Contrails (short for "condensation trails") are line-shaped clouds sometimes produced by aircraft engine exhaust. The combination of high humidity and low temperatures that often exists at aircraft cruise altitudes allows the formation of contrails. Contrails are composed primarily of water (in the form of ice crystals) and do not pose health risks to humans. Contrails have been a normal effect of aviation since its earliest days. Depending on the temperature and the amount of moisture in the air at the aircraft altitude, contrails can either evaporate quickly or they can persist and grow. Engine exhaust produces only a small portion of the water that forms ice in persistent contrails. Persistent contrails are mainly composed of water naturally present along the aircraft flight path.

Aircraft engines emit water vapor, carbon dioxide (CO2), small amounts of nitrogen oxides (NOx), hydrocarbons, carbon monoxide, sulfur gases, and soot and metal particles formed by the high-temperature combustion of jet fuel during flight. Of these emittants, only water vapor is necessary for contrail formation. Sulfur gases are also of potential interest because they lead to the formation of small particles. Particles suitable for water droplet formation are necessary for contrail formation. Initial contrail particles, however, can either be already present in the atmosphere or formed in the exhaust gas.
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For a contrail to form, suitable conditions must occur immediately behind a jet engine in the expanding engine exhaust plume. A contrail will form if, as the exhaust gases cool and mix with surrounding air, the humidity becomes high enough (or, equivalently, the air temperature becomes low enough) for liquid water to condense on particles and form liquid droplets. If the local air is cold enough, these newly formed droplets then freeze and form ice particles that make up a contrail. Because the basic processes are very well understood, contrail formation for a given aircraft flight can be accurately predicted if atmospheric temperature and humidity conditions are known.
(Source: Air Force Link)

Miriam Katz in Danish national newspaper Berlingske Tidende on 14-09-1999: 
"The frequency of days with a clear, blue sky has decreased significantly during the last decade. One important reason is due to the condensation trails left by jet engines. 
The globally growing aviation places a heavy blanket over the Earth, contributing to the greenhouse effect." 

Source: https://www.theozonehole.com/airtraffic.htm

Although most persistent contrails are associated with cirrus, the frequency of occurrence of persistent contrails when no cirrus was observed is ranging from a few to 15% over the Langley Air Force Base and from 1 to 6% averaged over the 19 surface sites of the Minnis et al. (1997) analysis. Therefore there is some potential for an increase in high-level contrail-cirrus occurrence due to aviation.
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Further discussion
We observe a tendency for larger cirrus occurrence increase in regions of high air traffic. But this tendency is no longer observed if we look in terms of regions of increase in air traffic (Figure 7). The reasons for this are not clear. It could be that the previous trends are fortuitous. It could also be due to a mismatch between the different fuel consumption inventories. The regions of decrease in fuel consumption include the eastern part of the North Atlantic Ocean, which is downstream a region of large increase in fuel consumption. It is conceivable that contrails or at least the soot emissions which are potentially responsible for an enhanced cirrus formation are advected from the source regions to the middle of the North Atlantic Ocean. In a similar manner the mid-latitude North Pacific Ocean contributes little to regions of increase in fuel consumption although it is located downwind an important flight corridor.
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Finally, our analysis is based on the vertically-integrated fuel consumption above 8 km. However, changes in the integrated fuel consumption are associated with changes in the flight level (see Figure 2c). As shown by Sausen et al. (1998), the altitude of air traffic is an important factor to determine the potential for contrail formation. A decrease in the mean air traffic altitude, as observed along the East coast of Asia, can induce a decrease in the contrail coverage (Sausen et al., 1998).
(Source: https://www-loa.univ-lille1.fr/~boucher/preprints/contrail/node6.html)

A potential source of recent climatic change in the north-eastern and midwestern United States is the artificial cloudiness generated by jet aircraft, known as contrails. Central Pennsylvania, like central Ohio, Indiana, and Illinois, lies beneath the flight route connecting major cities of the east and west coasts. At least for areas in the Midwest located beneath the flight route, there is evidence of changes in surface temperature consistent with an increase in higher-level cloudiness since the 1960s. The same may also be happening in the north-eastern U.S. In the fall season especially, the lack of lower-level cloud cover often permits jet contrails to be seen from the ground. It is not known how the regional climatic effects of jet contrails may modfy "global warming" arising from enhanced trace gas emissions.

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