«CHAPTER 10 BALLOON TECHNIQUES 10.1 BALLOONS 10.1.1 Main types of balloons Two main categories of balloons are used in meteorology, as follows: (a) ...»
10.1.1 Main types of balloons
Two main categories of balloons are used in meteorology, as follows:
(a) Pilot balloons, which are used for the visual measurement of upper wind, and ceiling balloons for the
measurement of cloud-base height. Usually they do not carry an appreciable load and are therefore
considerably smaller than radiosonde balloons. They are almost invariably of the spherical extensible type and their chief requirement, apart from the ability to reach satisfactory heights, is that they should keep a good spherical shape while rising;
(b) Balloons which are used for carrying recording or transmitting instruments for routine upper-air observations are usually of the extensible type and spherical in shape. They are usually known as radiosonde or sounding balloons. They should be of sufficient size and quality to enable the required load (usually 200 g to 1 kg) to be carried up to heights as great as 35 km at a rate of ascent sufficiently rapid to enable reasonable ventilation of the measuring elements. For the measurement of upper winds by radar methods, large pilot balloons (100 g) or radiosonde balloons are used depending on the weight and drag of the airborne equipment.
Other types of balloons used for special purposes are not described in this chapter. Constant-level balloons that rise to, and float at, a pre determined level are made of inextensible material. Large constant level balloons are partly filled at release. Super pressure constant-level balloons are filled to extend fully the balloon at release.
Tetroons are small super pressure constant level balloons, tetrahedral in shape, used for trajectory studies. The use of tethered balloons for profiling is discussed in Part II, Chapter 5.
10.1.2 Balloon materials and properties The best basic materials for extensible balloons are high quality natural rubber latex and a synthetic latex based upon polychloroprene. Natural latex holds its shape better than polychloroprene – which is stronger and can be made with a thicker film for a given performance. It is less affected by temperature, but more affected by the ozone and ultraviolet radiation at high altitudes, and has a shorter storage life. Both materials may be compounded with various additives to improve their storage life, strength and performance at low temperatures both during storage and during flight, and to resist ozone and ultraviolet radiation. As one of the precautions against explosion, an antistatic agent may also be added during the manufacture of balloons intended to be filled with hydrogen.
There are two main processes for the production of extensible balloons. A balloon may be made by dipping a form into latex emulsion, or by forming it on the inner surface of a hollow mould. Moulded balloons can be made with more uniform thickness, which is desirable for achieving high altitudes as the balloon expands, and the neck can be made in one piece with the body, which avoids the formation of a possible weak point.
Polyethylene is the inextensible material used for constant level balloons.
10.1.3 Balloon specifications The finished balloons should be free from foreign matter, pinholes or other defects and must be homogeneous and of uniform thickness. They should be provided with necks of between 1 and 5 cm in diameter and 10 to 20 cm long, depending on the size of the balloon. In the case of sounding balloons, the necks should be capable of withstanding a force of 200 N without damage. In order to reduce the possibility of the neck being pulled off, it is important that the thickness of the envelope should increase gradually towards the neck; a sudden discontinuity of thickness forms a weak spot.
Balloons are distinguished in size by their nominal weights in grams. The actual weight of individual balloons should not differ from the specified nominal weight by more than 10 per cent, or preferably 5 per cent. They should be capable of expanding to at least four times, and preferably five or six times, their unstretched diameter and of maintaining this expansion for at least 1 h. When inflated, balloons should be spherical or pear shaped.
The question of specified shelf life of balloons is important, especially in tropical conditions. Artificial ageing tests exist but they are not reliable guides. One such test is to keep sample balloons in an oven at a temperature of 80°C for four days, this being reckoned as roughly equivalent to four years in the tropics, after which the samples should still be capable of meeting the minimum expansion requirement. Careful packing of the balloons so that they are not exposed to light (especially sunlight), fresh air or extremes of temperature is essential if rapid deterioration is to be prevented.
Balloons manufactured from synthetic latex incorporate a plasticizer to resist the stiffening or freezing of the film at the low temperatures encountered near and above the tropopause. Some manufacturers offer alternative balloons for daytime and night time use, the amount of plasticizer being different.
where T is the total lift; V is the volume of the balloon; ρ is the density of the air; ρg is the density of the gas; and D is the diameter of the balloon, which is assumed to be spherical.
All units are in the International System of Units. For hydrogen at ground level, the buoyancy (ρ – ρg) is about 1.2 kg m–3. All the quantities in equation 10.1 change with height.
The free lift L of a balloon is the amount by which the total lift exceeds the combined weight W of the balloon and
its load (if any):
namely, it is the net buoyancy or the additional weight which the balloon, with its attachments, will just support without rising or falling.
