Ballooning around the world from east to west ?

Hot-air ballooning over ever-increasing distances has mushroomed in popularity in the 1990's. Such long-distance flights take advantage of the mid-latitude westerlies, which increase in strength with height in the troposphere. To be able to fly higher and for a longer time, these ballons have become increasingly complex in design. Meteorological support is also important: balloons are subject to the vagaries of the jet stream, and the only way in which they can change course is by changing altitude: wind changes in direction with altitude in a baroclinic atmosphere. For instance, detailed numerical trajectory models were run to simulate the possible paths by two transcontinental balloon flights in Australia in 1993. The actual paths, at levels of about 600 hPa, were in good agreement with the calculations based on NWP of the winds at various levels, apart from wind shear at the launch (1).

In 1997-99 several attempts were carried out to travel around the world by hot-air balloon, using westerlies at mid-latitudes in the northern hemisphere. The balloons traveled at altitudes up to 11 km, i.e. in the upper troposphere, near the polar or subtropical jet. At this altitude the balloon cabins must be pressurised. Because the upper-tropospheric westerlies are strongest in winter, all attempts were made in that season, in either hemispheres. Finally, in March 1999, Bertrand Piccard and Brian Jones completed the feat in the Breitling Orbiter III (Fig 1). They departed Switserland at a time when a deep trough was present over southern Italy. This carried them south over the Sahara. From that point much of their route was near the subtropical jet (Fig 2).

In January 1999, there was an aborted plan to rise in a huge unmanned helium-filled balloon to about 39 km, in the middle to upper stratosphere, where easterly winds of about 25 m/s can be expected across the globe between 5-25° S, at some longitudes even 35 m/s (125 km/h) (2). The launch was planned for summer at Alice Springs, in the centre of Australia. To fly at such altitude, the balloon's capacity must be enormous. That is because at 39 km, the ambient pressure is only 7.6 hPa (1000 exp(-39/8)). To generate the same lift as at sea level, the balloon's volume needs to be 131 times larger at 39 km (1000/7.6).

The notable feature of flying so high is that the winds are from east to west at this southern latitude and time of year. The reason is that stratospheric temperatures then can be 16 K higher over the South Pole than at the same height of 55 km over the North (Fig 3). (The Sun strikes all day in the south in summer, whereas in the north there is then almost total darkness for months.) Such a temperature difference creates a thermal wind from the east in the southern hemisphere, as explained in Section 12.4 in the textbook, and in Note 12.F.

Fig 3. January mean temperature (a) and zonal wind (b) between 10-80° S at 180° E, between 1000-10 hPa. The zonal wind between 200-10 hPa is shown in more detail in (c). Blue colors indicate negative (easterly) zonal winds. For reference, here are some approximate height conversions: 200 mb ~12.4 km, 100 mb ~16.6 km, 50 mb ~21 km, 10 mb ~37 km.

(1) Mills, G.A., W.K. Downey & F. Whitby 1994. Trajectory calculations in support of meteorological planning and forecasting for two transcontinental balloon flights. Aust. Meteor. Mag. 43, 29-39.

(2) Grose, S. 1999. It’s all to do with those corioles and meridionals. Canberra Times 18th January, p9.