article_snippet.sent: Condensation trails are similar to the fog that results when a person exhales on a cold day .
article_snippet.sent: Why do planes leave long white trails in the sky?
article_snippet.sent: These condensation trails form when humid exhaust from jet engines cools very quickly in colder and dryer air at higher altitudes.
article_snippet.sent: ’ Mr Smith said: ‘It's not unlike the fog that results when you exhale on a cold day.
article_snippet.sent: In other words, contrails are clouds.
article_snippet.sent: Water vapour, strange as it might seem, is a by-product of the combustion within jet engines, which is where the humidity comes from.