http://globalskywatch.com/chemtrails/ubbthreads.php?ubb=showflat&Number=14833#Post14833

Interesting part for equatorial Brazil regarding the ability to form "contrails" is seen at

minute 8 second: 5

before and after:

"For example, it is unlikely that a contrail will form in an Equatorial region where the air is warm all year." (Minute: 8 Second: 20)