What makes planes leave vapour trails?

Chris - The reason planes leave vapour trails is because they're burning hydrocarbons, chains of carbon atoms linked together with hydrogen round it. When this mixes with oxygen you make water and some carbon dioxide. The vapour trail is the water and because the plane's very high up in the air actually what you're getting is ice. Vapour trails are water vapour that often form ice crystals. They leave those nice miniature clouds behind in the wake of the plane. It is also possible to make much weaker vapour trails form without the plane's engines running. As the plane flies, the wings create a vortex of lower air pressure. This low pressure can lead to lower temperatures, where water vapour condenses forming vapour trails starting at the tips of the wings.