Underutilized observations for studying tropical climate variations: the
historical pilot balloon database

Michael Douglas^1^ John Mejia^2^, Raquel Orozco^2^, Javier Murillo^3^

^1^National Severe Storms Laboratory/NOAA, ^2^Cooperative Institute for
Mesoscale Meteorological Studies, ^3^Weather Decision Technologies

Norman, Oklahoma

Abstract

The longest-running atmospheric sounding database is that from pilot
balloon observations ("pibals"), whose routine observations date from
the early 20th century. Such observations are still made today, though
mostly in developing countries in Asia and Africa. Although pibals are
subject to limitations (cloudiness and darkness being the two most
obvious ones), their low cost has historically allowed for more frequent
and more widespread use than radiosondes. Much less well-known, and less
appreciated, is that pibals have some advantages over radiosonde winds
for studies of climate variability. The procedure for making a pibal has
not fundamentally changed in 100 years, unlike wind observations
obtained from rawinsonde systems -- which have used radiotheodolites,
Omega, LORAN and now GPS. Wind profiles obtained from these different
systems show somewhat differing characteristics, which can complicate
identifying historical trends in wind data.

Making effective use of historical pilot balloon data for climate
studies requires availability of the original angle data and some
metadata - such as the characteristics of the balloons and inflation
procedures. We discuss the difficulty in obtaining both of these; a
major effort at digitizing the global database of pilot balloon
observations will be required.

**Introduction**

Perhaps the simplest upper air wind measurement that can be made in
clear skies is that of a pilot balloon observation (Fig 1). A small
balloon, inflated precisely to produce a known (but approximate) ascent
rate, is tracked by a theodolite and the elevation and azimuth angles
measured at fixed intervals. With trigonometry the horizontal position
over the earth can be found and the horizontal displacement (and thus
wind) obtained. Although early measurements of this kind were made in
the late 1800's, the first suitable measurements were made around 1905.
By the end of WWI (\~1918) there were approximately 20 pilot balloon
stations in the US and similar observations were being made in a number
of countries.

Although an online NOAA history states that "The pibal was replaced by
the radiosonde in 1938" this is a considerable oversimplification of
what actually took place during this period
(<http://celebrating200years.noaa.gov/foundations/weather_obs/welcome.html#adv>).
Rawinsonde observations, with winds obtained by tracking the signal of
the radiosonde, did not become widely available until WW2 and a mean
wind map from June 1949 (Fig. 3) shows that pilot balloon observations
were still more numerous than rawinsonde observations. By 1956 (Fig. 4)
rawinsonde observation sites (\~63) were similar in number to pilot
balloon sites (\~65). (These maps of wind data over the US were
routinely published in Monthly Weather Review from 1938 to just after
1956.)

The pilot balloon history in the rest of the world is somewhat similar
to that in the US, except that in many countries the transition to
rawinsonde observations was not made smoothly. Pilot balloon
observations were relatively widespread in many parts of colonial Africa
and the Indian subcontinent in the 1940's and 50's. Upon independence,
many of the weather monitoring networks gradually deteriorated and many
pilot balloon stations ceased to operate. The exact history varied
greatly from country to country, with some maintaining extensive
networks up to the present (e.g. India, Fig. 5). Many others however,
ceased observations, or continued observations intermittently. East
African pibals were critical in describing the east African low-level
jet and its northward extension, the Somali jet (Fig. 6). Many Mexican
pilot balloon observations continued well into the 1960's and early
70's, and agreements between the US and Mexico specified the stations to
be operated, with some making 4 pilot balloon observations per day (Fig.
7).

Many of the current pilot balloon observations are not routinely
transmitted via the GTS and so it is not always easy to determine
whether observations are being made of not. For example, the IMD web
site states that there are 62 pilot balloon stations current in India
(Fig. 5). Yet few of the observations are internationally disseminated.
Many stations are listed as pilot balloon stations in current WMO
Station lists, but we know from personal experience that some of these
have not made observations for many years. There is a serious lack of
metadata about the stations. Hill (1979), in showing the locations of
pilot balloons he used in central Africa (Fig. 8), admitted that it used
mean winds from other analyses, since the original data was hard to
find.

**Value of historical pilot balloon data**

Since there have been few fundamental changes in pilot balloon
observation procedures since their advent, there should be little
variation in the uncertainties associated with the observations over
time. This is very unlike radiosonde measurements, where sensors have
changed as each new model appears. And there are very significant
variations between different manufacturers of radiosondes, so that a
major effort has been necessary to account for these historical changes
when studying long-term upper air temperature and humidity variations.
Even winds from radiosonde systems have changed with time. Early
radiotheodolites had certain error characteristics, losing balloons at
low elevation angles. Then Omega navigation windfinding required long
averaging intervals, thus smoothing the wind profiles. LORAN was higher
vertical resolution, but geographically limited. And although
GPS-windfinding is now very accurate, it has been subject to radio
interference from local sources. All radiosonde wind measurements are
subject to balloon drift characteristics, and even the high resolution
GPS winds include the sonde-balloon pendulum motion that must be
filtered. And 1-sec winds are of little use in most meteorological
applications anyway, as averaging is needed for most analysis and
weather prediction applications - does anyone use 5-sec winds from an
automatic surface station?

If the historical data can be recovered, pilot balloon observations
could be used to extend backwards in time by several decades our
description of circulation changes over parts of the world. Areas where
such observations have been extensive include North America, tropical
Africa, and south Asia.

