Hygrometry objectives:

1
ATMS 320 Meteorological Instrumentation

• Hygrometry objectives
• Understand how water vapor pressure is estimated
• Know the different definitions associated with
  humidity
• Learn the different methods for measuring
  humidity
• Appreciate various factors in choosing a humidity
  sensor

2
ATMS 320 Hygrometry

• The measurement of atmospheric humidity in the
  field has been and continues to be a challenge
• Low cost, low power consumption, and reliability
  requirements make it especially difficult for
  automatic weather stations

http//www.musichead.com.au/site/artistPhotos.asp?
actID20206
3
ATMS 320 Hygrometry

• Does the air really hold water vapor (like a
  sponge)?
• Lets look at a cartoon!

(courtesy F. Remer)
4
Evaporation

• Molecules in liquid water attract each other
• In motion

(courtesy F. Remer)
5
Evaporation

• Collisions
• Molecules near surface gain velocity by
  collisions

(courtesy F. Remer)
6
Evaporation

• Fast moving molecules leave the surface
• Evaporation

(courtesy F. Remer)
7
Evaporation

• Rate of evaporation
• Constant
• Function of water temperature

Twater
(courtesy F. Remer)
8
Evaporation

• Soon, there are many water molecules in the air

(courtesy F. Remer)
9
Evaporation

• Slower molecules return to water surface
• Condensation

(courtesy F. Remer)
10
Evaporation

• Rate of Condensation
• Variable
• Function of water vapor mass in air

(courtesy F. Remer)
11
Evaporation

• Net Evaporation
• Number leaving water surface is greater than the
  number returning
• Evaporation greater than condensation

Net Evaporation
(courtesy F. Remer)
12
Evaporation

• Rate at which molecule return increases with time
• Evaporation continues to pump moisture into air
• Water vapor increases with time

(courtesy F. Remer)
13
Equilibrium

• Eventually, equal rates of condensation and
  evaporation
• Air is saturated
• Equilibrium

(courtesy F. Remer)
14
Equilibrium

• At Equilibrium
• Rate of evaporation is a function of temperature

Twater
Evaporation f(T)
Rate of evaporation
Rate of condensation

(courtesy F. Remer)
15
Equilibrium

• At Equilibrium
• Rate of condensation depends on water vapor mass
• Also a function of temperature

Condensation f(T)
(courtesy F. Remer)
16
Equilibrium

• At Equilibrium

Tair Twater
(courtesy F. Remer)
17
ATMS 320 Hygrometry

• The pressure exerted by pure water vapor (es )
  is a function of the temperature of the vapor and
  liquid phases

derived by integration of the Clausius-Clapeyron
equation
(courtesy F. Remer)
18
ATMS 320 Hygrometry

• Variations on the saturation vapor pressure theme

Buck (1981)
Wexler (1976, 1977)
http//www.sheetmusicplus.com/store/
19
ATMS 320 Hygrometry
Buck (1981)
Correction for the air pressure enhancement
effect
This equation form is preferred because it is
easier to invert to Obtain the dew-point
temperature given the ambient vapor pressure
20
ATMS 320 Hygrometry
Equilibrium vapor pressure over a plane surface
of liquid water
Equilibrium vapor pressure over a plane surface
of ice
Equilibrium vapor pressure varies over two orders
of magnitude in the normal temperature range, one
might expect the accuracy of almost any humidity
instrument to decrease with decreasing temperature
.
21
ATMS 320 Hygrometry

• Humidity relationships
• Starting point (A)
• Saturation vapor pressure (B)
• Dew-point temperature (D)
• Wet-bulb temperature (C)

22
ATMS 320 Hygrometry

• Humidity definitions
• Be familiar with the definitions from Absolute
  humidity to Wet-bulb temperature and the
  accompanying formulas in Section 5.2 of the
  textbook.

http//www.onestopenglish.com/BookShop/BookShop/re
tail/dictionaries.htm
23
ATMS 320 Hygrometry

• Sensors that respond to humidity and report
  dew-point temperature are common
• The conversion leads to error since the
  relationship is non-linear and the inverted
  equations cannot be directly (analytically) solved

Sensor measures Td, converts to RH
Why is absolute error higher for lower dew-point
temps at a given RH?? Hint see EX on p. 91,92
24
ATMS 320 Hygrometry
Sensor measures RH, converts to Td
Why is absolute error higher for higher air temps
at a given Td?? Hint see EX on p. 92
25
ATMS 320 Hygrometry

