Comparisons between Vaisala RS90 and Snow White relative humidity measurements from
the WMO GPS radiosonde Comparison in Brazil (2001) and Ascension Island (1999)
Mr. R. Smout, Dr. J. Nash, Mr. D. Lyth and Mr. J. Elms
Met Office, Beaufort Park, South Road, Easthampstead, Wokingham, Berkshire, RG40 3DN, UK
Tel. +44 (0) 845 300 0300, Fax. +44 (0) 1344 85 5897, E-mail. radiosonde@metoffice.com
Introduction
The results presented in this paper are from two trials,
the first in support of aircraft research at Ascension
Island (1999) and then the WMO GPS trial held at
Alcantara in Brazil 2001. A main aim of the tests for
the Met Office was to evaluate and develop the
humidity sensing of the Vaisala RS90 and Meteolabor
Snow White chilled mirror hygrometer in a tropical
climate. The Snow White sensing system is
manufactured by Meteolabor, Switzerland and in these
tests was flown with Sippican MKII radiosondes.
Sippican, Meteolabor and Vaisala contributed to both
tests.
Operational Performance of Snow White
At Ascension, 17 Snow Whites were flown with the
Vaisala RS90 radiosonde and at Alcantara 21 SnowWhites. Snow White measures dewpoint or frost point
depending on the state of the film on the mirror. In
very dry conditions the peltier cooler on the Snow
White may not be able to cool sufficiently to maintain
the film. In this situation the system can be become
insensitive to humidity increase until the dewpoint rise
above the mirror temperature and the water film
reforms. In these tests, the loss of the film in flight was
not significant. Conversion of Snow White dewpoint to
relative humidity was computed using the RS90
temperature measurements. As a 1ºC error in
atmospheric produces around 6 per cent error in
relative humidity when computing relative humidity
from dewpoint, it is essential to have as accurate air
temperatures as possible. The change in phase of the
water surface on the Snow White mirror can be
detected by comparison with the RS90 measurements
on many flights. This usually occurs when the mirror
temperature is between -20ºC and -30ºC. A correction
was made after the basic Sippican in flight processing
was completed to ensure that relative humidity relative
to a water surface was computed for all heights.
Figure. 1 illustrates the good agreement in measuring
detailed vertical relative humidity structure as a
function of time into flight, obtained from the Vaisala
RS90 and Snow White systems in Alcantara. In Figure
2, the Snow White measurements showed a lot more
structure in the vertical on smaller scales than the
RS90. However, on this occasion the Vaisala ground
station software had smoothed out the structures, which
the RS90 sensor had sensed, see raw data.
Unfortunately, the Snow White sensors deployed at
Alcantara had a modified feedback control to speed up
the sensor. This led to instability in measurements at
lower temperatures, which had not occurred at
Ascension Island. These fluctuations were not
supported by raw RS90 measurements. The result was
that Snow White measurements could not be safely be
used as a reference below temperatures of -50ºC in the
Brazil test. The oscillations in dewpoint temperature
can be seen in the Snow White dewpoint trace between
minutes 30 and 60 into flight in Fig. 3. The oscillations
died away at about minute 60 and this leads to the
suspicion that at the lowest internal radiosonde
temperatures the power being delivered to the Peltier
cooler for the Snow White mirror may have been
inadequate to ensure cooling to the true frost point. In a
similar set of dewpoint measurements from Ascension
Island, see Fig. 6, the dewpoint trace was much less
noisy than the Alcantara measurements.
Large positive relative humidity anomalies (more than
a hundred percent) were produced by Snow White at
upper levels in early Alcantara flights. The Snow
White system heats the sensor area when it is
recognised that relative humidity is very high and rain
drops or ice crystals could enter the sample volume,
upsetting the film on the sensing mirror. The sensor
volume heating was going on for much too long when
it was initiated in thin cirrus cloud at Alcantara. This
caused the dewpoint temperature to be much too high
for up to 10 minutes after leaving cloud. Thus,
additional covers were used to cover the duct entrance
to the Snow White sensor area, during most of the
Alcantara trial. This made it difficult for ice crystals to
enter the sensor volume and trigger the heating.
