Atmospheric InstrumentationM. D. Eastin Measurement of Moisture (Humidity)

Atmospheric InstrumentationM. D. Eastin Outline Measurement of Moisture (Humidity) Review of Atmospheric Moisture Hygrometers Mechanical Psychrometer Electronic Chilled-Mirror Exposure Errors Ventilation Errors Drift Errors Precipitation Errors

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: 1. Relative Humidity (RH) The ratio (or percentage) of water vapor mass in a moist air parcel to the water vapor mass the parcel would have if it was saturated with respect to liquid water RH=relative humidity (ratio) e= vapor pressure (Pa) e s = vapor pressure at saturation (Pa) 2. Dewpoint Temperature (T d ) Temperature at which saturation (with respect to liquid water) is reached when an unsaturated moist air parcel is cooled at constant pressure where: T d =dewpoint temperature (K) T= temperature (K) R v =gas constant for water vapor (J kg -1 K -1 ) l v =latent heat of vaporization (J kg -1 K -1 ) RH=relative humidity (ratio) Review of Atmospheric Moisture

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Relative Humidity (RH): Theory: Clausius – Clapeyron Equation SI Unit: Ratio (0 → 1) Meteorology: Percentage (%) Instrument: Hygrometer Dewpoint Temperature: Theory: Clausius – Clapeyron Equation SI Unit: Kelvin (K) Meteorology: Fahrenheit (ºF)=ºC (9/5) + 32 Celsius (ºC)=K – Instrument: Hygrometer Review of Atmospheric Moisture Temperature T2T2 T1T1 e sw1 Vapor pressure TdTd Temperature Cools: T 1 → T 2 e sw2 e e sw (T)

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Atmospheric moisture decreases rapidly with altitude (~6–12 K / km) and can significantly vary by season (~30–40K from summer to winter) Upper-air hygrometers should exhibit a dynamic range→ –100ºC to +40ºC → 170K to 315K Review of Atmospheric Moisture

Atmospheric InstrumentationM. D. Eastin Definitions and Concepts: Horizontal variations in moisture are typically much smaller (~1 K / 100 km) except near fronts, dry lines, and thunderstorm outflows, but can vary more by season (~30–40K) Surface hygrometers should exhibit a dynamic range → –60ºC to +40ºC → 210K to 315K Review of Atmospheric Moisture

Atmospheric InstrumentationM. D. Eastin Mechanical Hygrometers – Basic Concept: Measure relative humidity by using a natural substance sensitive to moisture (called hygroscopic) Such substances change length as they acquire or lose moisture from the air 1.Hair 2.Cattle intenstine 3.Antlers Hair hygrometers Human hair increases in length by ~2% as atmospheric RH varies from 0% to 100% Was the most common instrument before electronic sensors were developed in the mid-1900s Rarely used today except as back-up instruments during power outages Hygrometers

Atmospheric InstrumentationM. D. Eastin Psychrometer – Basic Concept: Composed of two matched liquid-in-glass thermometers of similar make One is covered with wetted cotton (the “wet-bulb”) and measures temperature as moisture evaporates from the cotton One is not covered (the “dry bulb”) and measures ambient air temperature Together they measure the wet-bulb depression Requires a regular supply of air flow past the wet-bulb thermometer (provided by either a fan or a human) Requires a regular supply of distilled water to maintain a moistened wet-bulb wick 3. Wet-bulb Temperature (T w ):Temperature at which saturation with respect to liquid water is reached when an unsaturated moist air parcel is cooled by the evaporation of liquid water Hygrometers

Atmospheric InstrumentationM. D. Eastin Psychrometer – Practical Use: Theory and sources of error are well documented and easily checked / removed Excellent reference instrument for field use / calibration Rarely used today in an operational setting Assmann Psychrometer: No power source required (hand-held) Has radiation shields for each thermometer and forced ventilation Must maintain a regular supply of distilled water to the wet bulb Must ensure the dry bulb is free of dirt and dust Hygrometers

Atmospheric InstrumentationM. D. Eastin Electronic Hygrometers – Basic Concept: Measure relative humidity through changes in either electrical resistance or electrical capacitance Called hygristors Composed of hygroscopic polymer plate (often called the dielectric) separated by two thin electrodes Often operated as cyclical pairs: One is heated to remove condensed water while the other takes a measurement, and then switch operations. Resistance-based hygristors exhibit a highly non-linear response Capacitance-based hygristors exhibit a nearly linear response (used more often) Hygrometers

Atmospheric InstrumentationM. D. Eastin Electronic Hygrometers – Typical Specifications Accuracy ±3.0% (0 -90% RH) ±5.0% (90-100% RH) Resolution 0.1% Response Time 5-20 s Advantages Easy to automate Inexpensive Low power consumption Ideal for remote measurements (soundings) Disadvantages Temporary drift errors due to dust / salt contamination Permanent drift errors when exposed to SO 2 Large time lags (due to heating cycle) Accuracy degrades at high humidity Hygrometers Cyclical Hygristors (Capacitance)

