Chemtrails !!
What - No explanations?
Having just read through all previous posts to this thread I find it incredible that not one of you has tried to explain to the uninitiated just how a CONTRAIL forms. Let me have a go.
Water vapour in the earth's atmosphere is always present and is usually referred to as humidity. When humidity reaches 100% the air is saturated. The amount of water vapour that air can contain varies with temperature and pressure. If air at 100% humidity is cooled then some of the water vapour condences into water droplets to produce cloud or fog. The formation of the water droplets is assisted if there is present miniscule particles called condensation nucleii. If none of these paritcles are present then very still or undisturbed air can become super-saturated with the water vapour struggling to form water droplets.
Layers of high altitude air in the earth's atmosphere often become super-saturated. Along comes a piston or jet powered aircraft exhausting the products of fuel combustion which contain a great number of the mini particles and the super-saturated air takes immediate advantage to disgorge its over abundant invisible water vapour into visible water droplets/ice crystals called a CONTRAIL.
At high altitude and sub zero temperatures the transformations from water vapour goes directly to ice crystals and these will persist if the air is highly super-saturated to become a "Persisted Contrail". If the air is only marginally super-saturated then the the contrail will be non-persistent.
Military aircrew avoid contrailling levels like the plague as their aircraft position becomes all too obvious. Civil aircrew like contrails as other aircraft in their vicinity are more readily visible.
The cloud that forms around an aircraft flying fast through super-saturated air at low level has come to be called "ectoplasm" and is rarely persistent.