the great majority of them will just dissipate and evaporate. This is totally unlike Chemtrails which persist, spread and envelop the sky. .... Am I missing something?
Yes you are missing the fact that what you described are BOTH examples of normal characteristics of contrails.
Normal contrails can be short lived. Normal contrails can persist and spread and envelop the sky.
This has been a known characteristic for many decades.
How about you read this site in far more detail and have a careful read of the many threads to learn what normal contrails often do.
but these atmospheric science papers are a good start:
"The spreading of jet contrails into extensive cirrus sheets is a familiar sight. Often, when persistent contrails exist from 25,000 to 40,000 ft, several long contrails increase in number and gradually merge into an almost solid interlaced sheet."
from Peter Kuhn," Airborne Observations of Contrail Effects on the Thermal Radiation Budget" published 1970.
Journal of the Atmospheric Sciences Volume 27, Issue 6 (September 1970)
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1970)027<0937:AOOCEO>2.0.CO;2
“It is often observed that contrails spread considerably…Under favorable conditions, a lateral spread of kilometers is observed…If sufficient air traffic exists, an entire overcast of contrail cirrus may develop and persist for hours with rapid growth in the ice budget of individual contrails.”
http://cires.colorado.edu/science/groups/pielke/classes/atoc7500/knollenberg72.pdf
of this account from a book written in 1942/43:
Extract from ''Flight To Arras''
''The German on the ground knows us by the pearly white scarf which every plane flying at high altitude trails behind like a bridal veil. The disturbance created by our meteoric flight crystallizes the watery vapor in the atmosphere. We unwind behind us a cirrus of icicles. If the atmospheric conditions are favorable to the formation of clouds, our wake will thicken bit by bit and become an evening cloud over the countryside''.