atmospheric instrumentationm. d. eastin measurement of temperature

Atmospheric Instrumentation M. D. Eastin

Measurement of Temperature


Outline


• Review of Atmospheric Temperature

• Barometers• Liquid• Thermocouple• Thermistors

• Exposure Errors• Radiation• Precipitation• Minimization

• Surface / Soil Temperature


Definitions and Concepts:

Temperature: Mean kinetic energy of all molecules within a “system”(valid systems → air parcel, liquid water, soil)

Atmospheric temperature → Ideal Gas Law

where: p = atmospheric pressure (Pa)V = system volume (m3)N = number of moleculesk = Boltzmann’s constant (1.38 ×10-23 J K-1 molecule-1)ρ = density of air (kg m-3)

Rd = gas constant for dry air (J kg-1 K-1)T = temperature (K)

SI unit: Kelvin (K)

Meteorology: Fahrenheit (ºF) = ºC (9/5) + 32Celsius (ºC) = K – 273.15

Instrument: Thermometer

Review of Atmospheric Temperature

TRp dNkTpV



• Atmospheric temperature decreasesrapidly with altitude (~6–10 K / km)and can significantly vary by season(~30–40K from summer to winter)

• Upper-air thermometers should exhibita dynamic range → –80ºC to +50ºC

→ 190K to 325K




• Horizontal variations in surface temperatureare much typically smaller (~1 K / 100

km)except near fronts and thunderstorms,

but can significantly vary by season (~30–40K)

• Surface thermometers should exhibit adynamic range → –50ºC to +50ºC

→ 220K to 325K



Liquid Thermometers – Basic Concept:

•Directly measures temperature through thermal expansion of a liquid in a thin glass tube•Liquid is often alcohol (dyed red / black) or mercury (silver), but can be water (dyed blue)•Scale is marked on the glass tube

•Any change in volume (ΔV = V – V0) is directlyproportional to the change in temperature(ΔT = T – T0) via a cubic thermal expansivitycoefficient (α) for the liquid

•Since most liquid thermometers are cylindricalwith a glass bore of constant circular geometry,the sensitivity of the thermometer is defined by

where: r = radius of the borel = length of the bore

Thermometers

00 TTVV

20

r

V

T

l


Liquid Thermometers – Why Alcohol / Mercury / Water ?

•The liquid inside must exhibit the following characteristics

• Remain a liquid over the full dynamic range• Exhibit a well defined meniscus• Have sufficient expansivity to measure small changes in temperature (~0.1°C)

Liquid Thermometers – Typical Errors

1. Scale errors – due to a non-uniform bore

2. Thread errors – due to breaks (air bubbles) in liquid

3. Immersion errors – due to a temperature gradientpresent along the sensing bulb

4. Parallax errors – due to refraction within the glasswhen the observer’s eyes arenot level with the liquid meniscus

5. Exposure errors – will be discussed in detail later

Thermometers

Convexmeniscus

MercuryAlcohol

Water

Concavemeniscus


Liquid Thermometers – Typical Specifications

Accuracy ±0.2°CResolution 0.1°CResponse Time 30 s

Advantages

• Easy to use if stationary• Can be inexpensive• Calibration is simplest• No instrument drift

Disadvantages

•Not very portable•Sensitive to orientation•Difficult to automate•Lack of durability (easily broken)•Liquids can be a health hazard

Thermometers


Thermocouples – Basic Concept:

•Created when two dissimilar metals are welded / twisted / soldered together at two junctions•Such a physical connection will generate a small electrical voltage proportional to the temperature difference between the two junctions. •If one junction is kept at a known reference temperature, then a thermocouple can directly measure the temperature from the changes in voltage

Typical Choices for Metals:

1. Copper – Nickel (“Type T”)2. Chromium – Aluminum (“Type K”)3. Iron – Nickel (“Type J”)4. Nickel – Aluminum (“Type E”)

•Rarely used in the atmospheric sciences dueto the reference temperature requirement

•Much more common in residential / industrial

• Furnaces• Water heaters• Fireplaces• Manufacturing

Thermometers


Thermocouples – Typical Specifications

Accuracy ±0.1°CResolution 0.025°CResponse Time < 5 s

Advantages

• Can be very sensitive • Inexpensive and durable• Easy to automate

Disadvantages

•Reference temperature requirement•Non-linear response•Will experience drift if sensor junction

becomes coated with contaminants(soot, dust) or corrodes

Thermometers


Thermistors – Basic Concept:

•A semiconducting device designed such that its electrical resistance is highly sensitive to changes in temperature•Most commonly used temperature sensor•Metal is often platinum → chemically stable (or non-corrosive)

→ exhibits a minimal non-linear response

•Thin wire / coil of pure platinum (high-quality sensor)•Platinum film on a ceramic substrate (low-quality sensor)

