No sooner had meteorological instruments such as thermometers and barometers been invented than experimenters wanted to take them up into the sky to study the atmosphere. The easiest way to accomplish this was to carry them up a mountain. Perier was the first to do so when, in 1648, at the request of his brother-in-law, the mathematician Pascal, he took a barometer up a peak in the Auvergne mountains in France and found that atmospheric pressure fell by about 15 per cent during the 1000-metre climb.
The invention of the hot-air balloon offered another way to explore the sky. When the Montgolfier brothers’ made the first ascent in Paris, 1783, King Louis XVI decreed that it was too dangerous for people to fly in the balloon, so the first mammal into the sky was a sheep. It was accompanied in its cage by a cock, a duck and a barometer.
The lift for the balloon was provided by hot air coining from a fire lit underneath it. However, the Montgolfier brothers did not realise that it was heat which made the balloon rise (by making the air in the balloon less dense, and therefore buoyant compared to surrounding air). They thought that lift came from some unnamed gas given off by the fire, and that the amount of gas produced depended on what was being burnt. The best ingredients, in their estimation, were damp straw, wool, rotting meat, and old shoes. The stench of this concoction as it burnt occasioned a rapid retreat on the part of King Louis and Marie Antoinette, who watched proceedings from a distance. The flight was a great success, although brief, and an injury sustained by the cock was taken to be evidence of the dangers of the sky until several witnesses testified that the cock had been kicked by the sheep before take-off.
The first manned ascent took place two months later, in another Montgolfier hot-air balloon, followed a month later by a much longer flight in a balloon filled with hydrogen called “flammable air.”
Scientists were quick to exploit the opportunities ballooning provided. One of the earliest experiments took place in Birmingham in 1784, when James Watt released a hydrogen balloon with a fire cracker and time fuse. The intention was to discover if the reverberating sound of thunder was caused by echoes or successive explosions. After six minutes the cracker exploded with a sound satisfyingly like thunder. In 1804, Gay-Lussac ascended to 7000 m, and found that the atmosphere became drier with altitude, but that otherwise the chemical composition was the same as at the surface.
Perhaps the most famous of the scientific ascents took place in England in 1862, when Coxwell and Glaisher ascended to around 10,000 m in a hydrogen balloon, measuring temperature, humidity and pressure, every minute. The sudden ascent to high altitudes caused great distress to the two aeronauts as they ran low on oxygen.
Glaisher’s account of the flight is more famous for his dispassionate description of his symptoms than for the measurements he took. He first lost the use of his arms and legs, then his vision blurred and he could no longer hold his head upright. He then lost the power of speech, followed by his sight, before completely losing consciousness.
By this stage ice was forming on the ropes, and the balloon was in danger of rising so far that the men would die as was to happen to two French scientists a decade later. Coxwell was still conscious, although he had lost the use of his hands. He was able to grasp the release cord with his teeth and nod his head to let some hydrogen out of the balloon (above), so that they began to descend. Glaisher regained the use of his faculties after an interval of 15 minutes and resumed his temperature and pressure readings. For decades, these measurements remained the best set of observations taken in the sky.
High-altitude ballooning continued to make valuable contributions to atmospheric science into this century, including the surprising discovery that temperature stops falling above a certain height subsequently called the troposphere. However, from the point of view of meteorology, the most important development was the use of small unmanned balloons with lightweight recording instruments which descended by parachute after the balloon burst.
The invention of aeroplanes presented new possibilities for a exploring the sky. Initially frail, early aircraft avoided clouds and any sort of bad weather. However, as they were made more robust, they were able to survive increasingly boisterous encounters with weather.
During the Second World War, many advances were made both in aircraft and in exploration of the weather. Bombers flying at high altitude over Europe encountered and described the extremely strong winds of the jet streams. In the tropical Pacific, where weather observations had been sparse, the enormous increase in the number of wind observations from aircraft laid the basis for rapid advances in understanding tropical meteorology. But perhaps the greatest achievement was the penetration of tropical cyclones by US air force planes.
In 1945, a tropical cyclone struck the US Navy near Japan, sinking many ships and taking the lives of hundreds of sailors. This event helped focus attention on the need for tracking and forecasting the movement of tropical cyclones in order to mitigate their disastrous effects. Reconnaissance flights into the eye of tropical cyclones played a vital role in forecasting the movement of these storms for three decades, and, although the flights were dangerous, remarkably few aircraft were lost.
