Wow! is that ever low.
Do you have more pictures like this one; being that shallow of an altitude?
Would be great if you could get some pics of the planes with "fresh" (I use the term loosely) plumes in the same frame.

There is a somewhat accurate way to estimate/range plane altitude by the use of mildots if you can afford to get a spotting scope with such a reticle.

http://www.mil-dot.com/user-guide

"basically you have to find an airport and choose a plane on the tarmac and identify it ie: Boeing 747-400, Airbus A320-200s, 41 Embraer 190, Bombardier CRJ100/200s, CRJ700s, CRJ900s, Dash 8-200s
etc. not likely you are going to get to view a KC135 stratotanker – but all the specs and most importantly, dimensions (Key one being length) are available.
Range it as accurately as possible from a determined distance and record the mil. now you can extrapolate for higher altitudes.

Like I said – not 100% but will get you damn close if you have time to set up for when the aircraft is directly in line with you. The bonus is you can see that big juicy “persistent contrail” heading your way so you can estimate where your median for best range will be."

Even better if you can figure out how to mount it to a video recorder.