[capt. norm]

I hope you were joking.

First of all cirrus clouds are made up of ice crystals and they form at the edge of jet streams (the natural ones) when moisture (not carbon dioxide or ozone) freezes.

Carbon dioxide and ozone both freeze at temperatures sevral magnitudes colder than the temperature at the cirrus levels.

Cirrus forms where the air is cold enough for ice to form and not an inch lower.

[John H K]

You seem to be victim of the FR sydrome, common on science threads, of people babbling about things they know nothing about.

Contrails routinely form persistent cirrus clouds under the right atmospheric conditions.

There are atmospheric conditions, of course, where you don't even get a contrail, much less a persistent one that spreads out into a cirrus cloud.

At times you'll have waves where an aircraft flying level will have a "dashed line" contrail" as it moves into and out of areas favorable for forming one.

However, in many situations aircraft form persistent cirrus in areas that would otherwise be cloud free.

[Arthalion]

Here's an experiment for you. Next time the mercury drops below freezing, drag a big, forced air propane heater outside, fire it up, and watch the results. Unless the air is exceptionally dry, you should see a "cloud" form downwind of the heater as the frozen moisture in the air is vaporized by the heat and condenses into a local cloud.

Jet engines do the same thing, only on a greater scale.

The difference, or course, is that at cruising altitudes the temp is so low that these clouds quickly refreeze into ice clouds...which we typically call cirrus clouds (because that's what they look like), but which could just as easily be called "contrail clouds" if that term offends you. These "contrail clouds" can last for hours until they are finally dispersed by the wind, and in major flight corridors can actually combine to form a general overcast.