MAY 2004

NOTES AND CORRESPONDENCE

825

NOTES AND CORRESPONDENCE
Temperature, Humidity, and Pressure Response of Radiosondes at Low Temperatures
STEPHEN R. HUDSON AND MICHAEL S. TOWN
Department of Atmospheric Sciences, University of Washington, Seattle, Washington

VON P. WALDEN
Department of Geography, University of Idaho, Moscow, Idaho

STEPHEN G. WARREN
Department of Atmospheric Sciences, University of Washington, Seattle, Washington
28 May 2003 and 3 November 2003
ABSTRACTThe response of radiosondes to an instantaneous change of environment was studied by taking the instrumentsfrom a warm building into the cold environment at South Pole Station. After being initialized inside, theradiosondes were carried outside and placed on the snow surface, where they were left until they reported stablevalues of temperature, pressure, and relative humidity. Three models of radiosondes were tested: Vaisala RS80,
Atmospheric Instrumentation Research (AIR) 4A, and AIR 5A.
The reported temperature equilibrated to the outside conditions within 30 s. However, it frequently took 30
min before the relative humidity outside was accurately reported. Additionally, the reported pressure rose by
several hectopascals over a 5-min period when the sonde was taken outside. In the RS80s this bias was as large
as 10 hPa, and disappeared in about 30 min. In the AIR sondes, the maximum pressure bias was never muchover 2 hPa, but seemed not to diminish with time.
The RS80s were also tested to see if, once equilibrated to the outside conditions, they could respond to smallerchanges that would be encountered in flight. The results in this case indicate that, with some corrections fortime lag, the RS80 can provide accurate data at low temperatures if allowed to equilibrate initially.
The results of these tests together indicate that the quality of upper-air data in cold regions could be improved
if radiosondes are stored and prepared at ambient temperature or are given at least 30 min to equilibrate outside
after being prepared inside.

1. Introduction
Although new methods are under development forgathering upper-air meteorological data, the atmospheric science community still relies heavily on radiosondes.
Data from radiosondes are subject to many errors, including radiational heating and cooling of the sensor
arm, which affects both the temperature and humidity
measurements (Luers and Eskridge 1995; Wang et al.
2002); calibration errors (Miloshevich et al. 2001); andchemical contamination of sensors (Wang et al. 2002),
to name just a few. These errors are sometimes worse
when the instruments are used in cold environments, as
discussed by Miloshevich et al. (2001).
Corresponding author address: Stephen R. Hudson, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle,
WA 98195-1640.
E-mail: hudson@atmos.washington.edu

q 2004 American Meteorological Society

In addition to these steady-state errors, a time-dependent error can be introduced when the radiosondes
are prepared in a warm building and are not given adequate time to fully equilibrate to the outside surface
conditions prior to launch. Mahesh et al. (1997) showed
that, because the response time of the thermistor onradiosondes is several seconds, the preparation of radiosondes in a heated building at South Pole Station
leads to a significant error in the near-surface temperature data when the balloons are released immediately
after being taken outside. Radiosonde hygrometers canbecome sluggish at low temperatures (Antikainen andPaukkunen 1994), so it is reasonable to expect that the
problem Mahesh et al. described would be even worse
for near-surface humidity data. Additionally, Hirasawa
and Kizu (1999) showed that radiosondes prepared inside a warm building report erroneous pressure datawhen they are taken outside into cold conditions.

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Atmospheric humidity over the Antarctic Plateau isof interest not only for meteorological studies, but also
to determine the feasibility of sites for infrared astronomy; the latter was the motivation for the analysis ofSouth Pole radiosonde humidities by Chamberlin(2001). However, the response of radiosonde sensors tosudden, large changes in environmental conditions, as
encountered when taken from a heated building to cold
ambient conditions, has not been well characterized. To
further investigate the utility of radiosonde data fromSouth Pole Station for analyzing the climatology of relative humidity, we conducted tests to characterize thistype of response in radiosondes that have been used at
South Pole. These trials were carried out during the
2000/01 year-long field campaign of the South Pole Atmospheric Radiation and Cloud Lidar Experiment(SPARCLE; Walden et al. 2001).
To characterize this type of response, the radiosondes
were initialized inside a heated building, then taken outside and allowed to equilibrate to ambient conditions.
All three radiosonde models used routinely at South
Pole between 1991 and 2001 were tested in this way.
These were the Atmospheric Instrumentation Research,
Incorporated (AIR), models 4A and 5A and the Vaisalamodel RS80. In addition, the RS80 radiosondes were
tested to see if, once equilibrated to ambient conditions,they could quickly respond to the smaller changes intemperature, pressure, and humidity that are encountered on a balloon flight through the atmosphere.
The instruments and tests are described in section 2.
Section 3 contains the results of the tests. A discussionof the results and some recommendations for radiosonde
users are in section 4, followed by conclusions in section
5. Although the tests were conducted only at South Pole,the results suggest that changes in radiosonde launch
procedures in many cold climates could improve thequality of operational upper-air data in those regions.
2. Instruments and methods
The radiosondes included in this study were chosen
based both on their use at South Pole, by the South Pole
Meteorological Office (currently operated by Raytheon
Polar Services Company, Denver, Colorado) and by the
SPARCLE project, and on their availability at South
Pole in 2001. In all, five combinations of radiosondes
and ground stations were included.
From 1991 through 1996 the South Pole Meteorological Office (SPMO) used the AIR model 4A for theirroutine balloon flights. One of these older sondes, manufactured in 1994, was still unopened at South Pole in
January 2001. This sonde was used for three series oftests during the year. Between tests, the sonde was stored
in a sealed bag along with desiccant. In 1997 SPMO
began routinely using the AIR model 5A, which they
continued using until August 2001. Three series of tests
were also conducted using three separate 5A sondes.
Since August 2001, the SPMO has been using RS80