• Methods for measuring humidity
• Removal of water vapor from moist air
• Addition of water vapor to moist air
• Equilibrium sorption of water vapor
• Attainment of vapor-liquid or vapor-solid
  equilibrium
• Measurement of physical properties of moist air
• By chemical reactions

http//www.allergybuyersclubshopping.com/slanfinhu
mgf300.html
26
ATMS 320 Hygrometry

• Methods removal of water vapor from moist air
• Use a desiccant to absorb water
• Freeze out water vapor
• Separate moist air constituents using a semi
  permeable membrane
• After removal determine mass of water vapor and
  of remaining sample and calculate humidity

http//cgi.ebay.com/
a substance that promotes drying (e.g., calcium
oxide absorbs water and is used to remove
moisture)
27
ATMS 320 Hygrometry

• Methods addition of water vapor to air
• psychrometry

http//www.city.niagarafalls.on.ca/
28
ATMS 320 Hygrometry

• Addition of water vapor to air psychrometry
• Dry bulb sensor measures ambient air temperature
• Wet bulb sensor is covered with a wick moistened
  with water and measures a lower temperature,
  caused by evaporation of water into the ambient
  air stream
• Forced ventilation is required for optimum
  performance

29
ATMS 320 Hygrometry

• Sources of error in a psychrometer
• A lack of sensitivity and accuracy in wet- and
  dry-bulb thermometers
• Low ventilation rate
• Radiation incident on the temperature sensors
• Size, shape, material, and wetting of the wick
• Air flow from wet- to dry-bulb thermometer
• Dirty water used to moisten the wick

30
ATMS 320 Hygrometry

• Addition of water vapor to air psychrometry
• Conversion of wet- and dry-bulb temperatures to
  ambient vapor pressure

• Slope of curves give the static sensitivity
• Sensitivity increases markedly as the temperature
  increases for a given RH
• Sensitivity increases slightly as the relative
  humidity decreases for a given ambient temperature

31
ATMS 320 Hygrometry

• Addition of water vapor to air psychrometry
• Difficult to have an error of less than 1 RH for
  air temperatures below 10oC
• Hand-held sensors (e.g. Assmann psychrometer)
  must be held upwind of the observer
• Automated psychrometry is challenging
  (constrained by low cost, low power requirements,
  and reliability)

32
ATMS 320 Hygrometry
http//www.allroutes.to/logging/history.htm

• Equilibrium sorption of water vapor - hygrometers
• The process of sorption (absorption and/or
  adsorption) causes a material to expand or
  contract, alters electrical resistance or
  capacitance
• To be useful, the material must exhibit a change
  that is reversible and reproducible and detectable

First hygrometer wood (de Cusa, 1401-1464)
the accumulation of molecules of a gas to form a
thin film on the surface of a solid
33
ATMS 320 Hygrometry

• Equilibrium sorption of water vapor electric
  hygrometers
• Sorption sensors that take up water which causes
  a change in an electrical parameter such as
  resistance or capacitance

(top row)
(middle row)
Now a capacitor (formerly condenser) has the
ability to hold a charge of electrons. The number
of electrons it can hold under a given
electrical pressure (voltage) is called its
capacitance or capacity. Two metallic plates
separated by a non-conducting substance between
them make a simple capacitor.
http//www.electronics-tutorials.com/basics/capaci
tance.htm
34
ATMS 320 Hygrometry

• Electric hygrometers (cont.)
• Probe capacitance (Fig. 5-10) is converted to
  frequency and then to a voltage by electronics in
  the sensor probe.
• Non-linearity in probe capacitance (slope of line
  is not constant)

35
ATMS 320 Hygrometry

• Electric hygrometers (cont.) sources of drift
• Dust accumulation
• SO2 contamination
• Aging of electronic components

36
ATMS 320 Hygrometry

• Electric hygrometers sensor resistance (middle
  row of Fig. 5-9)
• Bulk polymer resistance where resistance
  decreases with increasing RH (Fig. 5-11)
• Difficult to maintain
• Difficult to measure the very high resistance for
  low values of RH (less accurate at RH values of
  below 20)

37
ATMS 320 Hygrometry

• Electric hygrometers carbon hygristor (bottom
  row of Fig. 5-9)
• Experiences a dimensional change in response to a
  change in RH
• Dimension (X) increases with increasing RH
• Increasing X also increases the distance between
  the carbon particles (increasing resistance)
• Subject to quite high drift rates
• Used only on radiosondes

