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pilot balloon:see weather balloonweather balloon,
balloon used in the measurement and evaluation of mostly upper atmospheric conditions (see atmosphere). Information may be gathered during the vertical ascent of the balloon through the atmosphere or during its motions once it has reached a predetermined maximum
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a small rubber balloon filled with hydrogen and released in free flight to determine wind velocity and direction at different altitudes or to establish the altitude of the lower boundary of clouds. The pilot balloon ascends at a nearly constant speed, calculated according to the lifting force and the weight of the balloon envelope. The altitude of the balloon is determined as the product of the vertical speed and the time between the moment of release and the moment of recording. The wind velocity and direction at different altitudes are computed according to simultaneous readings of the altitude of the balloon and the angles (elevation angle above the horizon and the azimuth) observed through an aerological theodolite.