VOLUME 21

radiosondes for their routine balloon launches. During
the year, four series of tests were carried out using their
RS80 system. In all four cases the data were received
by a Marwin ground station. Since 1993 Vaisala has
been producing RS80 sondes with two different humidity sensors: the A-Humicap and the H-Humicap.
While based on the same capacitive technology, the two
sensors differ in their chemical composition and dataprocessing algorithms (Miloshevich et al. 2001). Threeof the four SPMO RS80 sondes we tested had the newer
H-Humicap; the fourth had an A-Humicap. In the 2001/
02 summer, the SPMO began using the Vaisala RS90
radiosonde for some routine flights. However, as these
did not arrive at the station until after we had left, wehave no data for these newer instruments.
Finally, the SPARCLE project also used RS80 radiosondes, with the H-Humicap. The SPARCLE radiosondes transmitted through a TMAX-C board, and theirdata were transferred from the receiver to a personal
computer, via a modem. The TMAX board converts
frequency-encoded meteorological data from the Vaisala
RS80 radiosonde to a digital format with a measurement
frame rate of approximately 8 s. It was originally designed, and is still routinely used at South Pole Station,by the National Oceanic and Atmospheric Administration’s (NOAA) Climate Monitoring and Diagnostics
Laboratory (CMDL), to interface ozone sensors to an
RS80 for data transfer. Once reaching the computer, thedata were processed using TMCalc software (developed
by T. Thompson; information online at http://
63.228.74.201/tmax/index.html). In addition to sometests that were carried out specifically to characterizethese radiosondes, considerable data from SPARCLE’s
routine use of the RS80 radiosondes with TMAX boards
are also available to help characterize them; therefore,
this is the system for which we have the most data.
Although the RS80 sondes from the SPMO were powered by the water-activated batteries included in the
packaging from Vaisala, and those used by SPARCLE
were powered by lithium batteries, there was no noticeable difference between the data produced by thetwo systems.
The RS80 radiosondes measure temperature with asmall, temperature-sensitive capacitor, consisting of twoelectrodes separated by a ceramic dielectric. The humidity is measured with their Humicap technology,
which separates two electrodes with a thin polymer film.
Two different films are used, depending on whether itis the A-Humicap or the H-Humicap, but both cause thecapacitance to vary depending on the amount of water
absorbed by the polymer film and on the film temperature. The proprietary Vaisala routines correct for thetemperature dependence of the Humicap. Pressure ismeasured using a capacitive aneroid barometer, in which
two electrodes are separated by a distance that varies
depending of the volume of an expandable, partiallyevacuated cell. These sensors are briefly described inthe Vaisala’s brochure on the RS80 (Vaisala 1997), andMAY 2004

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NOTES AND CORRESPONDENCE

TABLE 1. Summary statistics of exponential decay time constants (s)for temperature response of RS80, AIR 4A, and AIR 5A sondes.

FIG. 1. RS80 temperature response upon being moved outside froma 1188C building when the outdoor air temperature was 2668C on
20 Sep 2001. The asterisks are the reported data, at 8-s intervals, thesolid curve is an exponential decay fit to the data, and the dash–dot
line approximates what would likely have been recorded with higherresolution data with a gap left near the transition.