Previous test work in the UK had shown that the design
of the Snow White sensor duct ensured adequate flow
of air past the sensor even when the duct entrance was
almost entirely covered. Excessive heating of the
sensor volume was reduced although not totally
eliminated in the remaining flights
Interpretation of in flight comparisons
Figure 3 shows dewpoint temperature measurement
from flight 11 as a function of time into flight. Here
minute 60 corresponds to a pressure of 88 hPa and is
close to the tropopause. Vaisala values of dewpoint
were computed from temperature and relative humidity
measurements and Snow White values are actual
measurements. Fig 4. Shows the corresponding
temperature and relative humidity measurements. On
this flight the RS90 and Snow White relative humidity
measurements were very close up till minute 50
(pressure 144 hPa, temperature -70ºC. After this the
RS90 appeared to be slower in response than the Snow
White, but as noted earlier the frostpoint reported by
the Snow White was probably too low because of
problems with the battery supply at the lowest
temperatures.
Figure 5 shows a similar temperature and relative
humidity plot from the trial on Ascension Island. At
minute 49 the Snow White shows a much larger
positive increment in relative humidity than the Vaisala
RS90. This was at a temperature of –65 ºC and a
pressure of about 170 hPa. At minute 72 the Snow
White's relative humidity increased and this is taken as
a symptom of the gradual failure of the peltier cooler to
deliver sufficient cooling to give the correct frostpoint.
When the remainder of the relatively reliable
comparisons are reviewed, the main discrepancies in
response between the two humidity systems seem to
start at about 150 hPa, and temperatures around –70 ºC.
This requires further investigation, once Snow White
has been improved to ensure that adequate cooling can
be supplied to pressures down to 50 hPa.
would be that the humidity sensors are sensing air at a
higher temperature than ambient after the air was
heated by passing over some of the surfaces
surrounding the sensors. Certainly, the RS90 sensors at
Brazil agreed with a Thygan reference before launch to
within 1 per cent in daytime and nighttime conditions.
On the ground , the surface winds were strong and the
air reaching the RS90 sensors was not usually passing
over any heated surface before encountering the sensor.
For the temperature band -15 to -30ºC, the conditions
differed between the tests, with high humidity not
encountered at Ascension. Systematic differences
between Vaisala RS90 and Snow White remained
lower than 4 per cent at night, whilst the Vaisala RS90
measurements were clearly lower by around 6 per cent
in the daytime comparisons.
Relative humidity differences between the Sippican
Hygristor used with Snow White Day and Snow White
Night are shown for the temperature band 0 to 15 ºC
(heights 3 to 5 km) in Fig. 7, and for the temperature
band –15 to –30 ºC (approximate heights 8 to 10 km)
in Fig. 9. The relative humidity differences between the
Vaisala Rs90 and Snow white for the same temperature
bands are shown in Figs. 8 and 10. Note the humidity
difference scales are quite different between the
Sippican and Vaisala RS90 plots. The thick black line
indicates nighttime comparisons whilst the light grey
lines are for daytime comparisons. The results from
Ascension Island are indicated by diamonds and the
result from Brazil are indicated by circles.
The reproducibility of the Sippican relative humidity
measurements is estimated at between 3 and 6 per cent
(better at high than low humidity) from the results in
Fig. 11, compared to better than 2 per cent for the
Vaisala RS90 measurements. Thus, the sample sizes in
these tests were not totally adequate to define longterm systematic bias for Sippican to the same degree of
certainty as with the Vaisala system. At low humidity
(less than 20 per cent) the Sippican sensors indicated
values 5 to 10 per cent higher than Snow White in
Brazil and about 10 per cent lower than Snow White in
Ascension. This is a clear demonstration of the main
weakness of the carbon hygristor sensor, that sensor
measurements are not reproducible at low humidity.
This problem was larger at the lower temperatures ,as
seen in Fig.9. At high humidity, the hygristors used
with the Snow White at night in Brazil had a strong
positive bias, whilst those hygristors used on the other
night flight without Snow white were within 5 per cent
of the Vaisala measurements. Thus overall the
hygristor sensors in Brazil appeared similar to those
used at Ascension.
Figures 11 and 12 show the standard deviations
associated with the comparisons between Sippican and
Vaisala RS90 and the Snow White. Here, the results
from the two tests have been combined together. Figs.