Today’s hourly high-quality satellite photographs have partially removed the need for flights into tropical cyclones on a routine basis, although they continue for research purposes. Heavily instrumented aircraft have also been used to investigate thunderstorms, flying close to and occasionally into them, measuring the airflow through them with Doppler radar and sampling the spectrum of water droplet and ice particle sizes, as well as measuring temperature and humidity.
In New Zealand a research programme to study the effect of the Southern Alps on the weather has been organised by the National Institute of Water and Atmospheric Research (NIWA) for this spring. Called SALPEX, the programme will include the use of an Australian Fokker F27 research aircraft, to be implemented by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). This aircraft is going to fly transects across the Alps and out over the Tasman Sea, measuring cloud particle types and size distributions ahead of, and near, active cold fronts approaching New Zealand in the westerly wind flow.
Balloons will be released from Hokitika, Christchurch and Timaru, carrying instruments to measure temperature, humidity and wind speed throughout the depth of the atmosphere. Similar instruments will be dropped by parachute from the research aircraft over the Tasman Sea. These observations will be supplemented by the normal daily observations taken by MetService. A number of universities will be involved in SALPEX, both from New Zealand and overseas. In particular, the Physics Department of Auckland University will he using a vertical-pointing radar to measure rainfall rates. A better understanding of how high-intensity rainfall occurs will help improve forecasting of flash-floods.
One of the problems the programme will focus on is “spillover” of rain to the eastern side of the Alps during northwest storms (see New Zealand Geographic, Issue 21). This spillover accounts for most of the water flowing into the lakes that provide the hulk of New Zealand’s hydroelectric power. Letting water out of the lakes in anticipation of massive new inflows also plays an important role in flood control downstream from the lakes.
Spillover seems to be favoured by strong northwest winds above mountain level, as well as by the presence of abundant ice particles. Because raindrops take some time to fall to earth, the faster the air is moving, the further the drops can be carried from where they originated. Hence raindrops created by the upward motion of air rising over the mountains will travel further downwind—away from the mountains—when the wind perpendicular to the mountains is stronger.
Because ice crystals fall more slowly than liquid raindrops, they can travel even further beyond the mountains.
A third factor that seems to be important in determining where the rain falls is the stability of low-level air. When the air is very stable, it resists upward motion. In such circumstances, low-level air approaching the South Island in a northwest airstream is blocked from rising over the Southern Alps. Instead, it is deflected to blow parallel to the mountains as a northeast airstream known as a harrier jet. What happens then is that the northwest wind further out over the Tasman Sea rises over the northeast airstream up against the land, almost as if the northeast barrier jet were part of the land. The effect is to shift the upward rain-generating movements in airmasses upstream, away from the mountains. When this happens, the heaviest rain falls close to the coast near Hokitika, rather than halfway up the Alps, and almost no rain makes it over the Main Divide to the eastern side of the Alps.
When a front in the westerlies approaches the South Island, the stability of the low-level air decreases, until finally the northeast barrier jet breaks down. Then the surface air blows straight on to the land from the northwest, and the air rises directly over the Southern Alps. This seems to coincide with a shift of the rainfall maxima to over the Main Divide, or just east of it, and a significant spillover occurs. A better understanding of the timing of the breakdown of the barrier jet will lead to better forecasts of spillover rainfall.
Northwest winds also bring warm temperatures to eastern districts of the South Island, and are sometimes strong enough to cause widespread destruction to forests and buildings. The SALPEX programme also intends to investigate this weather paticro, particularly the way in which the onset of the northwest gales is like a wave of air breaking in the sky, in a manner similar to an ocean wave breaking as it moves into shallow water.
Because the Tasman Sea is an area in which little weather and atmospheric data has been collected, the aircraft flights will also provide a unique opportunity to compare fronts in the Tasman Sea with fronts that have been studied in other parts of the world, as well as studying how the frontal clouds are changed by their interaction with the land. The studies of cloud physics will also contribute to global climate research programmes focusing on the properties of precipitating clouds of different types and their interactions with the land.