"Weather balloons" as was, back in the day.
Essentially two types, both Hydrogen, the gas was in a special brick shed away from risk.
The big ones carried a payload, and were lauched 0001 and 1200 as routine from about 10 UK sites to give wind [radar target tracked], temperature and humidy, the basis of the tephigram.
Supplementaries were flown, with radar target only, at 0600 and 1800, and "as required".
Every airfield with regular Met staff also had cloud base / low level wind-finder balloons, no payload, much smaller, and several colours chosen to maximise visibilty. These were tracked manually by theodolite and stopwatch, and the ace observers could compute winds in real time. Ace forecasters needed an hour.
One demo. at our college was regularly flown: three balloons, red, white, blue, from a known base line, and each tracked. The idea was to illustrate the large effect of low-level turbulence and dispersal ...... even after one minute, although all three were at the same height + / - a few feet, the horizontal scattering was huge.
And I won't begin to describe the naughties that can be achieved with hydrogen, candle or slow fuse, and various other ingenuities that should have got us a severe reprimand, if not the sack. I lost a set of eyebrows to H2 around 1970.
E & OE, it was a long time ago.