Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements
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measurement becomes imperative for synthesizing data from any set of observations,
whether it be area of cloud cover or a particle distribution having the moments
discussed above. It may be desirable under some circumstances to average particles
by updraft location—updrafts in the upshear of convective clouds have quite differ-
ent microphysical structure from the downshear, and combine the two averages for
remote-sensing comparison. Other combina tions may be required to achieve a
sufficient approach to reality.
From the viewpoint of idealizing ice particle formation and linking the physics of
the individual particle nucleation and g rowth with ambient conditions, specific
situations may be identified. A mountain wave lenticular cloud, for med under condi-
tions of high stability has the merit of having low turbulence levels with traceable air
trajectories from conservative variables. There is a well-defined rate of change of
relative humidity ahead of the cloud and rate of availability of water vapor in the
cloud itself, enabling specification of growth conditions. This situation also occurs
for orographic clouds in upslope flow, which can lead to continuous production of
uniform crystal type and size under quasi-steady conditions as in a lenticular cloud.
Conditions change along such trajectories over times of order several minutes as can
be judged from observation of the location of the cloud leading edge. Shallower
clouds formed in g ravity waves on frontal surfaces give somewhat longer growth
times. Of current interest is the ability to produce relatively low ice supersaturations
that persist over a long time, giving low nucleation rates of Cloud Condensation
Nuclei (CCN), which freeze homogeneously as they dilute at temperatures some-
what below 40
C. Small dropl ets freeze as single crystals and grow slowly under
ambient conditions in the form of crystals with uniform flat facets to give well-
defined and bright optical effects. The stability is very important since it maintains
conditions so that a conceptional model may be put together (Hallett et al., 1999).
Spectacular optical effects are reported extensively in polar regions and are known to
be associated with such pristine crystals—both plates and columns. Less well
reported are spectacular midlatitude summer displays at low temperatures and
high levels. A way of producing a low supersaturation is a rapid overrunning of a
cold layer by a warmer moister layer followed by a slow diffusion of properties from
one to the other. Such effects can be idealized by a time-dependent solution of the
heat=vapor diffusion equation in one dimension, with initial boundary conditions
being uniform temperature and mixing ratio with an initial sharp discontinuity at
the interface (Carslaw and Jaeger, 1959). Heat and vapor diffuse almost together
(vapor is a littl e faster) and since the saturation vapor pressure of ice is near expo-
nential with respect to 1=T (temperature in Kelvin), a pulse of supersaturation
spreads into the cold air with time constant:
X
2
D
;
X
2
k
where X is the distance from the initial interface, D is water vapor diffusivity in air,
and k the thermal diffusivity of air. For distances of tens of centimeters, the times are
of order 100 s; for meters of order a few hours. Shear can influence the boundary
conditions and change these times, but maintaining stability is important to the
716 MEASUREMENT IN THE ATMOSPHERE
process. This behavior results from the assumption of infinite lateral extent; in reality
the discontinuity may be linear or circular, giving a decreasing supersaturation as the
disturbance propagates. To maintain a crystal in growth conditions would require a
special combination of progressive gravity wave having such a diffusion supersa-
turation field but moving with the growing crystal. This is necessarily an infrequent
occurrence (the sky is not universally covered with spectacular optical events) but
may be sufficiently freque nt to explain the occasional occurrence of spectacular
optical displays under both high cirrus and boundary layer diamond dust conditions,
required for the local production of uniform crystal sizes and shapes. The detailed
structure of inversions is far from well known and can be extremely sharp—with
differences of 10 K some times extending over a meter or less. Such sharp disconti-
nuities would not appear in any standard sounding analysis. The spatial distribution
of supersaturation under these conditions is an interesting topic for further study.
The trail of defining and specifying cirrus cloud follows the process of nuclea-
tion, growth, and evaporation of individual crystals. It requires a knowledge of the
dynamic and radiation conditions under which they evolve and ultimately specifica-
tion on physical grounds of a spatial and temporal averaging domain. This domain is
related to the time and spatial resolution of the instruments both for in situ and
remote sensing. For multiple instruments, measurements of the same volume and
geometry of air by the different techniques is crucial to a combination of obser-
vations. All need to be considered in any numerical comparison, a difficult but not
impossible undertaking.
3 INSTRUMENTATION AND MODELING
Major advances in the atmospheric sciences often come from the development and use
of specific instr umentation. The barometer and measurement of pressure led to the
realization that weather systems existed. The development of telegraphy and radio
communication enabled the constr uction of weather maps wi th only a few hours delay.
