a fluid, this pressure, or force, is exerted equally in all directions.

  4. ATMS 320 – Barometry • If the wind is blowing, it exerts a dynamic pressure which introduces a static pressure error… http://news.bbc.co.uk/1/hi/in_depth/photo_gallery/3085722.stm

  5. ATMS 320 – Barometry • Physical principles employed for measuring atmospheric pressure: • Direct techniques • Indirect techniques http://www.rube-goldberg.com/html/pencil_sharpener.htm

  6. ATMS 320 – Barometry • Direct – mercury barometers: • Balance the force due to atmospheric pressure against the weight of a column of mercury

  7. ATMS 320 – Barometry • Direct – mercury barometers (cont.) • Difficult to automate • Not suitable for field experiments • Health risk

  8. ATMS 320 – Barometry • Direct – mercury barometers (cont.), why mercury? • High density (small column height) • Low vapor pressure (little evaporation into vacuum) • Chemically stable • Liquid for a wide range of temperatures http://www.gormangiftgallery.com/tormerbar.html

  9. ATMS 320 – Barometry Mercury barometer calibration equation

  10. ATMS 320 – Barometry • A nice web page to help with the understanding of how mercury barometers work… http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Barometer/Barometer.html

  11. ATMS 320 – Barometry • Main sources of error in a mercury barometer: • Dynamic wind pressure (alleviate via a static port) • Density of mercury (and of glass tube) are functions of temperature • Local gravity must be known accurately (a function of latitude) • Presence of air or water vapor in “vacuum” • Surface tension effects • Barometer must be kept vertical • Impurities in the mercury

  12. ATMS 320 – Barometry • Advantages of mercury barometers: • Simple in concept (can visualize how it works) • Easy calibration

  13. ATMS 320 – Barometry Getting the atmospheric pressure reading “right”… Brhf2-4 here… Thermal correction equation Altitude and latitude correction equation

  14. ATMS 320 – Barometry • Direct – aneroid (without fluid) barometers: • Balance the force due to atmospheric pressure against the restoring force of an “elastic” material (e.g. metal)

  15. ATMS 320 – Barometry y = deflection of diaphram center, t = diaphram thickness • Aneroid barometers: • By changing the shape of the material surface used in the aneroid barometer, we can improve the sensitivity of the barometer at high atmospheric pressures A yr B Brf2-6 here Curve A or B?? Curve A or B??

  16. ATMS 320 – Barometry • Other aneroid barometers: • Stacked aneroid cells (2-7) • Aneroid capsule (2-8) • Silicon diaphragm (2-9) • Bourdon tube (2-10)

  17. ATMS 320 – Barometry • Main sources of error in aneroid barometers: • Same exposure errors as mercury barometers (e.g. dynamic pressure) • Temperature-induced error • Error arising from defects or irregularities in the diaphragm material and/or shape • Sensitivity to pressure is non-linear • Diaphragm “creep” (causes drift, a long-term change in the sensor sensitivity)

  18. ATMS 320 – Barometry • Advantages of aneroid barometers: • Very small size • Readily automated • Insensitive to orientation, motion, and shock (portable) • No gravity correction required • Users not exposed to toxic materials

  19. ATMS 320 – Barometry • Indirect* – hypsometers: • Pressure sensor that utilizes the property of the decreasing boiling point of a liquid with decreasing pressure in order to determine pressure *A pressure measurement technique is call indirect if it does not respond directly to the force due to atmospheric pressure but, instead, responds to some other variable that is a function of pressure.

  20. ATMS 320 – Barometry • Hypsometers: • Must somehow provide heat to get liquid to boil • If the liquid has a boiling point below the air temperature, a heater is not required (Freon-13; 191.75 K) http://www.chefscatalog.com/store/catalog/silo.jhtml?itemId=cat000106&parentId=cat000000

  21. ATMS 320 – Barometry Hypsometer equations: Clausius-Clapeyron equation… Calibration equation… Transfer equation…

  22. ATMS 320 – Barometry The static sensitivity of hypsometers changes over the range of typical atmospheric pressures. If large static sensitivity is good, at what range of pressures do hypsometers perform “good”? at HIGH pressure or at LOW pressure ? Cast your votes now…

  23. ATMS 320 – Barometry A pressure observing network 1-2 punch!! (1) hypsometer on a radiosonde (2) aneroid barometer at low altitude

  24. ATMS 320 – Barometry • Main sources of error in hypsometers: • Sensitive to orientation of instrument • Extreme non-linearity at sea-level pressure

  25. ATMS 320 – Barometry • Advantages of hypsometers: • Small size • Can be automated • Reasonably portable • No gravity or temperature correction required • Simple physical concept (does require careful implementation) • No drift

  26. ATMS 320 – Barometry • All barometers are subject to dynamic wind effects (e.g., air flow, building air conditioning or ventilation). A static port is designed to reduce dynamic error for barometers located inside shelters. +Z tilt angle wind vector Static port: Must be located outside of the significant pressure field caused by the shelter. Field impacts 2.5 – 10 times shelter height. Should be kept at a tilt angle of less than 10 degrees.

  27. ATMS 320 – Barometry • Barometer project • Which type of barometer in Chapter 2 is most like your spaghetti sauce jar barometer? BAROMETER http://www.atomicmuseum.com/tour/manhattanproject.cfm

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