on the Web site of the British Atmospheric Data Centre(http://badc.nerc.ac.uk/data/radiosonde/radhelp.html).
Both types of AIR sondes used small thermistor beads
to measure the temperature, and capacitive aneroid barometers to measure pressure. The newer, 5A sondemeasured humidity with a thin polymer-film capacitancehygrometer, while the older, 4A sonde measured humidity with a carbon hygristor (R. Shellhorn, VaisalaInc., 2003, personal communication).
AIR radiosondes have not been widely used operationally (Connell and Miller 1995), so their characterization is of interest mainly because of their past use atSouth Pole. On the other hand, the RS80 is routinely
used not only at South Pole, but also at over 400 upperair stations around the world, as of December 2002
(WMO 2002).
The purpose of our investigation was to determine
the response time and other characteristics of the sensors’ response to environmental changes, not to determine their absolute accuracy. Therefore, no corrections
were applied to the data produced by the receiving systems. Also, as is routine at South Pole, no ground checkprocedures were performed beyond seeing that initialreports inside were accurate.
In order to characterize the radiosonde response tosudden, large changes in temperature and humidity, aradiosonde was initialized inside either a heated building, with the indoor temperature between 08 and 1258C,
or an unheated building, with a temperature of 2438 to
2558C. Shortly after being initialized it was taken outside, and placed on the snow surface or about 5 cm
above it, where it remained until it reported stable valuesof temperature, humidity, and pressure. Sometimes theradiosonde was then returned to the heated building,Type of sonde

RS80

4A

5A

No. of tests
Min
Median
Max

33
2.6
5.2
10.9

5
4.8
6.3
10.2

6
5.6
6.7
7.6

and allowed to equilibrate, and the procedure was repeated.
In all these tests the instrument was not ventilated,
except by any ambient wind. At the times of the tests,
outside temperatures ranged from 2248 to 2718C, andwere between 17 and 94 K colder than the temperatureinside the building.
In addition to the tests involving large changes, theRS80 radiosondes were also tested to see if, once equilibrated to cold outside conditions, they could respondto small changes in a timely manner. To do this, a pulley
was placed at the top of a 22-m tower at South Pole.
The radiosonde was attached to one end of a line thatran from the surface, through the pulley, and back tothe surface. The instrument was then raised from thesurface to about 20 m above the surface, where it washeld for 2 min, then lowered to the surface.
This procedure was carried out a total of 8 times, on
two winter days. In all the experiments on the tower,
the radiosonde used was an RS80 with an H-Humicap,
transmitting through a TMAX-C board. On both days
there was a strong surface-based temperature and humidity inversion, so that at the top of the tower thetemperature was 3–5 K higher and the relative humidity
was 3%–5% higher than at the surface (all differencesin relative humidity are given as absolute differences,e.g., 65% is 5% greater than 60%). The approximate
rate of ascent and descent of the radiosonde ranged from
0.4 to 1.0 m s 21 , significantly slower than the 3–6 m
s 21 typical of a free-launched balloon in the lower atmosphere.
3. Resultsa. RS80 response to large changes in temperatureand humidity
Figure 1 shows a representative response of temperature reported by an RS80, after being taken outsidefrom a warm building. In this case, the reported temperature decayed exponentially toward the outside temperature, with a time constant of 4.7 s. In all of the 33
cases in which data were recorded as an RS80 was
moved from inside to outside, the temperature responded in a fashion similar to Fig. 1. The exponential decay
time constants for these tests, summarized in Table 1,
have a median value of 5.2 s. The variability of the time
constants and the lack of correlation with thermal shock
are shown in Fig. 2a. Since most of the tests began at

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FIG. 2. Scatterplots of (a) e-folding time for the RS80 temperature response, (b) time t1 before the exponential decay in RS80 relative
humidity response, (c) e-folding time for the RS80 relative humidity response after time t1 , (d) time to reach maximum error in RS80 pressure
response, (e) maximum error in RS80 pressure response, and (f ) e-folding time for the RS80 pressure response, all vs the change in temperatureas the sonde was moved from inside to outside. The crosses indicate data collected as a sonde was moved outside from an unheated building;
all other data are plotted as dots.

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TABLE 2. Times associated with the RS80 relative humidity response curves in different situations. The time interval t 1 is the amountof time that passed from when the instrument was moved outside towhen the exponential decay began, with an e-folding time of t . Medians are given only for winter cases because of the small numberof tests in summer and unheated winter cases. All sondes had the HHumicap except for the one used in the unheated winter cases, which
had an A-Humicap.

