A weather or sounding balloon is a balloon ( specifically a type of high altitude balloon ) which carries instruments in the sky to send back information on atmospheric pressure , temperature , and humidity by means of a small measuring tool called a radiosonde . A weather or sounding balloon is a balloon ( specifically a type of high altitude balloon ) which 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 .