Atmospheric InstrumentationM. D. Eastin Measurement of Atmospheric Pressure.

Atmospheric Instrumentation M. D. Eastin

Measurement of Atmospheric Pressure


Outline


• Review of Atmospheric Pressure

• Barometers• Liquid• Aneroid• Hypsometer

• Wind Speed Correction – Significant Exposure Error

• Sea-Level Correction – Practical Application


Definitions and Concepts:

Static Pressure: Equal to the weight of the atmosphere in the vertical column above thesensor (located at any given altitude) per unit area

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: Pascal (Pa) = force exerted by 1 Newton over a 1 m2 area

Meteorology: Mean sea level pressure = 101325 Pa= 1013.25 hPa= 1013.25 mb

Instrument: Barometer

Review of Atmospheric Pressure

z

z

gdzp0



Station Pressure: Total pressure observed by a barometer (also called the barometric pressure or absolute pressure)

Sea-Level Pressure: Observed station pressure corrected to the altitude of mean sea levelby a universally agreed upon method (more on this later)

Differential Pressure:Pressure measured relative to ambient station pressure(the pressure inside an automobile tire)

Dynamic Pressure: A variety of differential pressure exerted by air flow (or winds)(stick your hand out the wind of a moving automobile)

where: Δp = differential pressure (Pa)ρ = density of air (kg m-3)V = wind or air speed (m s-1)


2

2

1Vp



•Static atmospheric pressure decreasesrapidly with altitude (~100 mb / 1 km)but horizontal variations are muchsmaller (~1 mb / 100 km) except nearsevere weather (hurricanes /

tornadoes)

•Surface barometers should exhibit adynamic range → 850 – 1050 mb



Liquid Barometers – Basic Concept:

•Directly measures atmospheric pressure bydetermining the height of a liquid column

•The liquid is often mercury

•The difference in height (h1) is related to thedifference in pressure (P2 – P1) [Figure A]

where: g = gravityρ = density of the liquid

•If one side is sealed and contains a vacuumthen the difference in height (h2) is a direct measure of the atmospheric pressure (Pa)[Figure B]

Barometers

112 ghPP

2ghPa


Liquid Barometers – Why Mercury?

•The basic equation for a sealed liquid barometer suggests that the height of the liquid (h2) 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 rangeof temperatures (-39ºC to +356ºC)

Disadvantages:

• Must be handled with extreme care• Very toxic if ingested or inhaled• Difficult to transport

Barometers

2ghPa


Liquid Barometers – In Practice

•The most widely used “operational” liquid thermometer 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 (CT)2. Local value of gravity (CG)3. Imperfections in the tube / scale (CX)

•Moreover, the Kew barometer must be kept

4. Vertical5. Inside a building (no wind) and away

from fans or air vents (limits any dynamic pressure error)

Barometers

XGTus CCCPP


Liquid Barometers – In Practice

•The detailed corrections for the Kew Pattern Barometer are as follows:

where: Pu = uncorrected pressure (hPa)T = local temperature (K)α = coefficient of expansion for mercuryβ = coefficient of expansion for the scale – typically made of brass

CX = 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 termsrelated to imperfections have been condensed into one term (CX) here.

Barometers

Xuus CT

TPP

gP

180665.9

hg

0003086.0

2cos0000059.0

2cos0026373.0180616.9

2


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 barograph2. Electronically – capacitance change

Advantages:

• Very small and inexpensive• Easily automated• No temperature correction required• No gravity correction required• Very portable• No toxic materials

Barometers


Hypsometers – Basic Concept:

•Boiling point of any liquid → Liquid Type→ Atmospheric Pressure

•Indirectly estimates pressure by maintaining a heated liquid-filled flask at a continuous boil.

•A number of fluids can be used:

1. Water2. Freon3. Carbon disulfide (C-S2)4. Carbon tetracloride (C-Cl4)

Advantages

• Can be small• Easily automated• No gravity or temperature corrections• No instrument drift

Disadvantages

•Not very portable and sensitive to orientation•Requires very-accurate temperature sensor•Most fluids are health hazards (except water)

Barometers


Exposure Error – Dynamic Pressure Correction:

• Only significant concern with measurements ofatmospheric pressure

• 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 onlysmall holes (static ports) to allow static pressure to equalize while minimizing air flow

Barometers

2

2

1Vp

Static ports


NOAAASOS

SurfaceStation

(operational)

Conformsto WMO

Standards

Most arelocated

at airports

Precipitation(heated)

(tipping bucket)(with)

(wind screen)

Wind Speed / Direction(cup anemometer)

(and wind vane)(at 10 m)

Temperatureand

Humidity(fan-aspirated)

(at 1.5 m)

Pressure(inside box)

(at 1.5 m)

Note the open and level terrain

away from obstructions

Barometers


In Practice – Sea Level Pressure Correction:

• Since atmospheric pressure is a strong function of altitude, any surface pressure measuredat 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:

where: P0 = sea-level pressure (hPa) Pz = station pressure (hPa) z = station elevation (m) T� = mean temperature between the station and sea level (K) g = gravity Rd = gas constant for dry air

•The correction effectively adds the expected additional weight of the air column between sea level and the station elevation

Barometers

TR

zgPP

dz exp0


Summary


• 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.

Atmospheric InstrumentationM. D. Eastin Measurement of Atmospheric Pressure.

Documents

Transcript of Atmospheric InstrumentationM. D. Eastin Measurement of Atmospheric Pressure.