Transcript Lecture #5: Instruments – Humidity

Measurement of Moisture (Humidity)
Atmospheric Instrumentation
M. 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 Instrumentation
M. D. Eastin
Review of Atmospheric Moisture
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
e
RH 
es
2. Dewpoint Temperature (Td)
RH = relative humidity (ratio)
e = vapor pressure (Pa)
es = vapor pressure at saturation (Pa)
Temperature at which saturation (with respect to liquid
water) is reached when an unsaturated moist air
parcel is cooled at constant pressure
 TR

Td  T 1  v ln RH 
lv


where:
Atmospheric Instrumentation
Td
T
Rv
lv
RH
=
=
=
=
=
1
dewpoint temperature (K)
temperature (K)
gas constant for water vapor (J kg-1 K-1)
latent heat of vaporization (J kg-1 K-1)
relative humidity (ratio)
M. D. Eastin
Review of Atmospheric Moisture
Definitions and Concepts:
Temperature Cools: T1 → T2
Relative Humidity (RH):
Clausius – Clapeyron Equation
Ratio (0 → 1)
Percentage (%)
Hygrometer
Dewpoint Temperature:
Theory:
SI Unit:
Meteorology:
Instrument:
Clausius – Clapeyron Equation
Kelvin (K)
Fahrenheit (ºF) = ºC (9/5) + 32
Celsius (ºC)
= K – 273.15
Hygrometer
Atmospheric Instrumentation
esw(T)
Vapor pressure
Theory:
SI Unit:
Meteorology:
Instrument:
esw1
Td
esw2
e
T2
T1
Temperature
M. D. Eastin
Review of Atmospheric Moisture
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
Atmospheric Instrumentation
M. D. Eastin
Review of Atmospheric Moisture
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
Atmospheric Instrumentation
M. D. Eastin
Hygrometers
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
Atmospheric Instrumentation
M. D. Eastin
Hygrometers
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
T  Tdry  Twet
• 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 (Tw):
Atmospheric Instrumentation
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
M. D. Eastin
Hygrometers
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
Atmospheric Instrumentation
M. D. Eastin
Hygrometers
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)
Atmospheric Instrumentation
M. D. Eastin
Hygrometers
Electronic Hygrometers – Typical Specifications
±3.0% (0 -90% RH)
±5.0% (90-100% RH)
Resolution
0.1%
Response Time
5-20 s
Accuracy
Advantages
• Easy to automate
• Inexpensive
• Low power consumption
• Ideal for remote measurements (soundings)
Cyclical Hygristors
(Capacitance)
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
Atmospheric Instrumentation
M. D. Eastin
Hygrometers
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
Atmospheric Instrumentation
No Dew
Dew Forms
M. D. Eastin
Hygrometers
Chilled-Mirror Hygrometers – Typical Specifications
±0.2°C (dewpoint)
±0.3°C (frost point)
Resolution
0.1°C
Response Time 1-10 s
Accuracy
Air
Flow
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
Atmospheric Instrumentation
M. D. Eastin
Exposure Errors
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
Atmospheric Instrumentation
M. D. Eastin
Exposure Errors
Drift Errors – Electrical Hygrometers:
• If the thin hygroscopic polymer plate becomes
coated (even partially) with a hygroscopic
contaminant (ex: soil, salt, SO2, NOX) 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 SO2 / NOX) 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
Atmospheric Instrumentation
M. D. Eastin
Exposure Errors
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)
Atmospheric Instrumentation
M. D. Eastin
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 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 Td (or RH) to prevent
super-saturation
RH sensor
wetting
in clouds
Super
adiabatic
layers
RH sensor
wetting
in clouds
Sub
adiabatic
Layer
Note the
dry bias
Atmospheric Instrumentation
M. D. Eastin
Exposure Errors
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 (Td > T) to saturation (Td = T), but
this assumes the thermometer does not simultaneously
experience precipitation exposure errors (???) and that
the actual ambient humidity is nearly saturated (???)
Clear Air
No Dew
Dew Forms
Atmospheric Instrumentation
Cloud / Precipitation Hydrometeors
No Dew
Dew Forms
M. 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 Instrumentation
M. 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, 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.
Atmospheric Instrumentation
M. D. Eastin