Matthias Rang

Report on the Weather Balloon Launch on 29 April 2022

As part of the "Working Days for Physicists and Physics Teachers", a weather balloon equipped to measure the Earth's thermal radiation was launched in front of the main entrance to the Goetheanum. We report here on the flight and the first look at the data collected.

The Thermal Equilibrium of the Earth in the Cosmos
Considered from the point of view of physical optics, the Earth's thermal balance is quite straightforward: the Earth is surrounded by vacuum and all heat exchange with the cosmos takes place through thermal radiation. The Earth is in thermodynamic equilibrium with the cosmos: whatever light and thermal energy is continuously supplied to the Earth by the Sun is continuously released by the Earth again in the form of light and thermal radiation. However, how this ultimately manifests as the surface temperature of the Earth is by no means easy to grasp. It depends in many ways on the atmosphere, the vegetation of the Earth, the oceans and several other factors that underlie the natural and anthropogenic greenhouse effect. It is also worth taking a look at the atmosphere from the point of view of physical optics. All the diverse processes in the atmosphere must ultimately be reflected in the transparency, scattering and absorption for the thermal radiation emitted from the earth's surface. If the atmosphere becomes more opaque, i.e. if scattering and absorption increase, the Earth's surface heats up to a temperature that allows the Earth to remain in thermal equilibrium with the cosmos.

The Weather Balloon, the Measuring Probe and the Planned Experiment

The aim of the experiment was to measure the thermal equilibrium between the Earth and the cosmos. For this purpose, a weather balloon carried a measuring probe into the stratosphere, to measure the descriptors of the air envelope (air pressure, air temperature and relative humidity), the heat radiation from the Earth into the cosmos, and the heat radiation reflected back to the Earth by the atmosphere through scattering, absorption and re-emission. Therefore, the probe had two sensors for thermal radiation. One sensor was oriented downwards and registered the thermal radiation emanating from the Earth's surface through the air mass located below the probe. The other sensor was oriented upwards and registered the thermal radiation emanating from the air mass above the probe through scattering, reflection and emission. The measured values were recorded by a home-made and a purchased data logger. In addition, a GPS tracker was placed on board so that the probe and the measured values could be recovered after the flight.

The Launch

In order to measure the heat radiation of the earth and its reflection from the atmosphere without the warming influence of the sun, the launch took place in the evening at 18:09 CEST. The amount of helium used for the balloon was calculated so that the balloon would rise to an altitude of 36,000 metres in about 120 minutes at a rate of ascent of 5 m/s and burst at this altitude or a few minutes later at an altitude of 37,000 metres. According to the plan, only 45 minutes would have been necessary for the return journey by parachute, since the balloon would spend the first kilometres in free fall due to the low air pressure in the higher atmospheric layers and be slowed down by the parachute only in the lower layers. After 3 hours at most, the balloon should have re-entered the mobile phone network so that the GPS tracker carried onboard could send position information again.

The Search

However, things turned out quite differently. A small team of conference participants and Natural Science Section staff set off for the target region where the weather balloon should have landed according to the forecast. To be on the safe side, we drove past the last location transmitted: the "Gempen", Dornach's local mountain, which is not far from the Goetheanum. The GPS tracker in the probe indicated that it had been stationary there for more than 30 minutes. It was somewhat peculiar: at the location transmitted, instead of the probe, we found a smoking pile of coal in the forest. We were sure it wasn't our balloon but the coincidence of the pile of coal and the GPS position transmitted, remained mysterious. We then waited in vain in the target region (the district of Lenzburg). Hours after the calculated landing time we still heard nothing from the probe. We had to assume that the weather balloon had been lost. It was not until the early hours of the following morning that we received a message from the GPS tracker again, according to which our balloon had been at the "Gempen", in the immediate vicinity of the Goetheanum, since the evening before. While we were preparing to search there again, a new message came: our balloon was on the "Wolfetsmatt", an alp south of Lake Lucerne, directly next to an alpine hut at an altitude of almost 1,500 metres. We were sceptical, as this location was not in the direction of the wind. Nevertheless we put our questions aside and set off. Up in the mountains, shortly before Wolfetsmatt, we saw a beautiful double rainbow despite the high sun, which felt much better than seeing a pile of coal the night before! And indeed, we found the probe with the parachute and burst balloon at the alpine hut. The Wolfetsmatt farmer had apparently found the balloon in a trough at about 10 am and taken it to the hut. Since we only received a real position report from our tracker at midday, picking up the probe from the trough was probably decisive in getting the GPS tracker back into a working mobile network. However, the question remained as to when and how the balloon found its way there!

The History of our Weather Balloon

Back at the Goetheanum, the data logger, which, in addition to the atmospheric measurements, had recorded the GPS position and altitude of the weather balloon every 2 seconds, gave us information about this. The balloon had risen as planned at just under 5 m/s and had been moving to the east. For unknown reasons, it abruptly stopped rising at 35,944 metres above sea level and was driven south for many hours at this altitude by the high-altitude winds of the stratosphere. How this is possible remains a mystery. At this altitude, due to the low air pressure of only about 5 hPa, the balloon should have inflated to about 12 metres in diameter. At low altitudes the balloon's internal pressure corresponds to atmospheric pressure because of the rather flabby balloon skin, i.e. there is hardly any pressure difference between internal and external pressure. At almost 36 kilometres altitude the balloon's interior would be under higher pressure compared to the thin external atmosphere, and the balloon skin stretched until just before it tears. It is conceivable that we did not seal the balloon tightly enough and that it released some pressure just at this altitude, just enough to maintain the altitude.

Trajectory of the Weather Balloon

After about 5.5 hours at this altitude at -50 °C (and below), the inside of the styrofoam probe had also cooled to -50 °C. Despite the special low-temperature lithium batteries, the supply voltage, as recorded by the logger, had dropped from 9V to below 5V. At first some sensors no longer provided consistent readings. Then our home-made data logger for radiation measurement went out of service. Some time later the purchased logger and finally probably all the electronics including the GPS tracker stopped working (fortunately for us, it later resumed operating)! In the stratosphere, the balloon was first driven south, crossed Lake Brienz near Interlaken at 22:40, turned around on the northern flank of the Jungfrau massif and headed north again over Lake Thun (01:40). Since at 02:04 the balloon had not yet arrived at the site after 194.6 km of flight, it must have continued its journey for at least another hour before it burst and landed by parachute on Wolfetsmatt. The much longer flight time of at least 9, or more likely 10 hours explains the unexpected finding place. Despite the tremendous cold, we have a complete data set from the nocturnal stratosphere until 23:14! We have a meaningful partial data set until 02:04 (including GPS position and altitude, which remained at 35,700 ±250 m throughout). If the flight had gone according to plan, we would only have a much smaller and less interesting data set of measurements!