I'm reading a paper about cometary impacts which has some information that bears on our topic and I think we can trust it since it is not being produced in a program to disinform relative to so-called chemtrails.
The first thing of interest is the main historically known large body which impacted the Earth: the Tunguska object. What is important to us in relation to chemtrails is that this object exploded in the atmosphere and for many nights afterward the sky was unusually luminous over Europe and western Asia,, allowing people to read newspapers at night.
When scientists finally made it to the remote site in 1927, no visible fragments of the exploded body were found. Later field work uncovered peculiar black, shiny, metallic spheres in the soil of numerous small, shallow, oval craters - 50 to 200 meters diameter - similar to the craters of the Carolina Bays. . These spheres were typical of extraterrestrial bodies having a very high iridium, nickel, cobalt and other metals composition.
Now, here is where the connection comes in: An unusually high content of these same metals were later found in Antarctic ice cores, but in the layer relating to the year 1912.
That is to say, it took four years for the stuff deposited in the stratosphere to precipitate onto Earth.
From the Antarctic data, global fallout of 7 million tons was estimated for the Tunguska body which means that it's diameter would have been about 160 meters.
The dust from such objects exploded in the stratosphere diffuses over the globe and it usually takes a period of about three to six months for this to happen. The time that it takes for anything in the stratosphere to get to the ground is about 3 years (or more) and that is "independent of the initial amount of dust {or whatever}." Also, there are so many atmospheric variables that it cannot be determined where the stuff will go.
That is, materials injected into the stratosphere do NOT precipitate onto the ground under which it is "laid" (as in so-called chemtrails) except, perhaps, almost accidentally, and much, much later.
Again, this has no bearing on spraying that takes place at much lower altitudes, as in the troposphere, but clearly, what is going on in the stratosphere is NOT "chemtrails."