Long, thin clouds can indicate several atmospheric conditions depending on their composition and altitude. This answer will focus primarily on one type of long, thin cloud: contrails.
Contrails: Jet Aircraft Exhaust Clouds
A common type of long, thin cloud is the contrail. These are narrow, elongated clouds that form as jet aircraft exhaust condenses in the cold air at high altitudes. According to the reference material, contrails are indicative of:
- Upper level humidity.
- Wind drift.
How Contrails Form
Contrails form through a process similar to how your breath becomes visible on a cold day. The water vapor in jet exhaust mixes with the extremely cold air in the upper atmosphere. This causes the water vapor to condense into ice crystals. These ice crystals then form the visible contrail.
What Contrails Tell Us
The persistence of a contrail can tell us about the humidity of the upper atmosphere:
- Short-lived contrails: Suggest relatively dry air. The ice crystals evaporate quickly.
- Long-lasting contrails: Indicate high humidity. The air is saturated enough that the ice crystals don't evaporate quickly and can even grow, spreading out to form cirrus-like clouds. These spreading contrails can contribute to cloud cover and potentially affect local weather patterns.
Wind Drift
Because contrails form at high altitudes, they are also affected by winds at those altitudes. The shape and direction of a contrail can therefore give an indication of the wind direction and speed at that altitude. If the contrail is curved or distorted, it indicates that the wind is changing direction or speed as you go higher.
Other Possibilities: Cirrus Clouds
While the question is focused on "long, thin clouds," it's worth noting that naturally occurring high-altitude cirrus clouds can also appear as long and thin streaks across the sky. These clouds are composed of ice crystals and often form ahead of weather systems. Their appearance can indicate changes in the weather.