The radiosonde, developed in the 1940s, opened our understanding of the upper
troposphere, the basis for description of the weather on a global scale, and forecasting
as a practical application. With the advent of major computing power (dependent on
the production of fast processing chips and even smaller but more powerful memory
devices by the semiconductor industry), opportunity to access very large databases
has emerged, together with the ability to perform rapid calculations in the use of
complex numerical models of a variety of atmospheric phenomena.
Cur rent models of a variety of processes in Earth’s atmosphere are an outcome of
this technology. Such models are necess arily limited in application by our ability
(or lack thereof ) to provide measurements of initial and subsequent conditions over
a sufficiently small grid length, both of the ther modynamic variables them selves
(e.g., temperature, dew point, mixing ratio, radiative flux) and of what may be called
structure-sensitive variables (cloud droplet nuclei, CCN, ice nuclei, trace NH
3
,
particle defect structure). The implication in the use of such models is that justifi-
cation of their prediction comes from a scheme that compares their output with some
set of measurements. This is called verification or perhaps validation (Hallett, 1996).
3 INSTRUMENTATION AND MODELING 717
This sounds very grand, but when we look at things more closely, we find that it may
be nothing of the sort. What it is is a check for consistency of the model against a set
of measurements. It should be realized that any ag reement may be coincidental and
requires justification as the progenitor of the model assesses the sensitivity to both
the physical assumptions together with the variability and rational for any para-
meterization of the measurement. This is the case for models on all scales in the
atmosphere and also for the weather forecast itself.
The atmosphere is not a controlled laboratory; it is a turbulent, highly variable
environment where parameterizations, sometimes overgenerous in their b elievability
(e.g., size distributions of aerosol or precipitation for the reasons discussed above)
may be no more than a figment of the modeler’s imagination. Hopefully, it represents
a space and time average that smoothes local effects—this is the nature of statistics.
But how much larger a scale—how long a time? The con clusion is that physical and
chemical concepts (still models in the generic terminology but based on our know-
ledge of the real world) need to guide closely what is real and what is not real in the
numerical model. This may lead to quite specific measurements—not to validate the
model (impossible) but to give some justification for the physical=chemical ideas on
which it is based. The concept of parameterization of any atmospher ic process needs
to be offset against the reality of an appropr iate space–t ime scale, whether it be
mixing at the top of a cumulus or transfer of some property (heat , momentum) in a
boundary layer or gravity wave.
The availability of suitable instruments is a necessary prerequisite to progress,
here defined as insight into useful ideas that can be used in a predictive sense. In a
completely unrealizable utopia, measurement ultimately provides a one-to-one
comparison between a model and a measured reality. From a theoretical basis,
self-consistency and logic necessarily have their place, but the reality is so complex
that the assumptions and approximations dominate. The ideas of a theme song of an
early radar conference ‘‘measure everything, everywhere, all the time’’ provide as
much of a hindrance to progress as does the model in the absence of a prescribed set
of measurements for its possible justification.
This brings us to the development of instruments for a specific purpose. A wide
variety of instruments can be bought ‘‘off the shelf ’’ for investigation of atmospheric
processes—providing a sufficient market has existed to encourage their development.
Thus, instrumentation is readily available, maybe for attacking yesterday’s problem.
But, what of today’s problems? What of tomorrow’s problems? This is where we need
to be perceptive in development of instrumentation to optimize the future develop-
ment of our subject. We need to train students not only in instrument design, construc-
tion, and use but in critical evaluation of such measurements. From a practical
consideration we have a challenge to develop instruments and techniques that give
us an edge for the evaluation of specific models and to enable a judgment to be
made on thei r utility. There is an impor tant caveat: The models themselves must be
designed to be amenable to such an approach through the predication of specific
instrumentation.
Thus, there is an important aspect of this concept in instrumentation evolution. If
a class of instrument is to be developed with a specific output in mind, it requires
718 MEASUREMENT IN THE ATMOSPHERE
input from the user whose ultimate aim is to justify an approach to a problem
through physicochemical insight; it requi res input from the engineer who makes
the final design; and it requires input from the design of the numerical model such
that there are very tight criteria for arguing that the model output makes sense in the
overall scheme of things. This concept applies to the choice of sensors and their
combination (as for turbulent flux measurement), it applies to the choice of wave-
lengths for assessing radiative flux budget both in solar and thermal infrared, and it
applies to characterizing shapes and sizes of nonsphe rical particles for interaction
with radiation fields of different wavelengths. The measurements made are to be
justified in terms of the whole app roach, as also is the model that is being tested for
consistency.
