An atmospheric balloon is a type of balloon that is filled with a GAS lighter than air, such as helium or hydrogen, and is used to carry instruments into the atmosphere. The term ATMOSPHERIC BALLOON can be broken down into three crossword solutions: SONDE (5 letters), SMOG (4 letters), and GAS (3 letters). A sonde is an instrument package that is attached to an atmospheric balloon and is used to measure various atmospheric parameters. SMOG is a type of air pollution caused by the burning of fossil fuels. GAS is a state of matter that is composed of molecules that are not bound together and can expand to fill any container.