Contrails can remain visible for very long periods of time with the lifetime a function of the temperature, humidity, winds, and aircraft exhaust characteristics.Yes, it is possible for contrails to remain visible for very long time, but scientist found that the condition required for trails that last for hours is very very rare.
Scientist often found that trails exist in a condition not suitable for very persistence trails. Some example:
Aviation and the Global Atmosphere
"The computed contrail coverage over southeastern Asia is only slightly smaller than that over Europe and North America. Although air traffic is much less extensive over southeastern Asia, this high contrail coverage may result because this region more often has ice-supersaturated air."
The above article speculate that the sky of southeastern Asia has more ice-supersaturated air. However, that speculation is denied by this article:
The Global Distribution of Supersaturation in the Upper Troposphere from the
Atmospheric Infrared Sounder
"In the subtropics, very little supersaturation is seen. Seasonally, supersaturation in the upper tropical troposphere follows major regions of convection (and the supply of humidity).
In middle and high latitudes, there is significant supersaturation near the tropopause, especially in the regions of storm tracks. Highest frequencies in the upper troposphere (Fig. 9) are found in spring and summer. In polar regions (Fig. 10), supersaturation is often seen near the surface as well, particularly in winter, and is quite high over the Antarctic ice cap."
Above article explain that strange things is happening. High contrails coverage is seen on a region with very little supersaturaion and much less air traffic.
Next article explain how the scientist observe contrail that form defying existing knowledge.
Measurements of the growth of the ice budget in a persisting contrail
"In order that the contrail be visible a temperature as much as several degrees bellow the critical temperature is generally required.
In this experiment, as shown in Fig. 5, we actually observed the contrail to form and grow at a temperature several degrees above the critical temperature as defined by appleman, or the laboratory contrail studies of Pillie and Jiusto."
Contrails formed by aircraft may appear to form a grid as the winds disperse the contrails.Like explained previously, wind can not explain grid that is formed not in sequence. Grid is often seen formed in random order, but will results in almost perfect gap between one trails to another.
Contrails are safe and are a natural phenomenon. They pose no health hazard of any kind.This is a complete lie. Aircraft exhaust output is not any safer than car exhaust output. Even the aircraft industries acknowledge that jet engine output is not healthy. And Volvoair even mention that aircraft exhaust output may have more impact because they created at very high altitude. Clouds created by toxic substance surely not healthy.
Many people notice the difference of health effects of natural clouds and contrail clouds. Some notice health problem immediately, not waiting for the aerosol to settle down.
I never heard research done on effect of contrail clouds to human.
The bacteria claimed to be DOD developed and patented is actually a common, naturally occurring bacteria.
While the bacteria can occur naturally, the chance of them can be sampled is never mentioned.
One thing that struct me most is how USAF consider contrails to be safe. Previous post explain that contrails and contrail clouds is concluded to be a much stronger promoter of global warming. If government really care about global warming, they should already done something to contrails and contrail clouds. But they have done nothing.