38
ATMS 320 Hygrometry

• Electric hygrometers summary
• Small and relatively inexpensive
• Require calibration
• Long lag times
• Significant hysteresis
• Sensitive to certain contaminants (e.g. SO2)

39
ATMS 320 Hygrometry

• Equilibrium sorption of water vapor - mechanical
  hygrometers
• Made from dimensionally variable materials (e.g.
  hair, skin, cotton, silk, nylon, paper, wood)
  mechanically coupled to an indicator or transducer

CAUTION!!
His appearance changes with the weather!!
40
ATMS 320 Hygrometry
Sasquatch

• Mechanical hygrometers - defects
• Drift
• Large hysteresis
• Large lag times

41
ATMS 320 Hygrometry

• Measurement of physical properties of moist air
• Refractive index
• Radiative absorption
• Thermal conductivity
• Viscosity
• Density
• Sonic velocity
• All vary with the amount of water vapor present

http//antwrp.gsfc.nasa.gov/apod/image/0102/sonicb
oomplane_navy_big.jpg
Photo credit Ensign John Gay
42
ATMS 320 Hygrometry

• Measurement of physical properties of moist air
  spectroscopic hygrometer
• Measures the attenuation of certain bands in the
  spectrum due to water vapor absorption
• 1000 to 3000 nm infrared band is ideal since
  there is small amounts of solar and earth
  background radiation
• Also, glass is transparent up to about 2800 nm
  and can be used to enclose the sensor source and
  detector

Beers law
43
ATMS 320 Hygrometry

• Spectroscopic hygrometer Beers law applies for
  measuring humidity if
• Water vapor were the only absorbing gas
  (satisfied for particular wavelengths)
• The instrument wavelength resolution were small
  compared to the absorption lines (satisfied if a
  laser source is used)
• Laser hygrometers are expensive and sensitive to
  orientation
• IR broadband hygrometers are used

44
ATMS 320 Hygrometry

• Spectroscopic hygrometer IR hygrometers
• Sources and detectors drift with time
• Windows change or get dirty
• Impacts source strength, I0
• Uses relative difference between two bands
  absorbing band (2600 nm) and non-absorbing or
  reference band (2300 nm). This subtracts out the
  apparent changing source strength

45
ATMS 320 Hygrometry

• IR hygrometer
• Two filters are rotated into the beam
• Filter wheel allows sampling at three different
  times during its rotation while the absorbing
  (VW), neither (VD), or reference (VR) filters are
  in the beam

transfer equation of IR hygrometer
where l W, D, or R
46
ATMS 320 Hygrometry

• IR hygrometer
• Normalized signal
• where S is the sensor static sensitivity
• Eliminates drift and is insensitive to variations
  in source strength

It is necessary to also measure air temperature
and pressure to obtain absolute humidity
47
ATMS 320 Hygrometry

• Spectroscopic hygrometer Lyman alpha
• Uses Lyman-alpha emission line of atomic hydrogen
  at 121.56 nm (UV) as the source radiation
• Windows made from magnesium fluoride
• Oxygen and ozone are weak emitters at this
  wavelength
• Motor and filter wheel not used
• Transmission of windows changes due to
  interaction of the magnesium fluoride with
  atmospheric constituents

http//www.mierijmeteo.demon.nl/mierij/research/ly
man.htm
48
ATMS 320 Hygrometry

• Spectroscopic hygrometer Lyman alpha (cont.)
• Drift rate correction requires a reference
  instrument
• Lyman alpha hygrometer is simpler, smaller, and
  much faster than the IR hygrometer
• Suitable for research aircraft and for tower
  measurements of turbulence (both requiring a very
  fast response)

http//www.mierijmeteo.demon.nl/mierij/research/ly
man.htm
49
ATMS 320 Hygrometry

• Attainment of Vapor-Liquid or Vapor-Solid
  Equilibrium dew- and frost-point hygrometer
• Operates by cooling a surface until vapor-liquid
  or vapor-solid equilibrium is achieved
• Frost or dew formation on the mirror is detected
  with a light-emitting diode and one or more
  photodetectors sense the change in light
  scattering when frost or dew forms on the mirror

Receives decreasing light if frost or dew has
formed
Receives light only if frost or dew has formed
50
ATMS 320 Hygrometry

• Dew- and frost-point hygrometer (cont.)
• Control unit uses the ratio of light received by
  the two detectors to determine if mirror heating
  or cooling is required
• Control system controls current through the
  thermocouple heat pump and regulates the mirror
  temperature to the point where dew or frost just
  begins to form
• Must avoid overshoot and still follow changes in
  ambient humidity quickly

