The past weekend some friends and I tried attaching a Windsond to a sky lantern. Sky lanterns are made of a light paper bag over a suspended piece of wax that burns to fill the bag with hot air, creating a small hot air balloon. Sky lanterns are very light-weight and don’t carry much fuel to keep them afloat — would it be able to lift a Windsond?
Winter had finally arrived in Sweden and it was about -6 °C with fresh snow covering the landscape. Beautiful as it was, a wind of 5 m/s made the open landscape less pleasant to experience this day. We huddled behind one of the cars to light up the sky lantern. The Windsond was attached with a thin thread.
After a couple of minutes, the sky lantern was filled in spite of the chilling wind and we let it go. It had a hesistant start, then rose by a full 2 m/s as the wind carried it away. It rose to 110 m AGL, then appeared to run out of steam and started to sink again. Unfortunately it never recovered and continued to descend back to ground level by about 1 m/s. The flight was over in 3 minutes but the sonde still travelled 1 km.
Since the flight was over so quickly, we didn’t get a chance to cut-down the sonde and we couldn’t choose the landing location. Knowing the GPS coordinates, with maintained radio contact and the sonde blinking and beeping, it was no problem to find the sonde after walking across a snowy field. The sonde must have been attached to the sky lantern all the way to the ground since the fall was so slow, but we didn’t find the sky lantern and may never learn what happened of it.
The sonde carried the new high-accuracy humidity and temperature sensor which performed perfectly. The sonde was also equipped with an extra memory that logged more data than the radio link has capacity to carry. The firmware was extended to log a number of parameters with timestamps; full GPS readings, different types of temperature readings inside and outside the sonde, pressure, battery voltage and error conditions (although none occurred).
All in all, the results are mixed. Windsond did prove light enough to be carried by a sky lantern. It’s not necessary to bring a helium canister and the price of helium and sky lantern is about the same. On the other hand, sky lanterns are sensitive to winds and high humidity. The open flame presents a risk of starting a fire on the ground. And not least, the altitude of a standard sky lantern is very limited.
A video clip of the launch: