How meteorologists use balloons to carry recording instruments several miles up into the atmosphere
CAPTIVE BALLOONS have been used to carry scientific instruments up into the fog at Kew Gardens, Surrey. The instruments measure such factors as temperature, wind currents and the height of the fog. Fog is one of the biggest difficulties in aerial navigation and the more the meteorologists are able to find out about its nature and formation, the more accurately are they able to forecast its occurrence.
A KNOWLEDGE of the atmospheric conditions which exist at varying altitudes is as necessary to the aeronautical engineer as it is to weather forecasters. The performance of an aeroplane is considerably affected by the temperature, humidity and density of the air through which it flies.
Although these factors obey certain laws, they are liable to variations which can be charted only by painstaking research. This research is going on constantly all over the world. Little is heard about it, for those engaged in the work are scientists who do not seek publicity. To them, research is a matter of routine. A section of the Meteorological Office of the Air Ministry specializes in the subject.
Occasionally, spectacular news is heard of the activities of these scientists. A balloon may soar into the stratosphere or an aeroplane may break the altitude record. Such achievements are merely incidental. An inspection of the gondola of the balloon or of the cockpit of the record-breaking aeroplane would reveal a mass of recording instruments for taking scientific observations of the conditions at the new ceiling.
These isolated events, important though they may be, do not really reflect the work of the upper air investigators. For every record altitude flight a thousand machines take the air with instruments suspended on their wing struts for measuring air humidities and temperatures. For every stratosphere balloon ascent, thousands of tiny pilot balloons and hundreds of instrument-carrying sounding balloons float upwards. Yet, despite the enormous amount of work that has been done, the upper air investigators are still faced by unsolved problems. It is no easy matter to chart the ever-shifting air strata and to gain knowledge of the causes of the ever-changing conditions. Until recently some of the difficulties encountered were insurmountable. For instance, it would be useless to send up the little pilot balloons for measuring wind velocities during cloudy or stormy weather. It would be obviously impossible to watch from the ground the movements of the balloons.
Hitherto, all such wind soundings have been taken in fine weather. Yet the conditions which prevail in the upper atmosphere during stormy conditions are at least as important. If meteorologists were able to make continuous observations, weather forecasting might at last become an exact science and flying safety would be greatly increased.
It is probable that within a few years continuous observations will be possible with new systems of “sky-sounding” by radio which have been invented, but these systems are not yet in general use.
In the meantime, the following three methods are used in routine observations: small pilot balloons for indicating the behaviour of upper winds; instruments carried by aeroplanes to measure air humidities, pressures and temperatures in the lower atmosphere; and self-recording instruments on free balloons to measure air conditions at great heights.
The small pilot balloons are made of pure rubber and not of cotton or silk, as are the large balloons. They have no valves, so that the hydrogen with which they are filled increases in volume
as the balloon floats higher and higher into the air until eventually the balloon bursts and descends. The greatest height thus attained so far is 125,505 feet, or nearly twenty-four miles.
Three sizes of pilot balloon are in use in Great Britain; these have diameters of approximately 15 in., 22 in. and 28 in. The smallest balloons are generally filled to rise at a rate of 400 feet a minute, and the larger ones to rise at speeds up to 1,000 feet a minute or more. A tail, comprising a length of thread and a sheet of paper, is fitted as an aid to height observations by means of a theodolite.
When the sky is patchy and alternates between clouds and clear blue, a red balloon is easiest to follow. Blue balloons are used when there are white skies, for these balloons, being more opaque, appear black in such conditions. In cloudless weather it is customary to use white balloons, as these reflect the rays of the sun and are seen as bright points of light.
For night ascents a small paper lantern containing a little candle of the kind used for Christmas trees is attached by a 12-feet length of thread.
The adventures of each pilot balloon during its short but useful life are followed by an observer on the ground, who watches it through the telescope of a theodolite. The height attained by the little balloon at a particular moment of time can be worked out from the apparent size of the tail, which is of known length, although a close enough approximation for most purposes can be gained from the fact that the rate of ascent is nearly constant.
By the use of two theodolites at two ground stations cross bearings for height measurements are possible. In Great Britain observations are sometimes made with only one theodolite.
As an indication of the painstaking research conducted with these pilot balloons, about two thousand ascents were made by the Meteorological Section of the British Army in France during the war of 1914-18 for the sole purpose of recording the changes in the velocities of the upper winds by night and by day.
It was discovered that at 1,000 feet the winds reached a maximum after midnight, with a well-marked minimum between nine and ten a.m. The variations at 2,000 and 3,000 feet were noted to be of the same general character, but there was no appreciable daily variation at 4,000 and at 6,000 feet.
From other pilot balloon observations, it was found that in the lower layers wind speed increased with height above the ground, whereas its direction veered, and that the variation of wind velocity with height depended upon the horizontal distribution of temperature.
It is by knowing such general principles as these that meteorologists can predict with considerable accuracy what is likely to be happening along the air routes when conditions at various points in the low layer adjacent to the ground are known.
