CONTRAILS A sign of change

It seems our spring time weather changes as fast as the jet planes flying overhead. In fact, we can look to the contrails produced by those jets as a signal for coming changes in our weather.

First it is important to understand what exactly contrails are. They form when water vapor is emitted as a byproduct from a jet engine. The water vapor condenses in the cold air and forms a cloud (made of ice crystals). Most contrails form in the upper levels of the atmosphere, above about 25,000 feet (more than 10,000 feet higher than Mt. Rainer).

If the atmosphere is dry then the cloud will dissipate quickly, or a contrail will not form at all. If there is a lot of moisture in the atmosphere then the contrail will last much longer. So the longer it lasts, the higher the humidity.

Increasing moisture in the upper levels of the atmosphere is usually a sign of an incoming weather system. I took these pictures during the early afternoon, notice the blue skies. By dinner time the skies were totally white and the front was just offshore with rain falling at the coast.



Another sign we have increasing moisture in the upper levels is when we see a halo or ring around the sun. In this case you can make out a faint rainbow at the base of the ring with contrails below.