7 to 12 indicate that the systematic bias between the
sensors tends to be larger than the random errors for
both sets of sensors.
Conclusion
The results from the tests between the Vaisala RS90
and Snow White in Ascension and Brazil indicate that
it should be possible to produce a working reference
for future assessment of radiosonde relative humidity
measurements by combining the two systems together,
given current technical problems are resolved.
In the temperature band 0 to 15 ºC, the Vaisala RS90
comparisons with Snow White were extremely
consistent between the two test sites. At night the
sensors agreed very closely at high humidity and the
Snow Whites were about 4 per cent lower than the
Vaisala measurements at relative humidity of about 20
per cent. Some of the difference at low relative
humidity would have originated from hysteresis in the
Vaisala sensing system. In the day the Vaisala
measurements were about 5 per cent lower than the
Snow White measurements at high relative humidity.
The measurements of the two systems in daytime cloud
suggest that most of the difference arises from a low
bias in the Vaisala measurements. A reason for this
The results demonstrate that there are significant daynight differences between operational relative humidity
sensor measurements.This should not be considered
surprising because of the methods of sensor exposure.
In the future it will be necessary to implement a
suitable radiation correction scheme for humidity
sensors, similar to that used for temperature sensors.
Comparison statistics from the middle troposphere
Improved accuracy of relative humidity measurements
in the tropics is a significant issue for progress in
numerical weather prediction. Further tropical
radiosonde tests will be required to ensure optimum
accuracy measurements for future operations.
Snow White
RS90
RS80
RS90 RAW
RS90
Snow White
Fig 1. Comparison of daytime RS90 and Snow–white
temperature and relative humidity measurements,
showing typical output from both RS90 edited (used
for the reported values) and raw data files before
smoothing algorithms were applied.
Fig 4. Comparison of temperatures and relative
humidity from the same flights as Fig. 3.
Snow White
RS90
Sippican
Snow White
RS90 edited
RS90 raw
Fig 2. Comparison of daytime RS90 and Snow–white
temperature and relative humidity measurements.
Snow White
Vaisala RS90
Vaisala RS80
Fig 3. Comparison of dew point temperatures measured
by the Snow-white and values computed from relative
humidty and temperature of the Vaisala radiosondes
Fig 5. Temperature and relative humidity comparison
between Snow White, Vaisala RS90 and Sippican
MKII from Ascension 1999.
Snow White
Vaisala RS90
Fig 6. Comparison of dew point temperatures measured
by the Snow White and values computed from the
Vaisala RS90.
Humidity Difference from Snow White (%)
Humidity Difference From Snow White (%)
15
10
5
0
-5
-10
Sippican MKII
-15
0
20
40
60
80
4
3
2
1
0
-1
-2
-3
-4
Vaisala RS90
-5
-6
100
0
20
Relative Humidity (%)
Humidity Difference from SnowWhite (%)
Humidity Difference From Snow White
(%)
Sippican MKII
5
0
-5
-10
-15
-20
40
60
80
100
Fig 8. Vaisala RS90 - Snow White Day
Night Differences from Ascension Island
& Brazil 0°C to 15°C
10
20
60
Relative Humidity (%)
Fig 7. Sippican MKII - Snow White Day
Night Differences from Ascension Island
& Brazil 0°C to 15°C
0
40
80
5
Vaisala RS90
3
1
-1
-3
-5
-7
100
0
20
40
60
80
100
Relative Humidity (%)
Relative Humidity (%)
Fig 10. Vaisala RS90 - Snow White Day
Night Differences from Ascension Island
& Brazil -30°C to -15°C
10
10
9
9
8
8
Standard Deviation (%)
Standard Deviation (%)
Fig 9. Sippican MKII - Snow White Day
Night Differences from Ascension Island
& Brazil -30°C to -15°C
7
6
5
4
3
2
7
6
5
4
3
2
1
1
0
0
0
20
40
60
80
100
Relative Humidity (%)
Fig 11. Standard Deviations for figures 7 & 8
Vaisala RS90 circles, Sippican MkII squares
0
20
40
60
80
100
Relative Humidity (%)
Fig 12. Standard Deviations for figures 9 & 10
Vaisala RS90 circles, Sippican MkII squares