Weather Matters!

Now you see them, now you don't ?:

Somedays you can see multiple trails and other days nothing - yet you know that the aircraft are out there?! The reason is all down to what is known as the 'zero' level. Contrails are human-induced clouds that only form at very high altitudes (usually above 8 km - about 26,000 ft) where the air is extremely cold (less than -40ºC). Cold air cannot hold as much water vapor as warm air. If the humidity is too high, as produced by the hot exhaust gases from the aircraft, the water vapor starts to condense. So, as you can see, weather really matters!

Practical Examples:

Identification & Formation Guide.
ID Different Types Of Trails. (PDF File)
Information For Predicting Trailing Levels.