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Which gas filled in balloons used for weather monitoring?

Which gas filled in balloons used for weather monitoring?
A weather or sounding balloon is a balloon that carries instruments aloft to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde. Balloons meant to stay at a constant altitude for long periods of time are known as Transosondes.

The balloon is usually filled with hydrogen due to lower cost, though helium can also be used. The ascent rate can be controlled by the amount of gas with which the balloon is filled. Weather balloons may reach altitudes of 40 km or more.