What you can see is water vapour.

Chemtrails are caused by hydrocarbon pollution, microscopic particles that are invisible to the naked eye, the moisture sticks to these particles, which are so small and light that they spread out when they hit a denser part of the atmosphere and create a blanketing effect.

The commercial airliners are ALWAYS making chemtrail pollution above a certain height, because there's less oxygen.

Less oxygen=less efficient combustion=more pollution

You only see chemtrails if there's enough moisture in the atmosphere for it to create a visible trail.