It can be shown by the principle of dynamic similarity that the rate of ascent V of a balloon in still air can be
expressed by a general formula:
(10.3) in which q and n depend on the drag coefficient, and therefore on the Reynolds number, vρD/µ (µ being the viscosity of the air). Unfortunately, a large number of meteorological balloons, at some stages of flight, have Reynolds numbers within the critical region of 1·.105 to 3.·105, where a rapid change of drag coefficient occurs, and they may not be perfectly spherical. Therefore, it is impracticable to use a simple formula which is valid for balloons of different sizes and different free lifts. The values of q and n in the above equation must, therefore, be derived by experiment; they are typically, very approximately, about 150 and about 0.5, respectively if the ascent rate is expressed in m min–1. Other factors, such as the change of air density and gas leakage, can also affect the rate of ascent and can cause appreciable variation with height.
In conducting soundings during precipitation or in icing conditions, a free lift increase of up to about 75 per cent, depending on the severity of the conditions, may be required. An assumed rate of ascent should not be used in any conditions other than light precipitation. A precise knowledge of the rate of ascent is not usually necessary except in the case of pilot- and ceiling balloon observations, where there is no other means of determining the height. The rate of ascent depends largely on the free lift and air resistance acting on the balloon and train. Drag can be more important, especially in the case of non spherical balloons. Maximum height depends mainly on the total lift and on the size and quality of the balloon.
10.2.2 Balloon performance
The table below lists typical figures for the performance of various sizes of balloons. They are very approximate. If precise knowledge of the performance of a particular balloon and train is necessary, it must be obtained by analysing actual flights. Balloons can carry payloads greater than those listed in the Ttable if the total lift is increased. This is achieved by using more gas and by increasing the volume of the balloon, which will affect the rate of ascent and the maximum height.
The choice of a balloon for meteorological purposes is dictated by the load, if any, to be carried, the rate of ascent, the altitude required, whether the balloon is to be used for visual tracking, and by the cloud cover with regard to its colour. Usually, a rate of ascent between 300 and 400 m min–1 is desirable in order to minimize the time required for observation; it may also be necessary in order to provide sufficient ventilation for the radiosonde sensors. In choosing a balloon, it is also necessary to bear in mind that the altitude attained is usually less when the temperature at release is very low.
For balloons used in regular operations, it is beneficial to determine the free lift that produces optimum burst heights. For instance, it has been found that a reduction in the average rate of ascent from 390 to 310 m min–1 with some mid size balloons by reducing the amount of gas for inflation may give an increase of 2 km, on average, in the burst height. Burst height records should be kept and reviewed to ensure that optimum practice is sustained.
Daytime visual observations are facilitated by using uncoloured balloons on clear sunny days, and dark coloured ones on cloudy days.
The performance of a balloon is best gauged by the maximum linear extension it will withstand before bursting and is conveniently expressed as the ratio of the diameter (or circumference) at burst to that of the unstretched balloon. The performance of a balloon in flight, however, is not necessarily the same as that indicated by a bursting test on the ground. Performance can be affected by rough handling when the balloon is filled and by stresses induced during launches in gale conditions. In flight, the extension of the balloon may be affected by the loss of elasticity at low temperatures, by the chemical action of oxygen, ozone and ultraviolet radiation, and by manufacture faults such as pinholes or weak spots. A balloon of satisfactory quality should, however, give at least a fourfold extension in an actual sounding. The thickness of the film at release is usually in the range of 0.1 to 0.2 mm.
There is always a small excess of pressure p1 within the balloon during ascent, amounting to a few hPa, owing to the tension of the rubber. This sets a limit to the external pressure that can be reached. It can be shown that, if the
temperature is the same inside and outside the balloon, this limiting pressure p is given by:
(10.4) where W is the weight of the balloon and apparatus; and L0 is the free lift at the ground, both expressed in grams.
If the balloon is capable of reaching the height corresponding with p, it will float at this height.
It is very important that radiosonde balloons should be correctly stored if their best performance is still to be obtained after several months. It is advisable to restrict balloon stocks to the safe minimum allowed by operational needs. Frequent deliveries, wherever possible, are preferable to purchasing in large quantities with consequent long periods of storage. To avoid the possibility of using balloons that have been in storage for a long period, balloons should always be used in the order of their date of manufacture.
It is generally possible to obtain the optimum performance up to about 18 months after manufacture, provided that the storage conditions are carefully chosen. Instructions are issued by many manufacturers for their own balloons and these should be observed meticulously. The following general instructions are applicable to most types of radiosondes balloons.
Balloons should be stored away from direct sunlight and, if possible, in the dark. At no time should they be stored adjacent to any source of heat or ozone. Balloons made of either polychloroprene or a mixture, or polychloroprene and natural rubber may deteriorate if exposed to the ozone emitted by large electric generators or motors. All balloons should be kept in their original packing until required for preflight preparations. Care should be taken to see that they do not come into contact with oil or any other substance that may penetrate the wrapping and damage the balloons.
Wherever possible, balloons should be stored in a room at temperatures of 15 to 25°C; some manufacturers give specific guidance on this point and such instructions should always be followed.