Atmospheric InstrumentationM. D. Eastin Chilled-Mirror Hygrometers – Basic Concept: Measure dewpoint temperature by cooling a small mirror until condensation (dew) first forms on the mirror surface and then recording the mirror temperature A regular supply of moist air passes by a small mirror which is electrically cooled and heated by a “Peltier device” The presence of dew is detected on the mirror surface by an LED optical sensor A reduction in detected light implies the light source was scattered by liquid drops (or dew) on the mirror surface Hygrometers No DewDew Forms

Atmospheric InstrumentationM. D. Eastin Hygrometers Chilled-Mirror Hygrometers – Typical Specifications Accuracy ±0.2°C (dewpoint) ±0.3°C (frost point) Resolution 0.1°C Response Time 1-10 s Advantages Can be automated Moderate response times Less expensive than IR hygrometers Minimal drift Ideal for airborne measurements Ideal for turbulence measurements Disadvantages Mirror must remain contaminant free Cloud and precipitation drops can produce large dewpoint errors High maintenance requirements Accuracy degrades at sub-freezing temperatures Air Flow

Atmospheric InstrumentationM. D. Eastin Ventilations Errors – Psychrometers: Insufficient air flow past the wet-bulb thermometer will prevent complete evaporative cooling to the desired wet-bulb temperature Wet-bulb depression will be too small Such an effect will produce ”too moist” relative humidity errors up to +10% Laboratory tests suggest ventilation flow and/or local wind speeds greater than 3 m/s are required Exposure Errors

Atmospheric InstrumentationM. D. Eastin Drift Errors – Electrical Hygrometers: If the thin hygroscopic polymer plate becomes coated (even partially) with a hygroscopic contaminant (ex: soil, salt, SO 2, NO X ) then its chemical properties will change and alter its response to ambient humidity → drift error Some drift errors (from soil / salt) can be corrected by cleaning the sensors with distilled water Other drift errors (from SO 2 / NO X ) cannot be corrected since the contaminant induces a permanent chemical change to the polymer plate → must be replaced often Most electrical hygrometers are placed in vacuum-sealed packaging upon manufacture (to eliminate any contact with contaminants before use), and then opened when used Exposure Errors

Atmospheric InstrumentationM. D. Eastin Precipitation Errors – Psychrometers: Any precipitation contacting the wetted cotton wick (or wet-bulb) will alter the chemical composition of the “water solution” (originally distilled water), which will alter the evaporation rate and wet-bulb depression Do not expose directly to precipitation (hand-held) Place inside a radiation / rain shield (automated) Exposure Errors

Atmospheric InstrumentationM. D. Eastin Precipitation Errors – Electrical Hygrometers: Any cloud / precipitation hydrometeors will “saturate” the hygroscopic polymer → positive humidity errors Place inside a radiation / rain shield (if possible) For exposed sensors (rawinsondes / dropsondes) cyclical pairs (heating cycles) can remove such errors if precipitation is light (or the cloud is thin) and both sensors are not simultaneously wetted Wetting of both RH sensors often produce “saturated sub/super-adiabatic layers” depending on whether (1) the thermistor was also wetted, and (2) how the sounding software adjusts T and T d (or RH) to prevent super-saturation Exposure Errors RH sensor wetting in clouds Super adiabatic layers RH sensor wetting in clouds Sub adiabatic Layer Note the dry bias

Atmospheric InstrumentationM. D. Eastin Precipitation Errors – Chilled-mirror Hygrometers: Any cloud / precipitation hydrometers introduced into the hygrometer air flow will also scatter light emitted from the LED and the instrument will warm the mirror to adjust → positive dewpoint errors Place inside a radiation / rainfall shield (if possible) Errors can be effectively reduced by adjusting any cases of super-saturation (T d > T) to saturation (T d = T), but this assumes the thermometer does not simultaneously experience precipitation exposure errors (???) and that the actual ambient humidity is nearly saturated (???) Exposure Errors No DewDew FormsNo DewDew Forms Clear AirCloud / Precipitation Hydrometeors

Atmospheric InstrumentationM. D. Eastin Summary Measurement of Moisture (Humidity) Review of Atmospheric Moisture Hygrometers Mechanical Psychrometer Electronic Chilled-Mirror Exposure Errors Ventilation Errors Drift Errors Precipitation Errors

Atmospheric InstrumentationM. D. Eastin References Anderson, P. S., 1995: Mechanism for the behavior of hydroactive materials used n humidity sensors, Journal of Atmospheric and Oceanic Technology, 12, 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, Buck, A. L., 1976: The variable-path Lyman-alpha hygrometer and its operating characteristics. Bulletin of the American Meteorological Society, 57, Cerni, T.A., 1994: An infrared hygrometer for atmospheric research and routine monitoring. Journal of Atmospheric and Oceanic Technology, 11, Fuchs, M., and C. B. Tanner, 1965: Radiation shields for air temperature thermometers. Journal of Applied Meteorology, 4, Gates, R.S., 1994: Dew point temperature error from measuring dry-bulb temperature and relative humidity. Transcripts of the American Society of Agricultural Engineering, 37, Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp. Muller, S.H., and P.J. Beekman, 1987: A test of commercial humidity sensors for use at automated weather stations. Journal of Atmospheric and Oceanic Technology, 4, Smedman, A. S., and K. Lundin, 1987: Influence of sensor configuration on measurements of dry and wet bulb temperature fluctuations. Journal of Atmospheric and Oceanic Technology, 4,