The proof is here,

In the 2009 Chicago Chemtrail ops the trails evenly spread out and merged into the adjacent Chemtrail over a short period of hours. They persisted as a visible even haze until it reached ground(approx) level, never dissipating as a haze for days.

The 'time to ground' was easy to calculate in this Chemtrail Operation which produced an unnatural fog that 'socked in' Chicago. 35,000 feet to ground level in two days.
Chemtrailing observation reported by me on GLP 11-22-2009 - Thread: Chem Trails in Chicagoland And the cloaking fog/cloud cover was for a(quietly scheduled) DHS and AF Joint task force OP for "terrorist air interdiction over highly populated cities(Chicago)", the public media reports are archived out there.