Thus, success in new sensor technology lies in elegance of design for specific
purposes by combining outputs to give meaningful quantities directly and measured
simultaneously in the same volume of air. This is critical and predicates an integrated
approach to instrumentation and model design at an early stage of any project in
investigation of specific atmospheric problems. An observed consistency between
model and measurement can then be used to give a better rationale both for the
combination of observations and in the predictive use of the model output.
4 COMPLEXITY AND DIVERSITY
Diversity of biological species may result from the spread of ecological niches
associated from a spread of microclimates. Measurements at widely separated loca-
tions miss such phenomena; remote sensing may miss such phenomena because of
lack of resolution. The complexity of natural phenomena as they occur in the atmos-
phere (Rind, 1999) may remain hidden if it is not realized that extremes of environ-
ment may be quite unrealistically represented; similar considerations may apply
elsewhere in our society (Arthur, 1999). Tails of distributions of such phenomena
may be quite unsuitable for extrapolation, assuming standard distributions, as has
been argued for years by the statistical community. This is certainly true for many
physical processes—extremes of temperature may lead to species elimination both
hot and cold. A few large drops lead to rainfall by coalescence and may or may not
be present depending on the presence or absence of distinctive physical processes.
Measurement of such phenomena must therefore proceed from a prior knowledge of
such mechanisms, necessary to suggest the nature of the instruments to be built and
used to verify (in its true sense) the physical processes responsible.
REFERENCES
Arthur, W. B. (1999). Complexity and the economy, Science, 284, 107–109.
Bailey, M. and J. Hallett (2002). Nucleation effects on the habit of vapour grown ice crystals
from 18 to 42
C, Q. J. R. Meteorol. Soc. 128, 1461–1483.
REFERENCES 719
Bentley, W. A., and W. J. Humphreys (1962). Snow Crystals, New York Dover (reprint of 1932
edition).
Brunt, D. (1917). The Combination of Observations, Cambridge, England, Cambridge
University Press.
Carslaw, H. S., and J. C. Jaeger (1959). Conduction of Heat in Solids, 2nd ed., Clarendon Press,
Oxford, UK.
Fitz Roy (Rear Admiral) (1863). The Weather Book: A Manual of Practical Meteorology,
London, Longman, Green, Longman, Roberts, & Green.
Hallett, J. (1996). Instrumentation and modeling in atmospheric science, Bull. Am. Meteorol.
Soc. 77, 567–568.
Hallett, J., W. P. Arnott, R. Purcell, and C. Schmidt (1998). A Technique for Characterizing
Aerosol and Cloud Particles by Real Time Processing, PM2.5: A Fine Particle Standard
Proceedings of an International Specialty Conference, Sponsored by EPA Air & Waste
Management Association, J. Chow, and P. Koutrakis, Eds., Vol. 1, pp. 318–325.
Hallett, J., M. P. Bailey, W. P. Ar nott, and J. T. Hallett (2002). Ice Crystals in Cirrus, Ch. 3,
pp. 41–77, New York, Cirrus, Oxford University Press.
Handbook (1956). Handbook of Meteorological Instruments, Part 1, Instruments for Surface
Observations, Meteorological Office, Air Ministry, London, Her Majesty’s Stationer y
Office, M.O. 577.
Korolev, A., G. A. Isaac, and J. Hallett (2000). Ice particle habits in stratiform clouds, Q. J. R.
Meteorol. Soc. 126, 2873–2902.
Lawson, P. R., A. V. Korolev, S. G. Cober, T. Huang, J. W. Strapp, and G. A. Isaac (1998).
Improved measurements of the droplet size distribution of a freezing drizzle event, Atmos.
Res. 47, 181–191.
Liu, Y., and J. Hallett (1998). On size distributions of cloud droplets growing by condensation:
A new conceptual model, J. Atmos. Sci. 55, 527–536.
Liu, Y., P. H. Daum, and J. Hallett (2002). A generalized systems theory for the effect of
varying fluctuations on cloud droplet size distributions, Am. Meteorol. Soc. ??, 2279–.
Mace, G. G., K. Sassen, S. Kinne, and T. P. Ackerman (1998). An examination of cirrus cloud
characteristics using data from millimeter wave radar and lidar: The 24 April SUCCESS
case study, G. Res. Lett. 25, 1133–1136.
Middleton, K. W. E., and A. F. Spilhaus (1953). Meteorological Instruments, 3rd ed., Toronto,
University of Toronto Press.
Nakaya, U. (1954). Snow Crystals, Natural and Artificial, Cambridge, Harvard University
Press.
Rind, D. (1999). Complexity and climate, Science 284, 105–107.
