Can you explain what causes the streaks in the cloud chamber? How are those particles made up in the high atmosphere?
Many common things like foods or rocks give off low, harmless amounts of radiation that can be detected by a cloud chamber. Can you design an experiment to test the radioactivity given off by different fruits or vegetables, or different kinds of rocks?
Are there different factors (indoors/outdoors, different weather, different altitudes) that cause your cloud chamber to detect more or less background radiation?