This is a suggestion to all who believe in chemtrails.....
Chemtrail theory is something that I strongly disbelieve in (at least to the extent in which it is portrayed). Some days (and nights) you will see these trails in the sky. Some days you will not. Why? Here is what you can do:
1) Make or buy a calendar.
2) Each day record the local weather conditions. In particular, note the barometric pressure - this can be found on the published synoptic charts for a country, continent, etc.
3) Record the cloud types observed during that day or in the presence of any trails you see. A cloud chart can be found here: http://asd-www.larc.nasa.gov/SCOOL/cldchart.html
4) Record on the calendar the days that you do see the trails.
5) Look for patterns in the observations you record. E.g. on days you see the trails, are they meteorologically similar? Are the pressures similar? Was there a dry or wet period following or preceeding the trails - and did this occur multiple times?
I would do this myself on a daily basis, only I live in an area where these trails are infrequently seen - perhaps a few times per year. However when I do see them, it is usually at a time when there is clear sky (but with sometimes high cloud, or forming high cloud), calm conditions, and a falling barometric pressure ahead of an approaching cold front that passes through usually a day later.
Vincit omnia Veritas: Truth conquers all.