•The non-linear relationship is defined by:

where:R0 = standard resistance (Ω)T0 = standard temperature (K) α = metal-specific coefficient β = metal-specific coefficient

Thermometers

20

0

0)(

)(1

TT

TTRR


Thermistors – Typical Specifications:

Accuracy ±0.1°CResolution 0.02°CTime Constant < 2 s

Advantages

•Very sensitive•Rapid response time•Easy to automate•Non-corrosive•Stable calibration (no drift)•Requires minimal electrical power for operation (ideal for sounding

systems or remote stations)

Disadvantages

•Non-linear response•Requires resistance to voltage

conversion for continuous data logging

Thermometers


Radiation Errors – Basic Concept:

• Solar radiation falling on a thermometer will cause the measurements to be greater thanthe true air temperature

• Such errors can be significant for even fine wire sensors if insufficient aspiration is availableto effectively and rapidly remove the radiant heat through convection

Radiation Error Magnitude in Direct Sunlight

• Sensor size (diameter)• Fraction of non-reflected radiation• Local wind speed

Exposure Errors


Wetting Errors – Basic Concept:

• A temperature sensor wetted via precipitation or dewfall will experience evaporational cooling if the local ambient air is unsaturated, causing the measurements to be cooler than the true air temperature

•Sensor behaves more like a wet-bulb thermometer until evaporation is complete

•Errors can exceed 10-20°C

Wetting Error Magnitude

• Local relative humidity (lower RH = larger errors)• Fraction of sensor wetted (less wetting = smaller errors)• Local wind speed (stronger winds = larger errors)

Exposure Errors


Exposure ErrorsMinimization – Fan-Aspirated Screens

•Shields from all direct sunlight•Prevents reflected solar radiation•Maintains regular free passage of air (ventilation flow > 3 m/s)•Reduces conduction heat sources (from buildings and sensor mounts)•Protects sensors from precipitation wetting

Rain shield

Sensor

ConcentricAir Intakes

Fan

Surface Temperatures:

•Actual surface temperatures can vary significantly from the overlying air temperature•Strong function of surface material type and its solar absorption and emission properties

Surface Type ΔT DifferenceWater 0-5ºC coolerSoil 0-10ºC warmerGrass 0-5ºC coolerTrees 0-5ºC coolerConcrete 5-10ºC warmerAsphalt 10-50ºC warmerRoof 10-50ºC warmer

•Surface materials can have stronginfluence on surface heat fluxesand the formation of Urban HeatIslands (UHIs) which often resultin urban air temperatures beingup to 10ºC warmer than rural airtemperatures (primarily at night)


Surface / Soil Temperature


Surface / Soil TemperatureDaytime Surface Temperatures in the Central Business District (CBD) of Sacramento (CA)

Soil Temperatures:

•Temperatures vary as a function of (1) depth, (2) time of day, and (3) soil type•Measurements at various depths are obtain from a vertical array of sensors

•Diurnal variations are suppressed with soil depth and the maximum

temperature occurs later



Soil Heat Flux:

•The direction and magnitude of heat transfer (called heat flux) can be determined usinga heat flux plate composed of two temperature sensors separated by an insulated resin

•Remember the direction of heat transfer is always from warmer toward cooler

•Larger temperature differences(ΔT = T2 – T1) imply strongerheat fluxes

•If T1 > T2 then there is andownward heat flux (G↓)

•If T1 < T2 then there is anupward heat flux (G↑)

•Heat fluxes with bare soil are oftendirected away from the surfaceupward into a cooler atmospheredownward into cooler soil layers




Summary


• Review of Atmospheric Temperature

• Barometers• Liquid• Thermocouple• Thermistors

• Exposure Errors• Radiation• Precipitation• Minimization

• Surface / Soil Temperature


References

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.

Cheney, N. R., and J. A. Businger, 1990: An accurate fast-response temperature system using thermocouples. Journal of Atmospheric and Oceanic Technology, 7, 504-516.

Fuchs, M., and C. B. Tanner, 1965: Radiation shields for air temperature thermometers. Journal of Applied Meteorology,4, 544-547.

Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp.

Kent, E. C., R. J. Tiddy, and P. K. Taylor, 1993: Correction of marine air temperature observations for solar radiation effects. Journal of Atmospheric and Oceanic Technology, 10, 900-906.

Ney, E. P., R. W. Mass, and W. F. Huch, 1961: The measurement of atmospheric temperature. Journal of Meteorology,18, 60-80.

Richardson, S.J, F. V. Brock, S. R. Semmer, and C. Jirak, 1999: Minimizing errors associated with multiple radiation shields. Journal of Atmospheric and Oceanic Technology, 16, 1862-1872.

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