pilot balloon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small balloon used to determine wind velocity.

from The Century Dictionary.

• noun A small balloon used for ascertaining the direction of air-currents at different levels: most frequently used before ascents of manned balloons.
• noun A small balloon sent up in advance of a larger one to ascertain the direction and strength of the wind.

from the GNU version of the Collaborative International Dictionary of English.

• A small, unmanned balloon sent up to indicate the direction of air currents.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun meteorology A small unmanned balloon whose ascent is followed by a theodolite to obtain data for the computation of the speed and direction of winds in the upper air.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun meteorological balloon used to observe air currents