Stankov, B. B. (1998). Multisensor retrieval of atmospheric properties, Bull. Am. Meteorol. Soc.
79, 1835–1839.
Weickmann, H. K. (1947). Die Eisphase in der Atmosphare, Reports and translations, Ministry
of Supply (A) Vbkenrode (H. M. Stationery Office, London, 1947, Volkrende R&T).
720
MEASUREMENT IN THE ATMOSPHERE
CHAPTER 36
INSTRUMENT DEVELOPMENT IN
THE NATIONAL WEATHER SERVICE
JOSEPH W. SCHIESL
1 INTRODUCTION
Meteorological instrumentation is a very challenging arena. The first difficulty in
the desig n of these instruments is finding a common set of requirements upon which
the various users can agree. For example, hydrologists are generally happy with a
precipitation resolution of 2.5 mm (0.1 in.) while meteorological modelers prefer a
resolution of 0.25 mm (0.01 in.) for forecast verification.
Instruments must survive the very elements that they are measuring. There are the
temperature and humidity extremes, high winds, heavy rains, ice accretion, sand and
dust, solar insolation, ultraviolet radiation , salt spray, and altitude variations. Also,
there are the nonmeteorological elements like corrosion, fungus, insects, birds, radio
frequency interference, power line surges, pollen, and jet fuel. Many laborator y
measurement technologies do not survive the passage to the outdoors with its diverse
conditions.
There are many hundreds of instruments that have been used by the National
Weather Service (NWS) and its predecessor organizations since 1870. Some were
short lived while others that existed before the NWS are still in use. The instruments
that emerged with the more interesting histories are those that measure the meteoro-
logical parameters in which the public is most interested, i.e., temperature,
precipitation, and wind. The public wants to know if they have to dress for hot or
cold, do they need an umbrella or boots, or will the wind blow their hats off or tip
the trashcans?
Not all the instruments will be addressed, but those discussed will capture the
flavor of the challenge of the times. There are many books available for those
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
721
wishing to know a compl ete history or the detailed theor y of operation of meteoro-
logical instruments.
2 TEMPERATURE AND HUMIDITY
Air temperature readings have and still are made by liquid-in-glass thermometers.
These simple devices are based on the principle that materials expand or contract
with temperature changes. The liquids are either mercury or alcohol, depending on
the range of temperatures to be measured. Mercury can only be used to 38.9
C
where it reaches its freezing point.
Regardless of the sensor used, the temperature of the ambient air cannot be
measured properly if the sensor is not adequately exposed. The sensor must
be protected from direct and indirect solar radiation while still being exposed to a
free flow of ambient air. To provide this protection, shelters or screens are
constructed of wood and painted white with louvered sides for ventilation.
The roof is double layered to provide air insulation. The door to the shelter faces
north, in the Northern Hemisphere, to minimize the possibility of direct radiation
when the door is opened. See Figures 1 and 2. Normally, temperatures measured
in these shelters are representative of the ambient air. However, when the solar
radiation is high and the winds are below 5 knots, the shelter temperature has
been shown to be up to 3
C (5.4
F) higher than the ambient temperature. To over-
come most of this bias, some shelters are aspirated by a fan rather than depending on
natural aspiration.
Liquid-in-glass thermometers do not provide continuous temperature records, so
the possibility of capturing the maximum and minimum temperature is very remote.
A maximum thermometer is made by placing a constriction in the base of the bore
Figure 1 Cotton Region temperature shelter. See ftp site for color image.
722
INSTRUMENT DEVELOPMENT IN THE NATIONAL WEATHER SERVICE
just above the bulb. A temperature increase causes the liquid to rise, but the force of
gravity is insufficient to allow the liquid to subside. After the maximum temperature
is read, the mercury is shaken down below the constriction. See Figure 3.
In the minimum thermometer, a glass barbell-shaped object is placed in
the alcohol column. The thermometer is placed in a near horizontal position with
the bulb side down. When the alcohol contracts with decreasing temperatures, the
meniscus pushes the barbell down. The top of the barbell stays at the minimum
temperature position when the alcohol expands. After the minimum temperature is
read, the barbell is tipped back to the meni scus. The minimum thermometer can also
Figure 2 Cotton Region temperature shelter with door open. See ftp site for color image.
Figure 3 Maximum (bottom) and minimum (top) thermometers in Townsend support. See
ftp site for color image.
2 TEMPERATURE AND HUMIDITY 723
be used to obtain the cur rent temperature without disturbing its primary function.
See Figure 3.