51
ATMS 320 Hygrometry

• Dew- and frost-point hygrometer (cont.) issues
• Difficult to measure mirror temperature without
  interfering with the detection of frost or dew
• Control unit must be capable of detecting rapid
  change
• Water-soluble matter on the mirror can lower
  dew-point
• Formation of very small droplets increases
  dew-point
• Presence of supercooled water leads to error

52
ATMS 320 Hygrometry

• Attainment of Vapor-Liquid or Vapor-Solid
  Equilibrium saturated salt solution (dewcel)
• Contains a saturated solution of lithium chloride
  (LiCl) applied to a glass cloth that surrounds a
  temperature measuring device
• A heater raises the solution to, and maintains it
  at, the equilibrium temperature

http//eol.jsc.nasa.gov/EarthObservatory/EffectofD
roughtonGreatSaltLake.htm
53
ATMS 320 Hygrometry

• Chemical reactions
• Remove the water vapor by use of a chemical
  reagent and then weigh the resulting water

http//www.drbrainzlab.com/drbrainzfiles/gallerydo
c_drfrankenstein.html
54
ATMS 320 Hygrometry

• Factors in choosing a humidity sensor
• Cost
• Accuracy
• Maintenance required
• Speed of response
• Power consumption

http//sargentwelch.com/product.asp_Q_pn_E_WLS4274
05FEA
55
ATMS 320 Hygrometry

• Psychrometers - issues
• Low-cost when hand-held
• High-cost when automated
• Automation made difficult by wick contamination
  and providing an adequate supply of pure water to
  the wick
• Most sources of error can be readily detected
• Requires adequate ventilation
• Less susceptible to drift

http//sargentwelch.com/product.asp_Q_pn_E_WLS4274
05FEA
56
ATMS 320 Hygrometry

• Sorption sensors - issues
• Small size
• Low cost
• Moderate accuracy
• Low power consumption
• Maintenance requirements mainly are coping with
  drift
• Problems rarely detectable by visual examination
• Intercomparison between other stations needed to
  check for drift

http//sargentwelch.com/product.asp_Q_pn_E_WLS4274
05FEA
57
ATMS 320 Hygrometry

• Spectroscopic hygrometers - issues
• High cost
• High power consumption
• Potentially high maintenance requirements
• Lyman-alpha has very fast response and subject to
  severe drift
• IR absorption hygrometers have very limited
  response times
• Limited applications

http//sargentwelch.com/product.asp_Q_pn_E_WLS4274
05FEA
58
ATMS 320 Hygrometry

• Chilled-mirror dew-point sensors - issues
• High cost
• High power consumption
• Slow response times
• High maintenance requirements
• High accuracy
• Wide range of observable dew-points (at very low
  dew-points)
• Well suited for laboratory applications

http//sargentwelch.com/product.asp_Q_pn_E_WLS4274
05FEA
59
ATMS 320 Hygrometry

• Exposure of humidity sensors
• Air temperature is almost always a requirement,
  so the exposure of humidity sensors is closely
  related to the exposure of air temperature
  sensors (Chap. 4)
• Psychrometers require good ventilation and
  shielding from solar radiation

http//www.nps.gov/yell/
60
ATMS 320 Hygrometry

• Psychrometers
• Require good ventilation and shielding from solar
  radiation
• Avoid contamination by salts which inhibit
  evaporation (problem for sites near the ocean)
• Cannot work in freezing weather

61
ATMS 320 Hygrometry

• Sorption sensors
• Shield from radiation
• Avoid forced aspiration
• Dust filter
• Keep liquid water away from sensor and dust
  filter
• Sensitive to contamination by some gases (e.g.
  SO2)

http//cyranosciences.com/technology/sensor
62
ATMS 320 Hygrometry

• Spectroscopic hygrometers
• Same exposure requirements as for measuring air
  temperature
• Lyman-alpha requires another kind of hygrometer
  to provide drift compensation

63
ATMS 320 Hygrometry

• Chilled-mirror hygrometers
• Mirror contamination problems seriously
  compromise the performance of this sensor in the
  field (contamination cannot be completely avoided
  for any reasonable outdoor exposure)
• Requires a lot of attention from skilled
  technicians

http//www.nyad.com/principles_of_moisture.htm
64
ATMS 320 Hygrometry

• Exposure issues general
• Humidity at the measurement site may differ from
  the value at surrounding stations due to
  localized rainfall, proximity to a lake or river
  upwind of the site, and changes in nearby farming
  practice.

http//www.ehabweb.net/clake.html