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

Atmospheric Instrumentation M. D. Eastin

Measurement of Moisture (Humidity)


Outline


• Review of Atmospheric Moisture

• Hygrometers• Mechanical• Psychrometer• Electronic• Spectroscopic• Chilled-Mirror

• Exposure Errors• Ventilation Errors• Drift Errors• Precipitation Errors


Definitions and Concepts:

1. Vapor Pressure (e) ** In the atmosphere, water vapor behaves like an Ideal Gas

Partial pressure associated with all H2O molecules in a parcel

where: e = vapor pressure (Pa) ρv = density of water vapor (kg m-3) Rv = gas constant for water vapor (J kg-1 K-1)

T = temperature (K)

2. Mixing Ratio (w) ** Mass of water vapor per unit mass of dry air:

where: w = mixing ratio (kg/kg) ρd = density of dry air (kg m-3)

Rd = gas constant for dry air (J kg-1 K-1)p = total atmospheric pressure (Pa)lv = latent heat of vaporization (J kg-1 K-1)

Td = dewpoint temperature (K)

Review of Atmospheric Moisture

TRe vv

d

v

ρ

ρw

ep

e

R

Rw

v

d

dv

v

T

1

273.15

1

Rexp11.6e

l

** Difficult to observe



3. Saturation Mixing ** Mass of water vapor per unit mass of dry air at saturation Ratio (ws)

where: ws = saturation mixing ratio (kg/kg) es = vapor pressure at saturation (Pa) ρv = density of water vapor (kg m-3) ρd = density of dry air (kg m-3) Rv = gas constant for water vapor (J kg-1 K-1) Rd = gas constant for dry air (J kg-1 K-1)

p = total atmospheric pressure (Pa)lv = latent heat of vaporization (J kg-1 K-1)T = temperature (K)

4. Specific Humidity (q) ** Mass of water vapor per unit mass of moist air:

where: q = specific humidity (kg/kg)w = mixing ratio (kg/kg)


d

vs ρ

ρw

s

s

v

ds ep

e

R

Rw

T

1

273.15

1

Rexp11.6e

v

vs

l

** Difficult to observe

w

w

1

q



5. Relative Humidity (RH) ** The ratio (or percentage) of water vapor mass in a moistair 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)

es = vapor pressure at saturation (Pa)

6. Dewpoint Temperature (Td) ** Temperature at which saturation (with respect to liquid water) is reached when an unsaturated moist air

parcel is cooled at constant pressure

where: Td = dewpoint temperature (K)T = temperature (K)

Rv = gas constant for water vapor (J kg-1 K-1) lv = latent heat of vaporization (J kg-1 K-1)

RH = relative humidity (ratio)


se

eRH

** Easy to observe

1

ln1

RHl

TRTT

v

vd



Relative Humidity (RH):

Theory: Clausius – Clapeyron EquationSI Unit: Ratio (0 → 1)Meteorology: Percentage (%)Instrument: Hygrometer

Dewpoint Temperature:

Theory: Clausius – Clapeyron EquationSI Unit: Kelvin (K)Meteorology: Fahrenheit (ºF) = ºC (9/5) + 32

Celsius (ºC) = K – 273.15Instrument: Hygrometer


Temperature

T2 T1

esw1

Va

po

r p

res

su

re

Td

Temperature Cools: T1 → T2

esw2 e

esw(T)



• Atmospheric moisture decreasesrapidly with altitude (~6–12 K / km)and can significantly vary by season(~30–40K from summer to winter)

• Upper-air hygrometers should exhibita dynamic range → –100ºC to

+40ºC→ 170K to 315K




• Horizontal variations in moisture are typicallymuch smaller (~1 K / 100 km) except nearfronts, dry lines, and thunderstorm

outflows,but can vary more by season (~30–40K)

• Surface hygrometers should exhibit adynamic range → –60ºC to +40ºC

→ 210K to 315K



Mechanical Hygrometers – Basic Concept:

•Measure relative humidity by usinga natural substance sensitive tomoisture (called hygroscopic)

•Such substances change length as theyacquire or lose moisture from the air

1. Hair2. Cattle intenstine3. Antlers

Hair hygrometers

•Human hair increases in length by ~2% as atmospheric RH varies from 0% to 100%

•Was the most common instrumentbefore electronic sensors weredeveloped in the mid-1900s

1.Rarely used today except as back-upinstruments during power outages

Hygrometers


Psychrometer – Basic Concept:

•Composed of two matched liquid-in-glassthermometers of similar make

•One is covered with wetted cotton (the “wet-bulb”)and measures temperature as moisture evaporatesfrom the cotton

•One is not covered (the “dry bulb”) and measuresambient air temperature

•Together they measure the wet-bulb depression

•Requires a regular supply of air flow pastthe wet-bulb thermometer (provided byeither a fan or a human)

•Requires a regular supply of distilled waterto maintain a moistened wet-bulb wick

7. Wet-bulb Temperature (Tw): Temperature at which saturation with respect to liquid water is reached when an unsaturated moist air parcelis cooled by the evaporation of liquid water

Hygrometers

wetdry TTT


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


Electronic Hygrometers – Basic Concept:

•Measure relative humidity throughchanges in either electrical resistance

or electrical capacitance•Called hygristors

•Composed of hygroscopic polymer plate(often called the dielectric) separatedby two thin electrodes

•Often operated as cyclical pairs: Oneis heated to remove condensed waterwhile the other takes a measurement,and then switch operations.