If consideration is being given to generating a reanalysis product based
solely on surface observations over the northern hemisphere, the
addition of pilot balloon data would certainly be an important addition
to such a reanalysis. Modern data assimilation systems should be able to
use the wind information to generate much more reliable atmospheric
structures than from surface data alone. Whether through
data-assimilation based reanalyses or diagnostic studies of the original
data from pibal networks, the historical tendencies of circulation
characteristics can be diagnosed over a longer period than can
temperature tendencies from radiosondes. This may be useful for many
climate studies.

Since pilot balloon observations are still widespread across south Asia,
this would probably be the most suitable region to test the usefulness
of diagnosing long-term changes in atmospheric circulation from *only*
pilot balloon observations. Such a diagnosis could be compared with
reanalysis-based changes to determine the sensitivity of the changes to
changes in initial data quantity and type.

Feasibility of obtaining the raw observations

There is an ongoing need to digitize the historical paper archives that
exist of pilot balloon observations in many, many countries. It is not
enough to compile GTS-reports, which are a very large underestimate of
the observations actually made. The raw angle observations are
essential, so that the winds can be examined for errors and only then
winds calculated.

Historical procedures to calculate winds were either manual or
calculator-based, neither of which could easily detect and correct
observational errors. In fact, errors are virtually impossible to detect
when using the programmable hand calculators widely used in Africa
today. And errors in entering the data cannot be detected.
Unfortunately, the poor quality of GTS pibal data has led to a certain
disregard for the quality of such observations. And unfortunately for
data impact studies, the demise of widespread pilot balloon observations
occurred mostly prior to the widespread development of numerical weather
prediction activities. There were virtually no pilot balloon
observations taking place over North America by the time NWP models had
developed to a point where they could show a positive impact from
assimilating higher spatial-resolution wind soundings (Fig. 10). The
impact of such observations could still be assessed with regional NWP
models over parts of south Asia, where pibals are numerous. And the
impact of pibals on future higher-resolution reanalyses is sure to be
positive in much of Africa and Asia.

To obtain and digitize the historical pilot balloon database would be a
very major undertaking, and one that is likely to be successful only if
funded by the international community. Understandably, it is a low
priority for most weather services, since the value of such observations
is primarily for climate research. The urgency of the digitization is
apparent to anyone who has visited data archives of a meteorological
service in Africa. The need to obtain metadata on the procedures,
balloons, and historical changes to the networks should also be
documented urgently. Quite literally, knowledge of this is dying off, as
few observers remain from the early years of pilot balloon observations.

References:

Hills, R.C., 1979: The Structure of the Inter-tropical Convergence Zone
in Equatorial Africa and its relatonship to East African rainfall.
Transactions of the Institute of British Geographers, New Series, Vol.4,
No. 3, pp329-352.

![](media/image1.jpeg){width="2.2930555555555556in"
height="3.020138888888889in"} ![](media/image2.jpeg){width="3.3125in"
height="2.323611111111111in"}

Fig. 1. Pilot balloon launch (in 1930's??,from NOAA web page) and in
2004 (northern Mexico). The *advantage* of the same technology...

![](media/image3.jpeg){width="3.5569444444444445in"
height="2.5034722222222223in"}

Fig. 2. Pilot balloon stations (red) around 1919.

![](media/image4.jpeg){width="4.899305555555555in"
height="3.745138888888889in"}

Fig. 3. Pilot balloon observations -- mean winds for August 1939. Some
radiosonde observations are available for the isotherm analysis -- but
winds are obtained by tracking of the radiosonde balloons.

![](media/image5.jpeg){width="4.782638888888889in"
height="3.6840277777777777in"}

Fig. 4. Pilot balloon (black) and radiosonde (red) mean winds at 700 mb
for June 1949. More pibals than rawins.

![](media/image6.jpeg){width="4.964583333333334in" height="3.25625in"}

Fig. 5. Pilot balloon (black) and radiosonde (red) mean winds for August
1956. Still many pibals are present, though rawinsondes are now more
numerous.

![](media/image7.png){width="3.1465277777777776in"
height="3.5180555555555557in"}

Fig. 6. Current Indian pilot balloon network according to the Indian
Meteorological Department web page.

![](media/image8.jpeg){width="2.479861111111111in"
height="3.2131944444444445in"}

Fig. 7. Mean 1.5 km June winds from pibal stations in east Africa from
the period 1950's-1970's. Dates vary with stations, but 23 stations are
present in this analysis. *Only 1-2 radiosonde stations are functioning
in this same area today.*

![](media/image9.jpeg){width="5.506944444444445in"
height="3.5069444444444446in"}

Fig. 8. A table showing Mexican upper air stations specified for the
period 1966-68 (actual observations not known). Note 4-times daily
pibals at some sites.

![](media/image10.jpeg){width="4.5569444444444445in"
height="3.3472222222222223in"}

Fig. 9. Pilot balloon stations (blue) in central Africa in the \~
60-70's. In this region there were 40 pibal and 8 radiosonde (red)
stations. From Hills, 1979.

![](media/image11.jpeg){width="3.1444444444444444in"
height="1.7305555555555556in"}

Fig. 10. A schematic suggesting why pilot balloon observations did not
have a major impact on numerical weather prediction activities. The peak
of pilot balloon activity occurred several decades before the advent of
NWP (ascending line), and by the time NWP could benefit from dense pibal
networks (90's?) there were few available. Bottom axis is year of 20^th^
century, vertical axis is number of observations. Pibals decayed more
rapidly in US than worldwide.