No. of tests
Inside temp (8C)
Outside temp (8C)
Min t 1 (s)Median t 1 (s)Max t 1 (s)Min t (s)Median t (s)Max t (s)FIG. 3. RS80 humidity response for the same case as in Fig. 1.
The relative humidity indoors was about 47% and outdoors was about
53% (101% w.r.t. ice). The dots are the reported data, at 8-s intervals,
and the solid curve is an exponential decay fit to the data.

temperatures between 1108 and 1208C, scatterplots
versus outside temperature were very similar to thoseshown in Fig. 2, and are not shown here.
The relative humidity response of the RS80 was not
as well behaved as the temperature response. Figure 3
shows the reported relative humidity from the same experiment as in Fig. 1. The time constant of the exponential decay varied considerably throughout the year,
but the shape of the curve in Fig. 3 is representative of
those from all the tests, regardless of temperature ortype of RS80. When taken outside, the reported humidity initially drops, then exponentially decays toward
the value of the outdoor relative humidity. The initial
decrease in reported humidity could be caused by athermal lag in the hygrometer, which would lead to thehygrometer being warmer than the ambient temperature,
leading to an improper temperature correction being applied in the calculation of relative humidity.
In Fig. 3, the exponential decay does not begin until
about 70 s after the radiosonde is taken outside; thedecay to equilibrium has an e-folding time constant of
320 s. Table 2 summarizes the responses that were observed in different conditions. In tests done in the summer, the time interval from when the radiosonde wastaken outside to when the exponential decay began
ranged from 10 to 30 s, and the e-folding time constantranged from 13 to 20 s. In winter, the time before theexponential decay began ranged from 5 to 120 s, andthe e-folding time ranged from 30 to 420 s; neither time
showed any apparent correlation with the outside airtemperature or with the magnitude of the thermal shock.
During the winter there were also two cases in which
the radiosonde was taken outside from a building at
2438C when the outside temperature was 2608C. Although this was the only time a sonde with an A-Humicap was used, the response to this situation was similar

Summer

WinterUnheated
winter5
18 to 124
224 to 225
10
—
30
13
—
20

25
12 to 124
245 to 271
5
30
120
30
140
420

2
243
260 to 261
65
—
90
50
—
120

to other winter cases, with times before exponential decay began of 65 and 90 s, and e-folding times of 50
and 120 s.
Figure 2b shows the variability in the times before
the exponential decay began, and Fig. 2c shows thevariability in the e-folding times for the relative humidity response. It is also clear from these two figuresthat there is no correlation between temperature change
and either of these values for the many cases with temperature changes between 60 and 100 K.
A final interesting note on the relative humidity regards the accuracy of the equilibrated measurement. In
Fig. 3 we can see that, after equilibration, the reported
relative humidity was 52.6%. At the ambient temperature of 266.48C, this corresponds to a relative humidity
with respect to ice (RH i ) of 100.9%, which is close to
the values reported by a nearby frost-point hygrometerat the same time: 102%–105%. Typically, the RS80s
used by SPARCLE equilibrated to within 65% of the
RH i that was reported by a nearby frost-point hygrometer, even when supersaturated. Relative humidities withrespect to ice were reported as high as 123%, and were
frequently between 100% and 110%. All conversions
between RH i and relative humidity with respect to water(RH w ) were done using the equations for saturation vapor pressure over water by Wexler (1976) and for saturation vapor pressure over ice by Hyland and Wexler(1983), to be consistent with Vaisala’s calibration (Miloshevich et al. 2001).
Not all of the RS80 radiosondes were able to perform
this well. SPARCLE’s radiosondes were packaged witha new sealed protective cover over the sensor arm, which
Vaisala began using on all RS80s in June 2000. This
cover eliminated a dry bias that was caused by contamination of the hygrometer resulting from the packaging,described by Wang et al. (2002). The RS80s used by
SPMO did not have this new sealed cover. Their RS80s
with the H-Humicap generally showed a dry bias, andnever reported supersaturated conditions. In the one test

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TABLE 3. Times associated with the RS80 pressure response curves
in different situations. The time interval t1 is the time elapsed before
the reported pressure reached its maximum value; dP is the difference
between the maximum reported pressure and the actual pressure; t
is the exponential decay time constant of the response after the maximum was reached. Medians are given only for winter cases becauseof the small number of tests in summer and unheated winter cases.

FIG. 4. Pressure reported by an RS80 after being taken outside forthe same case as in Figs. 1 and 3. The dots show the reported data,
and the solid curve is an exponential decay fit to the latter part ofthe data.

that was done using an SPMO RS80 with the A-Humicap, at a temperature of 2618C, and RH w (from thefrost-point hygrometer) of about 60%, the sonde showeda dry bias of about 30%, or roughly a factor of 2, consistent with Miloshevich et al. (2001).
b. Response of the RS80 pressure sensor
As reported by Hirasawa and Kizu (1999), the RS80s
showed a response in the reported pressure when taken
outside from a warm building. Figure 4 shows that, upon
being moved outside from a building at 1188C when
the outside temperature was 2668C, the reported pressure initially increased, until reaching a value about
8–9 hPa higher than the actual pressure. After about 300s the pressure began to recover, and then exponentiallydecayed toward the actual pressure, with a time constantof 330 s. While the pressure bias always decreased tonear zero after 30–40 min, it frequently recovered to avalue slightly higher than was reported before being
taken outside, as seen in Fig. 4. Going back into thewarm room, the response was the mirror image of Fig.
4: a decrease of 8 hPa and a slow recovery.
This kind of response is seen in the pressure data at
all temperatures at which the tests were conducted. The
characteristics of the response in different conditions
are summarized in Table 3. In the cases in which theradiosondes were moved between a heated building andoutside during the winter, the time for the reported pressure to rise to its maximum ranged from 175 to 450 s,with a median and mean of 300 s. The maximum error
in the reported pressure in these cases ranged from 3.5
to 10 hPa, with a median and mean of 6 hPa, and thee-folding time in the latter part of the response ranged
from 230 to 600 s, with a median and mean of 380 and
385 s.
In the two tests done using an unheated building in

