I believe that it was common to subtract 11 to 14deg C from MINTRA for jet aircraft - this predicts short non-persistent trails (but persistent in high humidity) More than14deg C below MINTRA one can expect long persistent trails, irrespective of the humidity.
Which is bl@@dy useful for an artist whose equipment probably consists of a piece of seaweed and a wet finger held up to the wind.
She appears to have (somewhat understandably) given up in despair!