•Resistance-based hygristors exhibita highly non-linear response

•Capacitance-based hygristors exhibita nearly linear response(used more often)

Hygrometers


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 SO2 •Large time lags (due to heating cycle)•Accuracy degrades at high humidity

Hygrometers

Cyclical Hygristors(Capacitance)


Spectroscopic Hygrometers – Basic Concept:

•Measure specific humidity through the absorption of infrared (IR) or ultraviolet (UV) radiation

•Utilize a short-path broadband sourcewhich includes one wavelength that isabsorbed by atmospheric water vaporand one wavelength that is not.

•A rapidly-rotating “chopper wheel” is usedto alternate measurement between thetwo wavelengths

•Atmospheric specific humidity is directlyrelated to the difference in receivedradiation by the detector

•Operate in one of two wavelength bands

1. 1-2 μm (IR band)2. 120-122 nm (UV band or Lyman–α)

Hygrometers


HygrometersSpectroscopic Hygrometers – Typical Specifications

Accuracy ±0.5 g/m3

Resolution 0.1 g/m3

Response Time < 1 s

Advantages

• Can be automated• Very fast response times• Ideal for airborne measurements• Ideal for turbulence measurements

Disadvantages

•Suffer from large drift errors•Large power consumption•Very expensive•High maintenance requirements

SpectroscopicHygrometer(Lyman – α)


Chilled-Mirror Hygrometers – Basic Concept:

•Measure dewpoint temperature by coolinga small mirror until condensation (dew)first forms on the mirror surface andthen recording the mirror temperature

•A regular supply of moist air passes bya small mirror which is electrically cooledand heated by a “Peltier device”

•The presence of dew is detected on themirror surface by an LED optical sensor

•A reduction in detected light implies thelight source was scattered by liquid drops (or dew) on the mirror surface

Hygrometers

No Dew Dew Forms


HygrometersChilled-Mirror Hygrometers – Typical Specifications

Accuracy ±0.2°C (dewpoint)±0.3°C (frost point)

Resolution 0.1°CResponse 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

AirFlow


Ventilations Errors – Psychrometers:

• Insufficient air flow past the wet-bulbthermometer will prevent completeevaporative 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


Drift Errors – Electrical Hygrometers:

• If the thin hygroscopic polymer plate becomescoated (even partially) with a hygroscopiccontaminant (ex: soil, salt, SO2, NOX) then itschemical properties will change and alter itsresponse to ambient humidity → drift error

• Some drift errors (from soil / salt) can be correctedby cleaning the sensors with distilled water

• Other drift errors (from SO2 / NOX) cannot be correctedsince the contaminant induces a permanent chemicalchange to the polymer plate → must be replaced often

•Most electrical hygrometers are placed in vacuum-sealedpackaging upon manufacture (to eliminate any contact withcontaminants before use), and then opened when used

Exposure Errors


Drift Errors – Spectroscopic Hygrometers:

• If any optical lens for the IR / UV sourceor detector accumulates a non-watersubstance (dust, salt, pollutants) then some attenuation of the source radiationwill occur, altering the ambient water vapor measurement → drift error

• Regular cleaning of the optical lens andchopper wheel is required

• Some units have built-in corrections thatwill account for modest attenuation

• Most units deployed for field work requireat least monthly (sometimes weekly / daily)cleanings depending on ambient air quality

Exposure Errors


Precipitation Errors – Psychrometers:

• Any precipitation contacting the wettedcotton wick (or wet-bulb) will alter thechemical composition of the “watersolution” (originally distilled water), which will alter the evaporation rateand wet-bulb depression

•Do not expose directly to precipitation(hand-held)

•Place inside a radiation / rain shield(automated)

Exposure Errors


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 sucherrors 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 thermistorwas also wetted, and (2) how the soundingsoftware adjusts T and Td (or RH) to preventsuper-saturation

Exposure Errors

RH sensorwetting

in clouds

Superadiabatic

layers

RH sensorwetting

in clouds

Subadiabatic

Layer

Note thedry bias


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 casesof super-saturation (Td > T) to saturation (Td = T), butthis assumes the thermometer does not

simultaneously experience precipitation exposure errors (???) and that

the actual ambient humidity is nearly saturated (???)

Exposure Errors

No Dew Dew Forms No Dew Dew FormsClear Air Cloud / Precipitation Hydrometeors


Summary


• Review of Atmospheric Moisture

• Hygrometers• Mechanical• Psychrometer• Electronic• Spectroscopic• Chilled-Mirror

• Exposure Errors• Ventilation Errors• Drift Errors• Precipitation Errors


References

Anderson, P. S., 1995: Mechanism for the behavior of hydroactive materials used n humidity sensors, Journal of Atmospheric and Oceanic Technology, 12, 662-667.

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.

Buck, A. L., 1976: The variable-path Lyman-alpha hygrometer and its operating characteristics. Bulletin of the American Meteorological Society, 57, 1113-1118.

Cerni, T.A., 1994: An infrared hygrometer for atmospheric research and routine monitoring. Journal of Atmospheric and Oceanic Technology, 11, 445-462.

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

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, 687-688.

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, 731-735.

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, 668-673.

Documents

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