Measurement of Atmospheric Pressure
Outline Measurement of Atmospheric Pressure Review of Atmospheric Pressure Barometers Liquid Aneroid Wind Speed Correction Significant Exposure Error Sea-Level Correction Practical Application
Definitions and Concepts: Review of Atmospheric Pressure Static Pressure: Equal to the weight of the atmosphere in the vertical column above the sensor (located at any given altitude) per unit area p = z= z= 0 ρgdz where: p = atmospheric pressure (Pa) ρ = density of air (kg m -3 ) g = acceleration due to gravity (m s -2 ) z = altitude (m) SI unit: Meteorology: Instrument: Pascal (Pa) = force exerted by 1 Newton over a 1 m 2 area Mean sea level pressure = 101325 Pa = 1013.25 hpa = 1013.25 mb Barometer
Definitions and Concepts: Review of Atmospheric Pressure Station Pressure: Sea-Level Pressure: Total pressure observed by a barometer (also called the barometric pressure or absolute pressure) Observed station pressure corrected to the altitude of mean sea level by a universally agreed upon method (more on this later) Differential Pressure: Dynamic Pressure: Pressure measured relative to ambient station pressure (the pressure inside an automobile tire) A variety of differential pressure exerted by air flow (or winds) (stick your hand out the wind of a moving automobile) p = 1 ρ V 2 2 where: Δp = differential pressure (Pa) ρ = density of air (kg m -3 ) V = wind or air speed (m s -1 )
Review of Atmospheric Pressure Definitions and Concepts: Static atmospheric pressure decreases rapidly with altitude (~100 mb / 1 km) but horizontal variations are much smaller (~1 mb / 100 km) except near severe weather (hurricanes / tornadoes) Surface barometers should exhibit a dynamic range 850 1050 mb
Liquid Barometers Basic Concept: Directly measures atmospheric pressure by determining the height of a liquid column The liquid is often mercury The difference in height (h 1 ) is related to the difference in pressure (P 2 P 1 ) [Figure A] P 2 P1 = ρgh1 where: g = gravity ρ = density of the liquid Barometers If one side is sealed and contains a vacuum then the difference in height (h 2 ) is a direct measure of the atmospheric pressure (P a ) [Figure B] P a = ρgh 2
Liquid Barometers Why Mercury? The basic equation for a sealed liquid barometer suggests that the height of the liquid (h 2 ) is inversely proportional to the liquid s density (or mass). Advantages: Large density (13595 kg m -3 at 0ºC) (column can be of reasonable height) Low vapor pressure (0.021 Pa at 0ºC) (no evaporation into the top vacuum) Easily purified and chemically stable Remains a liquid over a wide range of temperatures (-39ºC to +356ºC) Disadvantages: P a = ρgh 2 Must be handled with extreme care Very toxic if ingested or inhaled Difficult to transport Barometers
Barometers Liquid Barometers In Practice The most widely used operational liquid barometer is the Kew Pattern Barometer Has a mercury column with a vernier dial for high resolution (±0.01 mb) measurements Kew barometers are calibrated for use at T = 0ºC and g = 9.80665 m s -2 However, such conditions are rarely met in practice corrections are needed 1. Temperature effect on density (C T ) 2. Local value of gravity (C G ) 3. Imperfections in the tube / scale (C X ) P = P + C + C + C s u T G X Moreover, the Kew barometer must be kept 4. Vertical 5. Inside a building (no wind) and away from fans or air vents (limits any dynamic pressure error)
Barometers Liquid Barometers In Practice The detailed corrections for the Kew Pattern Barometer are as follows: P s ( α β ) g T = Pu Pu + CX 9.80665 1+ αt ( 2φ ) ( 2φ ) 1 0.0026373 cos g = 9.80616 0. 0003086 h 2 + 0.0000059 cos where: P u = uncorrected pressure (hpa) T = local temperature (ºC) α = coefficient of expansion for mercury β = coefficient of expansion for the scale typically made of brass C X = imperfections correction (hpa) provided by the manufacturer ϕ = latitude (degrees) h = station elevation (m) Note: The top equation differs slightly from one provided in your text (on page 127) since all terms related to imperfections have been condensed into one term (C X ) here.
Aneroid Barometers Basic Concept: Station pressure is directly measured by an evacuated capsule or diaphragm (an aneroid) made of an elastic material that can distorted in response to atmospheric pressure changes Simple barometers contain one aneroid capsule High-quality precision barometers contain multiple aneroid capsules linked in series Aneroid distortion is monitored either 1. Mechanically via a barograph 2. Electronically capacitance change Advantages: Very small and inexpensive Easily automated No temperature correction required No gravity correction required Very portable No toxic materials Barometers
Barometers Exposure Error Dynamic Pressure Correction: Only significant concern with measurements of atmospheric pressure p = 1 ρ V 2 2 Every effort should be made to minimize the barometer s exposure to wind or moving air (even air flow inside a building can produce a non-negligible dynamic pressure) Most pressure sensors are housed inside a structure (building, instrument box) with only small holes (static ports) to allow static pressure to equalize while minimizing air flow Static ports
Barometers Precipitation (heated) (tipping bucket) (with) (wind screen) Temperature and Humidity (fan-aspirated) (at 1.5 m) Pressure (inside box) (at 1.5 m) Wind Speed / Direction (cup anemometer) (and wind vane) (at 10 m) Note the open and level terrain away from obstructions NOAA ASOS Surface Station (operational) Conforms to WMO Standards Most are located at airports
Barometers In Practice Sea Level Pressure Correction: Since atmospheric pressure is a strong function of altitude, any surface pressure measured at a station located above sea level will largely depict that station s elevation. In order to discern the spatial structure of synoptic weather systems, surface pressures observed above sea level, must be corrected down to sea level: P 0 = P exp z g z R T d where: P 0 = sea-level pressure (hpa) P z = station pressure (hpa) z = station elevation (m) T = mean temperature between the station and sea level (K) g = gravity = gas constant for dry air R d The correction effectively adds the expected additional weight of the air column between sea level and the station elevation
Summary Measurement of Atmospheric Pressure Review of Atmospheric Pressure (different types) Barometers Liquid (basic concept, advantages, disadvantages) Aneroid (basic concept, advantages, disadvantages) Hypsometer (basic concept, advantages, disadvantages) Wind Speed Correction Significant Exposure Error Sea-Level Correction Practical Application
References Akyuz, F. A., H. Liu, and T. Horst, 1991: Wind tunnel evaluation of PAM II pressure ports. Journal of Atmospheric and Oceanic Technology, 8, 323-330. Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp. Brock, F. V., K. C. Crawford, R. L. Elliot, G. W. Cuperus, S. J. Stadler, H. L. Johnston, M.D. Eilts, 1993: The Oklahoma Mesonet - A technical overview. Journal of Atmospheric and Oceanic Technology, 12, 5-19. Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp. Jones, B. W., 1992: The elimination of temperature effects in micro-barometers. Journal of Atmospheric and Oceanic Technology, 9, 796-800. Liu, H., and G.L. Darkow, 1989: Wind effect on measured atmospheric pressure. Journal of Atmospheric and Oceanic Technology, 6, 5-12. Richner, H., J. Joss, and P. Ruppert, 1996: A water hypsometer utilizing high-precision thermocouples. Journal of Atmospheric and Oceanic Technology, 13, 175-182. Snow, J. T., M. E. Akridge, and S. B. Harley, 1992: Basic meteorological observations for schools: Atmospheric pressure. Bulletin of the American Meteorological Society, 73, 781-794.