Wind speeds and directions are, however, related to the navigation rather than to the design of aircraft. The soundings for obtaining more detailed information about the great heights are invaluable to designers who are trying to produce aircraft having high performances at these altitudes.
The first research was being conducted a long while before the first power-driven aeroplane managed to fly. Nearly two hundred years ago kites were sent up 10,000 feet to take continuous records of temperature, pressure and humidity, as well as of wind velocities.
Stratosphere’s Varying Depth
Later, as many as eight kites in tandem figured in these pioneer air soundings. Even greater heights were reached and it was established that temperature was more variable at 20,000 feet than at ground level. It was discovered also that temperature decreased with height at the average rate of about three degrees Fahrenheit for every thousand feet.
Until free balloons were used it was not discovered that this law held good only up to a certain height, and that a limit was reached where the temperature remained constant, or even slightly increased for a time with height.
THEODOLITE READINGS ARE TAKEN to determine accurately the position of a small hydrogen-filled balloon at each 500 feet of height reached by it. From this information the direction and speed of the wind at various heights can be computed. The greatest height attained by one of these pilot balloons before the decrease in atmospheric pressure caused it to burst was nearly twenty-four miles.
The lower region of the atmosphere is known as the troposphere. This has a varying depth; the depth is about ten miles at the Equator and about six miles at the Poles. The higher region is the stratosphere, also of varying depth ; the surface separating the two regions is the tropopause. Above the stratosphere are various layers.
Although the laws relating to the atmosphere are well known and have been demonstrated, there remains a great deal to find out about the variations which occur in a horizontal direction in varying conditions of weather. That is why almost daily in most countries balloons are being sent into the air to take soundings with their finely adjusted automatically recording instruments. These balloons are larger than the pilot balloons used for measuring wind speeds; like the pilot balloons, they burst at a certain height and come back to earth.
The earlier practice was to fit the balloon with a wicker framework to protect the instruments when the balloon eventually fell to the ground. Nowadays an increasing use is being made of release gear and small parachutes.
By means of an aneroid trigger the mechanism can be set to operate at a certain height. When this height has been reached the parachute and instruments detach themselves and float down, leaving the balloon to drift to greater heights and eventually to burst.
For sky soundings made at sea an ingenious idea was formerly used. Two balloons were joined together in tandem, the upper balloon being given twice the lift of the lower one. Attached to them were the instruments and a small sea-anchor float made of canvas.
After the apparatus, with the balloons, had been released, it would all float upwards until it reached a great height, at which the upper and larger balloon would burst. The apparatus would then descend until the sea anchor float touched the sea. Then the smaller balloon would take up the lift and keep the instruments clear of the water and also act as a marker to indicate the position.
The automatic release system using a parachute has the great advantage that, when the altitude at which the soundings are to be taken has been reached, the instruments at once come down and the observers do not have to wait until the balloon’s bursting height has been reached.
At bursting height the balloon may have drifted many miles away from the observing vessel, thus necessitating hours of additional steaming.
Recently, there have been further considerable advances, in which radio is used. Most of the earlier experiments were conducted in Soviet Russia and Germany, but the United States were also early in the field.
Small ultra-short-wave wireless transmitters are used. Weighing only a few pounds and operated by batteries, one of the transmitters is fixed to the self-releasing framework to which a parachute is attached.
Miniature Radio Transmitters
A simple mechanism driven by clockwork causes the transmitter to send out a series of signals which is varied in accordance with the different heights, temperatures and humidities recorded by the instruments. The observers on the ground are able to note the atmospheric conditions merely by listening to these signals.
This is probably the greatest step forward yet made in sounding the upper skyways. It is even possible by this method to measure wind speeds, for all that has to be done is to follow the positions of the balloon with two direction-finding stations taking cross-bearings.
With, on the one hand, a tabulated list of the heights of the balloon with the exact moments in time when these heights were reached, and, on the other hand, a tabulated list of accurately plotted positions, the exact behaviour of the balloon can be worked out in great detail. As the balloon is at the mercy of the winds, the behaviour of the winds becomes known in detail in so far as they affect the balloon’s journey.
Another enormous advantage is that clouds and stormy conditions do not affect the observations. Observations can even be made on cloudy nights.
It is not an exaggeration to say that the soundings which can be carried out with these radio balloons will double our knowledge of the upper strata of the skies and enable us to chart them with that accuracy which will be needed before stratosphere flying can become commercially successful.
There are still problems to be solved. There is the difficulty of ventilating the measuring instruments used on the balloons and of protecting them from the direct radiation of the sun. The effects of this radiation are considerable in the lower densities of air at the great heights and a simple screening is not effective.
RELEASING A BALLOON for wind direction observations from the aircraft carrier Courageous. Larger balloons carrying recording instruments are also released at sea. When these balloons have reached a certain height, a release gear detaches the instruments which float down on a parachute. The parachute is retrieved and the readings of the instruments noted. The balloon continues to rise until the reduction of atmospheric pressure causes it to burst.
Until free balloons were used it was not discovered that this law held good only up to a certain height, and that a limit was reached where the temperature remained constant, or even slightly increased for a time with height.