'Contrails are condensation trails (sometimes vapour trails): artificial clouds made by the exhaust of jet aircraft or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air. They are the cloud-like trails of water vapour that can be seen in the wake of airplanes, either from their exhaust or sometimes from their wing tips.
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Contrails are created in one of two ways:
1. First, the airplane's exhaust increases the amount of moisture in the air, which can push the water content of the air past saturation point (see saturation or dew point). This causes condensation to occur, and the contrail to form.
Airplane fuel such as petrol/gasoline (piston engines) or paraffin/kerosene (jet engines) consists primarily of hydrocarbons. When the fuel is burned, the carbon combines with oxygen to form carbon dioxide; the hydrogen also combines with oxygen to form water, which emerges as superheated steam in the exhaust. For every gallon of fuel burnt, approximately one gallon of water is produced, in addition to the water already present as humidity in the air used to burn the fuel. At high altitudes this steam emerges into a freezing environment, (as altitude increases, the atmospheric temperature drops) which lowers the temperature of the steam until each individual droplet freezes into tiny ice crystal. These millions of tiny ice crystals form the contrails. The time the steam/water droplets take to freeze accounts for the contrail forming some way behind the aircraft's engines.
2. The wings of an airplane cause a drop in air pressure in the vicinity of the wing (this is partly what allows a plane to fly). This drop in air pressure brings with it a drop in temperature, which can cause water to condense out of the air and form a contrail.
Exhaust contrails tend to be more stable and long-lasting than wing-tip contrails, which are often disrupted by the aircraft's wake.'
Since contrail formation is related to the dew point, they will appear or not in correlation with changes in the temperature and the concentration of water in the air through which the airplane flying.