Other methods were used to record temperatures. One consisted of two bonded
strips of metal with differing coefficients of expansion. Temperature changes caused
the bonded strip to deform because of the different expansion coefficients.
This motion was transformed by levers to make an ink trace on a chart called a
thermograph.
In the nineteenth century, scientists were aware that the resistance of a material
was a function of its temperature. This relationship was used in reverse to determine
temperature. Electrical thermometers in which the resistance of the element
increases with the temperature are known as resistance temperature devices. Those
in which the resistance of the element decreases with increasing temperature are
called thermistors. The small size of the electrical thermometer elements, usually
less than an inch in length, allows them to have quick response times.
Measuring the amount of water vapor in the air accurately, over a wide range of
temperatures outdoors, is one of the most challenging of meteorological parameters.
Some terms used to express this measurement are wet-bulb temperature, dew-point
temperature, vapor pressure, specific humidity, and relative humidity. The
instruments used to measure water-vapor content are called hygrometers.
Many materials with which the public is familiar are coarse versions of
hygrometers, although the specific values of moisture content are not apparent
with them. Many materials expand with increasing humidity, and this is exhibited
by sticking doors and windows. Certain fibers expand significantly with humidity,
which caused problems in the non-air-conditioned b uildings of the garment industry
at the turn of the century. As the sewing threads expanded in high humidity, a point
was reached where the thread could not pass through the sewing machine needle eye
easily, and the thread snagged and broke. Humidity readings were taken to determine
the proper needle eye size for that condition. This expansion property was used in
one of the first hygrometers to measure humidity. The material used was human hair.
As strands of hair changed in length with humidity changes, their movement was
exaggerated with levers causing an inked pen arm to move. This record of humidity
on a clock-driven drum was called a hygrograph. The problem with this instrument
was that each hygrograph had to be calibrated individually and often, depending on
the type of hair. Corrections also had to be made to separate the expansion caused by
temperature versus that caused by the humidity.
One of the more common manual instruments used to measure the amount of
water vapor in the air is the sli ng psychrometer. It consists of two liquid-in-glass
thermometers mounted on a metal strip. One of the thermometer bulbs is wrapped in
muslin. The muslin is dipped in distilled water and both thermometers on the strip
are whirled, which evaporates water from the wetted bulb. The temperature decreases
to a reading that will remain constant, even with further evaporation. This tempera-
ture is called the wet-bulb temperature. The amount of heat given up by the bulb is a
function of the water vapor in the ambient air. If the dry-bulb and the wet-bulb
temperature are the same after whirling, the air is saturated and the humidity is
100%. The difference in temperatures is called the wet-bulb depression. With
724 INSTRUMENT DEVELOPMENT IN THE NATIONAL WEATHER SERVICE
psychrometric tables, the dry-bulb and wet-bulb temperatures can be used to deter-
mine the dew-point temperature, the humidity, and the amount of water vapor in the
air per unit volume. Variations of this instrument were used, where the aspiration
was provided by a motor-driven fan rather than by hand slinging.
The NWS, in addition to using the sling psychrometer to determine temperature
and wet bulb, uses another instrument to measure both the temperature and the dew
point. It is called the hygrothermometer and is in use at hundreds of airports across
the United States. See Figure 4. The data are used in support of aircraft operations at
airports and by meteorologists for forecasting. These systems were first deployed in
the 1960s.
The dry-bulb temperature sensor is a resistance temperature device. The dew
point is determined by a resistance temperature device imbedded in a mirror.
A light-emitting diode provides a light source, and the dew point is determined
by the ratio of the light received by two phototransistors. One meas ures the direct
reflectivity from the source light, while the other measures the indirect. The heating
and cooling of the mirror is effected by a Peltier device. See Figure 5. As the mirror
is cooled, the formation of dew=frost on the mirror will cause an increase in the
indirect sensor level. Cooling continues until the indirect sensor level is about 80%
of the direct sensor level and the servo-loop equilibrium is achieved. This is the dew-
point temperature. The device electronically compensates for a loss of mirror
reflectivity as contaminates accumulate on the mirror surface. Another feature of
this system is a circuit used to detect the reduction of flow through the instrument.
This reduction can be caused by blockage (leaves and insects) or complete or partial
failure of the fan. Specific mirror-cleaning procedures are critical to its proper
operation. These procedures involve the use of isopropyl alcohol, lacquer thinner,
and distilled water. The criticality of following specified procedures was emphasized,
when a nonprescribed brand of cotton-tipped swabs was used to apply these
Figure 4 Hygrothermometer enclosure. See ftp site for color image.
2 TEMPERATURE AND HUMIDITY 725