Measurement of Temperature Atmospheric Instrumentation M. D. Eastin

Outline Measurement of Temperature Review of Atmospheric Temperature Thermometers Liquid Thermistors Exposure Errors Radiation Precipitation Minimization Surface / Soil Temperature Atmospheric Instrumentation M. D. Eastin

Review of Atmospheric 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 molecules k = 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) + 32 Celsius (ºC) = K – 273.15 Instrument: Thermometer Atmospheric Instrumentation M. D. Eastin

Review of Atmospheric Temperature Definitions and Concepts: Atmospheric temperature decreases rapidly with altitude (~6–10 K / km) and can significantly vary by season (~30–40K from summer to winter) Upper-air thermometers should exhibit a dynamic range → –80ºC to +50ºC → 190K to 325K Atmospheric Instrumentation M. D. Eastin

Review of Atmospheric Temperature Definitions and Concepts: Horizontal variations in surface temperature are much typically smaller (~1 K / 100 km) except near fronts and thunderstorms, but can significantly vary by season (~30–40K) Surface thermometers should exhibit a dynamic range → –50ºC to +50ºC → 220K to 325K Atmospheric Instrumentation M. D. Eastin

Thermometers 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 directly proportional to the change in temperature (ΔT = T – T0) via a cubic thermal expansivity coefficient (α) for the liquid Since most liquid thermometers are cylindrical with a glass bore of constant circular geometry, the static sensitivity (S) of the thermometer is where: r = radius of the bore l = length of the bore Atmospheric Instrumentation M. D. Eastin

Thermometers 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 gradient present along the sensing bulb 4. Parallax errors – due to refraction within the glass when the observer’s eyes are not level with the liquid meniscus 5. Exposure errors – will be discussed in detail later Convex meniscus Mercury Alcohol Water Concave Atmospheric Instrumentation M. D. Eastin

Thermometers Liquid Thermometers – Typical Specifications Accuracy ±0.2°C Resolution 0.1°C Response 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 Atmospheric Instrumentation M. D. Eastin

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 Atmospheric Instrumentation M. D. Eastin

Thermometers Thermistors – Typical Specifications: Accuracy ±0.1°C Resolution 0.02°C Time 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 Atmospheric Instrumentation M. D. Eastin

Exposure Errors Radiation Errors – Basic Concept: Solar radiation falling on a thermometer will cause the measurements to be greater than the true air temperature Such errors can be significant for even fine wire sensors if insufficient aspiration is available to 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 Atmospheric Instrumentation M. D. Eastin

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) Atmospheric Instrumentation M. D. Eastin

Exposure Errors Minimization – 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 Concentric Air Intakes Fan Atmospheric Instrumentation M. D. Eastin

Surface / Soil Temperature 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 Difference Water 0-5ºC cooler Soil 0-10ºC warmer Grass 0-5ºC cooler Trees 0-5ºC cooler Concrete 5-10ºC warmer Asphalt 10-50ºC warmer Roof 10-50ºC warmer Surface materials can have strong influence on surface heat fluxes and the formation of Urban Heat Islands (UHIs) which often result in urban air temperatures being up to 10ºC warmer than rural air temperatures (primarily at night) Atmospheric Instrumentation M. D. Eastin

Surface / Soil Temperature Daytime Surface Temperatures in the Central Business District (CBD) of Sacramento (CA) Atmospheric Instrumentation M. D. Eastin

Surface / Soil Temperature 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 Atmospheric Instrumentation M. D. Eastin

Surface / Soil Temperature Soil Heat Flux: The direction and magnitude of heat transfer (called heat flux) can be determined using a 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 stronger heat fluxes If T1 > T2 then there is an downward heat flux (G↓) If T1 < T2 then there is an upward heat flux (G↑) Heat fluxes with bare soil are often directed away from the surface upward into a cooler atmosphere downward into cooler soil layers Atmospheric Instrumentation M. D. Eastin

Summary Measurement of Temperature Review of Atmospheric Temperature Thermometers Liquid Thermistors Exposure Errors Radiation Precipitation Minimization Surface / Soil Temperature Atmospheric Instrumentation M. D. Eastin

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