No. of tests
Inside temp (8C)
Outside temp (8C)
Min t1 (s)Median t1 (s)Max t1 (s)Min dP (hPa)Median dP (hPa)Max dP (hPa)Min t (s)Median t (s)Max t (s)Summer

WinterUnheated
winter

1
124
225
450
—
450
3.3
—
3.3
N/A*
—
N/A*

25
12 to 124
245 to 271
175
300
450
3.5
6.0
10.0
230
380
600

2
243
260 to 261
275
—
400
0.4
—
1.6
N/A**
—
N/A**

* Too little data to characterize recovery.
** Data were not exponential. Correct pressure was reported about
1500 s after sonde was moved outside.

winter, 275 and 400 s elapsed before reaching maximum
errors of 0.4 and 1.6 hPa. The data after the maximum
error were more linear than exponential, but about 1500s passed before they fully recovered, similar to the timerequired in the other tests.
Unfortunately, we did not notice this strange behaviorof the pressure sensor until winter (and were unawareof the work of Hirasawa and Kizu), and we departed
South Pole in late spring, so we do not have completed
experiments at higher temperatures. There was one casein the previous summer in which a radiosonde was taken
outside to 2258C and left stationary long enough toreach the peak in the pressure response curve. After 450s the reported pressure was 3.3 hPa higher than it was
initially; it was taken inside too soon to get enough data
to characterize the recovery. In two other summer cases,
at 2248 and 2258C, the pressure error reached 2.1 and
2.2 hPa, but had not started to recover before being
moved.
Figures 2d–f show the variability in the three characteristics of the pressure response: time to reach peakerror, magnitude of peak error, and e-folding decay time.
We can see that, from the data available, it appears thetimes t1 and t that characterize the pressure response
are not functions of thermal shock over the range thatthese data cover. The maximum error in reported pressure, on the other hand, seems to increase with increasing thermal shock. Figure 5 shows the maximum error
versus temperature change for all cases, including thosewhere the sonde was moved from outside to inside. Here
we can see a linear relationship between the maximum
pressure error and the magnitude of the thermal shock,such that dP 5 20.070(dT) 1 0.575, where dP is the
maximum pressure error in hectopascals and dT is the

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NOTES AND CORRESPONDENCE

FIG. 5. The maximum error in pressure reported by an RS80 (dP)vs the difference between starting and ending temperature, for allcases, including those that went from outside to inside (positive temperature changes). The results of a linear regression on the data arealso shown.

change in temperature in kelvins. This equation accounts for 87% of the variance in these data.
Hirasawa and Kizu (1999) conducted similar tests
with RS80 radiosondes at Dome Fuji Station on the East
Antarctic Plateau, whose climate is similar to that at
South Pole. They presented results from eight tests, withtemperature differences ranging from 46 to 75 K. The
character of the pressure response reported for their tests
was similar to what was seen at South Pole, with the
exception that the maximum pressure errors reported by
the Dome Fuji tests were generally only 50%–75% ofwhat was observed at South Pole. The Dome Fuji tests
were carried out much like those at South Pole, except
that, when outside, the sondes were in a snow cave,rather than on the snow surface. The ambient pressurein the Dome Fuji tests was between 610 and 630 hPa,
while at South Pole it was between 660 and 700 hPa.
To explain the pressure error we must discuss thedesign of the instrument. The aneroid cell in the RS80
provides pressure information by measuring the volumeof an expandable, partially evacuated chamber. Since
this volume will be affected by both the external pressure and the temperature of the aneroid cell, an adjustment must be made to account for the temperature of
the cell. Significant errors in reported pressure are possible from this type of sensor when temperature gradients develop across the aneroid cell and associatedelectronics (WMO 1996). It seems likely that this pressure error is a result of such temperature gradients thatdevelop in response to the sudden change in ambient
temperature. Figure 6 shows how this error in pressure
seems to be closely related to the rate of change of thisinternal temperature with time. This explanation provides hope that the pressure response may be less of aproblem operationally, when the sonde is well ventilated.

