These are just common figures you find in scientific literature pertaining to atmospheric science. Just at a quick glance, 8000 m is often cited as the height where you begin to see cirrus clouds. I know at least one textbook that calls out -40 C as the temperature where you start seeing persistent contrails. RH 70% is also mentioned from a number of sources. Wikipedia cites 8,000m, -36.5 degrees, and RH 60%, as when contrails "
usually forms". (Source:
http://en.wikipedia.org/wiki/Contrail)
Generally, you will tend to see contrails with these conditions, so they are not necessarily "pseudoconditions." I think the problem that a lot of chemtrail believers have is that they tend to assert that
thou must have all these conditions for a contrail to form, which is blatantly false if only they understood
why height, temperature, and RH are significant. In short,
they take these numbers at face value, and often out of context.
For example, while a lot of contrails do form at heights greater than 8,000 meters (~26,000 feet), it may totally escape some people that they can form at lower altitudes if the temperature was colder (heck, here in Canada, it's not unusual seeing -40 at 5,000 feet on a cold winter day, or even at the surface). I am pretty sure most people don't even realize that the colder the temperature gets, the less moisture the air can carry before that moisture is condensed, and thus visible to the naked eye. There is also the factor that temperatures
tend to decrease as you go up in altitude (exception is an inversion), which is a fact that often gets left behind when advocating chemtrails.