It is harder for scientists to do a controlled experiment when they are studying events that occur in natural settings. Scientists need laws like the combined gas law to deal with situations in which multiple variables are changing. Balloons like the one in Figure 14 are used by scientists to gather data about Earth's atmosphere. The balloon is filled with hydrogen or helium. It carries a package of weather instruments up into the atmosphere. The instruments measure temperature, pressure, and water content at different levels in the atmosphere.

What will happen to the volume of the weather balloon as it rises through the atmosphere? Both pressure and temperature decrease as the altitude increases in Earth's atmosphere. A decrease in external pressure should cause the balloon to expand to a larger volume. A decrease in temperature should cause the balloon to contract to a smaller volume. Whether the balloon actually expands or contracts depends on the size of the changes in pressure and temperature.

Figure 14 These scientists are releasing a weather balloon into the atmosphere. The balloon is designed to burst when it reaches an altitude of about 27,400 meters.
Drawing Conclusions What happens to the pressure inside a weather balloon as it rises?

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