Ever wonder why airplanes sometimes leave a trail that looks like a cloud?
The contrails, as they are known, are scientifically explainable.
How they form:
Jet exhaust has water vapor in it.
The air at jet level is very cold.
When invisible water vapor is added to very cold air, the water vapor condenses into visible cloud particles.
A "cloud" forms.
Why they are there on some days and not on others:
In order for the jet contrail, or "cloud" created by the jet's exhaust, to remain visible, two conditions must be present:
1) Enough moisture must be in the level of the atmosphere where the jet flies to prevent immediate evaporation of the contrail.
2) There can't be lots of turbulent wind that would cause the contrail to dissipate.
Some common questions:
Q1: How can there be moisture at jet level (see #1 above) when the sky is bright blue and clear?
A1: Moisture can exist without clouds. Clouds are present when the relative humidity is 100%. If the relative humidity is high, but less than 100%, the contrail won't evaporate away quickly. Nor will there be visible natural clouds.
Q2: How can there be sufficient moisture at jet level to sustain contrails while the relative humidity report is very low?
A2: The relative humidity that we report on television is at about 4 feet above the ground. The atmosphere is like a sandwich; it has many layers. It can be dry in one layer and moist in another. The relative humidity at ground level is not connected to the relative humidity thousands of feet high where jets fly.
If you have questions, please feel free to email me at the email address in the byline above.