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Watching the weather
Watching the weather

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3.1 Radiosondes

Hot-air balloonists care a great deal about how the wind changes with height. It offers them their only means of steering. The launch is a particularly critical phase in ballooning as there is precious little room for manoeuvre, so just before committing to a launch the pilot usually arranges for a brightly coloured, helium-filled balloon to be released. As it rises it traces the local air movement giving a vivid impression of how the wind speed and wind direction vary with height.

  • Why is it possible to have a stable atmospheric environment in the troposphere, where the air temperature is observed to fall by about 5 °C per kilometre as height increases, in apparent contradiction of the 'hot air rises' maxim?

  • Air from lower in the troposphere, although warm when at its original level, would cool to a lower temperature and so become denser than its surroundings if it were raised.

This situation is indeed stable, unless the temperature falls by more than about 10 °C per kilometre for unsaturated air. Only if the temperature falls with height even more rapidly than 10 °C per kilometre, would any rising air still be warmer than its surroundings, even after cooling, and continue to rise. In that case, convection would cause the warmer, lower air to rise until the overall change of temperature with height is such that the atmospheric environment is stable.

A balloon filled with air tends to sink because of the weight of the skin of the balloon and because of the weight of the extra air blown into it in order to inflate it. A soap bubble appears a great deal more buoyant as its skin is relatively light and very little extra air is used in its inflation, but unless caught on an upward-moving air current a soap bubble also sinks. Balloons filled with helium or hot air on the other hand tend to float upwards. The reason is that they are inflated with gases that are less dense than the surrounding air. If the total weight of skin and gas is less than the weight of surrounding air that would otherwise occupy the same space, then it inevitably follows that the balloon will float upwards.

From the start of systematic weather observations in the mid-nineteenth century it was recognised that data from above the ground would be useful in predicting how the weather might develop. Putting a thermometer and a barometer on a kite was easily done, though reading them was more of a challenge. By stacking many kites, or using a hot-air balloon, it was possible to generate enough lift to get a person aloft in a basket complete with instruments and record book. But, as you might imagine, this was not the basis for routine observation.

The practical solution to probing the atmosphere with airborne instruments is to carry them on a helium-filled balloon large enough also to carry a radio transmitter and its power source. This arrangement is called a radiosonde ('sonde' is a French/international word meaning probe). The weight and value of the instrumentation posed serious challenges before the advent of microelectronics in the latter half of the twentieth century, but today the radiosonde is virtually a disposable automatic observation system, with the investment in the recovery of instruments being driven as much by tidiness as by economy. A modern radiosonde is shown in Figure 13. Radiosonde balloons will normally reach an altitude of about 25-30 km (i.e. into the stratosphere), at which point the balloon expands so much that it bursts. A small parachute then slows the descent of the instrument payload to prevent impact damage on landing. During its flight a radiosonde transmits data on temperature, pressure and humidity for 90 minutes or so. The balloon itself can also be tracked to map wind speed and direction; nowadays this is often done by an onboard Global Positioning System (GPS).

Figure 13 A radiosonde being launched.

There are more than 800 sites around the world that launch radiosondes daily at 0000 and 1200 UTC and, as with other meteorological observations, radiosonde data are shared through the WMO.