David Shirley, of Wellington, asks :-

Why are con trails formed behind jet aircraft and why are they only seen on some days.

Erick Brenstrum, a meterologist with Metservice, responded.

Aircraft condensation trails form because water vapour is one of the by-products of the combustion of aviation fuel. The water vapour is violently ejected from the aircraft engine as one of the hot gases comprising the exhaust. Initially the exhaust is too hot to allow the gaseous water to condense, but as it turbulently mixes with the surrounding air it cools rapidly.

Once the exhaust has moved behind the plane to a distance of about twice the plane's wingspan it has been diluted by about 100 times its volume in surrounding air. As well as cooling the air, and so making cloud likely to form, the turbulent mixing also lowers the water content, making cloud less likely to form.

Whether or not a condensation trail appears depends on the extent of these opposing trends, and on the temperature and humidity of the air the plane is flying through. Generally the environmental air needs to be about minus 30C or colder; otherwise the water from the exhaust will not produce a large enough change in the air's relative humidity for contrails to form. Consequently, the plane needs to be flying above about seven kilometres altitude.

The air also needs to be fairly humid. In fact, contrails are usually observed when the air at high altitude has enough humidity so that there are other high clouds in the sky.