831

FIG. 6. The error in pressure reported by an RS80 vs the rate ofchange of the internal temperature with time (dTint /dt) for the samecase as is shown in Fig. 4.

Hirasawa and Kizu (1999) cited a personal communication with Vaisala indicating that strain on the metal
leads in the aneroid cell (used to measure its volume
through capacitance), caused by contraction of the metalin the cold, can lead to pressure errors at low temperatures. Perhaps this explains the roughly 0.5-hPa bias
often seen in the data after the sonde equilibrated.
This error in reported pressure will lead to an apparent
temperature error in a sounding since the temperature
measured at a particular pressure level will appear tohave come from a lower height (higher pressure). To
estimate the magnitude of the error that could be caused
in a temperature sounding by this pressure response, apolynomial was fit to data similar to those shown in
Fig. 4, from a day with an outside temperature of
2618C, and from which the original pressure had been
subtracted. This polynomial was then used to correct
the pressure data from a routine sounding taken at 0000
UTC 16 October 2001, when the surface temperature
was 2618C. No other data corrections were applied.
In performing this calculation, it was assumed thatthe error in the reported pressure during the flight was
the same function of time that it was during a groundtest at the same surface temperature. At least two factors
are likely to reduce this error during a flight: the increased ventilation, and the fact that the sonde does notremain at a constant temperature during flight. Thus,
these results probably represent a maximum possible
error.
The magnitude of the resulting error is clearly highly
dependent on the shape of the temperature profile and,in particular, on the rate of change of temperature withheight. The sounding used here represents a typical winter sounding at South Pole.
The reported profile is shown in Fig. 7a, and the error
estimation in Fig. 7b. The surface pressure at the timeof this sounding was 671 hPa. The surface data in the

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FIG. 7. (a) The temperature profile from the sounding at 0000 UTC 16 Oct 2001, made using an RS80, and (b) the estimated error in thetemperature profile that results from the pressure error.

profile were entered from surface observations, andwere excluded from the correction. Although the maximum pressure error (not shown) did not occur until thesonde had passed 550 hPa, we can see that the maximum
apparent temperature error, nearly 2 K, occurs near thesurface, where the magnitude of the lapse rate is verylarge. The pressure bias does not fully disappear until
the sonde reaches about 250 hPa.
c. AIR response to large changes

FIG. 8. Relative humidity reported by an AIR 4A sonde after being
taken outside from a 148C building on 18 Jan 2001, when the outdoor
air temperature was 2268C. The plus signs are the recorded data andthe solid curve is an exponential decay fit to the latter data.

The response of the temperature data reported by the
AIR 4A and 5A sondes, after being taken outside, wassimilar to that of the RS80s shown in Fig. 1. The exponential decay time constants for the temperature response of the AIR sondes are summarized in Table 1.
At temperatures above 2408C the 4A sonde reported
reasonable values of relative humidity within about 2
min of being taken outside. Figure 8 shows the responseof the reported relative humidity for a case when thesonde was moved outside when the ambient temperature
was 2268C. The reported RH w quickly begins to in-

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833

NOTES AND CORRESPONDENCE

TABLE 4. Characteristics of the AIR 4A relative humidity response
to large changes in ambient conditions.
Date

OutsideT (8C)

t1
(s)

t
(s)

RH w
(%)

Actual
RH w (%)

18 Jan 2001
18 Jan 2001
18 Jan 2001
8 Feb 2001
8 Feb 2001
7 Aug 2001

227
226
227
239
239
268

40
75
30
125
100
250

60
40
30
70
30
4000

76
70
76
69
65
35

73
73
73
59
59
55

t1 5 time before exponential decay began.
t 5 e-folding time constant.
RH w 5 final reported relative humidity w.r.t. water.
Actual RH w 5 RH w reported by a nearby, stationary instrument.

crease, at first with the rate of change increasing withtime. After about 75 s, it follows an exponential decay
toward the value of the outdoor relative humidity, witha time constant of 40 s. The shape of this response curveis similar to all of those for tests at temperatures above
2408C. Table 4 gives the response times for the various
tests that used the 4A sonde, as well as the value towhich the relative humidity equilibrated.
The final row in Table 4 shows data from the one test
that used the AIR 4A sonde in the winter. In this case,the equilibration not only took well over an hour, butalso, unlike the other 4A cases, reached a value muchdrier than that reported by other instruments at the sametime.
Although the older (4A) sonde seems capable of reporting reasonable relative humidity data to temperatures at least as low as about 2408C, the newer AIR
5A sonde was grossly inaccurate at all temperaturesencountered at South Pole, as Chamberlin (2001) also
concluded. When taken outside, these sondes typicallyshowed a strong dry bias and very slow response, or
quickly stabilized at a relative humidity of 1%. Thisbehavior was seen in the ground tests as well as in
SPMO’s routine flights with the 5A sondes.
As with the RS80s, the pressure reported by the AIR
sondes was affected by large temperature changes. Figure 9 shows the reported pressure as an AIR 5A sonde
was taken repeatedly inside and outside, between temperatures of about 1108 and 2638C. In this case thepressure increased by about 2 hPa when taken outside,and decreased by the same amount when taken inside.
Unlike the RS80s, there is no indication in this case thatthe data recover to the correct value after being taken
outside. The AIR sondes also frequently reported spurious data when used at the surface, as is seen in Fig.
9 near 5, 15, and 50 min. It is clear that the responsewhen the sonde was moved inside was not the mirror
image of when it was moved outside, but instead showedsome peculiar timing, especially after the final transition
shown in Fig. 9. It is not known what caused this timing
while inside, but at least some of it may have been aresult of changing conditions in the building, due to the

