Howto talk:Add contrails
Looking for information on contrails I found an interesting diagram describing the necessary conditions for contrail formation in a standard atmosphere on page 2 in this paper: Ulrich Schumann, Formation, properties and climatic effects of contrails, 2005.
It seems that the altitude at which contrails can occur is not only has a lower limit but also an upper limit. With the assumptions made about fuel and its combustion, contrails would in a standard atmosphere occur between 8.2 - 19 km (27,000 - 62,000 ft) for saturated air (RH = 100%) and between 10.2 - 14 km (33,000 - 50,000 ft) for dry air (RH = 0%).
I did also find other papers through Google (search phrase: contrail formation properties) but they was way beyond my abilities, as this one in large parts also were.