# A weather balloon is filled with a fixed mass of gas at ground level, where the temperature is 17 °C and the pressure is high. As the balloon rises into the upper atmosphere, the surrounding pressure decreases and the temperature drops to −33 °C. Explain what happens to the volume of the gas inside the balloon as it rises, considering the effects of both the pressure change and the temperature change.

> WJEC A-Level Physics (Wales) — 4.1 Thermal physics · Explain · 5 marks

> Weather balloons are used to carry scientific instruments high into the atmosphere. They are only partially inflated at ground level before release.

## Mark scheme (5 marks)

1. As pressure decreases (at higher altitude), the volume of the gas increases
2. This is because pressure and volume are inversely related (for a fixed mass of gas at constant temperature)
3. As temperature decreases, the volume of the gas decreases
4. This is because at lower temperatures the gas particles have less kinetic energy / move more slowly, so they exert less pressure on the walls, reducing the volume
5. Overall the volume increases because the effect of the large pressure decrease outweighs the effect of the temperature decrease (which is why balloons expand / are only partially inflated at launch)

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)
- [fixed mass of gas](https://www.gradenine.co.uk/glossary/fixed-mass-of-gas)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-weather-balloon-is-filled-with-d6cb8d2e) · Published by Druglandscape Ltd.