evidence: Aeroplanes sometimes leave a trail of condensed water – a contrail – in their wake depending on factors such as altitude and weather conditions. The water condenses as the hot exhaust from the engine hits air, which is cold at high altitude. Some people have pointed to these as evidence of chemtrails but they are water vapour. There is no evidence of chemicals including barium salts deliberately deposited in the atmosphere to counter climate change or for any other reason.