Plenty types of met phenomena can cause that. Temperature inversions, mountain waves, etc, etc, etc. Suggest you have a look at met data collection programmes such as SONEX or POLINAT carried out by NASA or the DLR where aircraft take samples from the atmosphere and analyse the chemicals within them. Oh hang on ... they're part of the conspiracy probably
Show us pictures of these chem sprayers so we can debunk the chemwierdos argument or are they so secret that ordinary cameras can't pick them up ??
(Actually we lied .... there is a switch in the cockpit
there was a photo which appeared on this site once coupled with the aircraft flight manual details
)