FIG. 9. The pressure reported by an AIR 5A sonde on 6 Jun 2001
as it was repeatedly moved between a heated building and outside;
the outside temperature was 2638C. Dots show the recorded data,and solid vertical lines indicate when the sonde was moved insideor outside.

heater being on (the indoor temperature varied between
258 and 1258C while the sondes were inside).
The data shown in Fig. 9 are the most extreme example of this response that we saw in the AIR sondes,
but similar responses, of smaller magnitudes, were seen
in most cases that tested the AIR 4A or 5A sondes inthis way. The results of these tests are shown in Table
5. After a transition from 08 to 2688C a 4A sonde
showed an increase in reported pressure of 1.7 hPa. In
the unheated winter cases, the 4A sonde showed pressure increases of 0.7 hPa, while the 5A sonde showedsmall decreases in reported pressure. At temperatures
between 2258 and 2408C the 4A sonde showed a pressure increase of 0.1–0.5 hPa when taken outside froma heated building. There are no similar summertime data
for the 5A sonde.
TABLE 5. The change in reported pressure (dP) from AIR sondes
when taken from inside to outside.
Date

InsideT (8C)

OutsideT (8C)dP(hPa)

AIR-5A sondes
6 Jun 2001
6 Jun 2001
16 Oct 2001
16 Oct 2001

16
13
242
243

263
263
261
261

2.2
1.8
20.1
20.3

AIR-4A sondes
18 Jan 2001
18 Jan 2001
18 Jan 2001
8 Feb 2001
8 Feb 2001
7 Aug 2001
7 Aug 2001
7 Aug 2001

12
14
14
15
11
0
255
255

227
226
227
239
239
268
268
268

0.3
0.1
0.1
0.5
0.4
1.7
0.7
0.7

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d. RS80 response to small changes after equilibration
Figure 10 shows a sample of the data collected while
an RS80 descended from near the top of the 22-m tower.
In the case shown, the data begin after equilibration to
the conditions at the top of the tower, and they extend
through the equilibration to surface conditions. The vertical lines in the figure show when the descent, which
took 34 s, began and ended.
The pressure generally responded to within the levelof the noise by the time the ascent or descent was completed. This is apparent in Fig. 10a, where the reportedpressure 1 s before the end of the descent was approximately equal to the pressure reported while at the surface. Furthermore, it is encouraging that, in all cases,
the pressure differences between the top and bottom of
the tower agreed well with what is expected from the
hypsometric equation. For the case in Fig. 10, using alayer-averaged temperature of 2618C and a surfacepressure of 691.5 hPa, we would expect the pressure at
20 m to be about 2.2 hPa lower than at the surface,similar to what was observed.
Figure 10b shows that the sonde responds to the temperature change with a lag. There is one data point,
reported 7 s after the descent was completed, that is still
indicating a temperature somewhat warmer than the later data, suggesting that it took the sonde 8–15 s, after
reaching the surface, to equilibrate to the temperaturechange. This delay was seen in all of these tests, but,given that the time between successive data points isnearly as long as the response time, it is not possible
to characterize the response precisely.
The relative humidity response is shown in Fig. 10c.
Full equilibration of the relative humidity was typicallyreached about 15–20 s after the descent ended. As withthe temperature data, precise characterization of this response was not possible.
4. Discussion
The data presented in section 3 require some caveats,
and will hopefully inspire more work on this subject.
A better, more robust characterization of the radiosondes
would be possible if more radiosondes were used tocollect a larger set of data, and if these experiments
were repeated under more controlled conditions, while
ventilating the instruments as they would be during
flights. Given that the data shown here often were collected with little ventilation, the results may be closer
to the maximum errors that occur when radiosondes areprepared in a heated building than to the average errors.
Clearly the data in section 3d would be more applicable if ascent and descent rates were closer to thoseexperienced in a typical radiosonde flight. While thefaster ascent rate of a balloon causes more ventilationof the instrument, it also carries it through the changed
ambient conditions more quickly. These opposing factors make it difficult to know whether the lag experi-

FIG. 10. The reported (a) pressure, (b) temperature, and (c) relative
humidity w.r.t. ice as an RS80 was lowered from approximately 20
m above the surface to the surface on 19 Jul 2001. The dots indicate
the data, at 8-s intervals, and the vertical solid lines show the times
when the descent began and ended.

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835

NOTES AND CORRESPONDENCE

enced in operational use of the RS80s is likely to bebetter or worse than that observed in these experiments.
Despite the less-than-ideal conditions just noted,
these data provide not only some valuable information
applicable to radiosonde preparation procedures aroundthe world, but also another reason to be cautious whenusing archived radiosonde data in climate studies (Elliottand Gaffen 1991).
Although the data were collected at South Pole, theirapplication should not be limited to the Antarctic Plateau. The conditions at South Pole during the summer
are similar to those found at many continental, midlatitude, Northern Hemisphere stations in winter. Furthermore, Arctic, sub-Arctic, and coastal Antarctic stations
experience these conditions during long portions of theyear, and at times have conditions similar to those insome of the winter cases presented here.
Ideally radiosondes would be stored and prepared inambient conditions to eliminate any bad data resulting
from preparation in a climate controlled building. It isevident from the tests using an unheated building thateven relatively small differences between inside andoutside conditions should be avoided, indicating thatany shelter that is used would need to be unheated andwell ventilated.
Working in extreme cold can obviously be difficult.
Not only is it hard on the observer to prepare the balloon
and sonde in very cold temperatures, but also the cold
can make preparation of water batteries and connection
and manipulation of rubber-insulated battery leads impossible. Therefore there are likely times and places
where the storage and preparation of radiosondes will
have to be done inside a heated building. If this is thecase, these data suggest that the instrument should beplaced outside for at least 30 min prior to launch toallow for equilibration. While not shown here, data collected when sondes were moved from outside into abuilding showed similar responses in pressure, but of
opposite sign. They also indicated that the humidity
sensor collects a large amount of frost very quickly,
causing erroneous relative humidity reports. Thus, thesonde should not be brought back inside after equilibration, even for a very short period to attach it to theballoon.
The data presented in section 3d indicate that the
RS80s are capable of responding reasonably quickly tochanges encountered in the atmosphere once they haveequilibrated to the initial shock of being taken outside.
This indicates that by providing a time lag correction
to the data, as in Mahesh et al. (1997), and by following
the recommended preparation procedures, these radiosondes are capable of providing more accurate data thanwith current procedures, even in a cold boundary layer
with a steep inversion. Such procedural changes could
contribute to a large improvement in upper-air data fromcold regions.
It seems apparent that while procedural changes maylead to improved data quality, it is still necessary tocontinue pursuing improved technologies that are inexpensive enough to be incorporated into future radiosondes. Decreasing the effect of thermal shock on the
instruments and shortening their response times are improvements important for making radiosondes fully suitable for operational and research use.
5. Conclusions
Vaisala RS80, AIR 4A, and AIR 5A radiosondes were
tested at South Pole to characterize their response tobeing taken outside in cold conditions after being prepared in a warm building. The results show that, while
the reported temperature can equilibrate in under 30 s,
the reported relative humidity and pressure take much
longer to equilibrate after the temperature change.
Radiosonde hygrometers are notoriously slow to respond at low temperatures, so it was not surprising tofind that a half hour was frequently required for the
instruments to report accurate humidity measurements
after being taken outside. It is perhaps less well known,
however, that it also takes this long for the radiosondes
to report an accurate pressure.
While these results were discouraging, results fromthe tower experiment indicate that the RS80 is capableof accurately reporting changes of pressure, temperature, and humidity in the atmosphere, once equilibrated
to ambient conditions. These responses were not always
as rapid as the changes were encountered, but they did
occur fast enough that the data could likely be correctedwith a time lag correction.
It appears that either storing and preparing radiosondes in ambient conditions, or allowing them at least
30 min to equilibrate after being taken outside from awarm building, could make a substantial improvementin the quality of upper-air data collected at cold stations.
Acknowledgments. We thank Dar Gibson, MeghanPrentiss, and Nathan Tift, meteorologists at South Pole,for giving their time and equipment to help us in thisstudy. Richard Brandt provided us invaluable technicalassistance, especially with the TMAX boards. The Hirasawa and Kizu (1999) paper was translated from Japanese to English by Dr. Madelon Mottet; copies of thistranslation are available from Stephen Hudson. The
comments from three anonymous reviewers were helpful and appreciated; one of the reviewers alerted us to
the paper by Hirasawa and Kizu. This research was
supported by NSF Grant OPP-97-26676.
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