Definition
A high-altitude cloud type, typically forming above 20,000 feet AGL, composed almost entirely of ice crystals and characterized by thin, wispy, fibrous, or feather-like strands. Cirrus clouds indicate fair weather at the time of observation but can signal an approaching change in weather, such as a warm front or jet stream activity.
Plain English
A thin, wispy cloud made of ice crystals that forms very high in the sky. It looks like fine streaks or feathers and usually means good weather right now, though it can be an early sign that weather is changing.
Context Anchor
Seen in aviation weather study, weather briefings, and cloud identification when a pilot is learning to recognize high cloud types.
Derivation
From the Latin cirrus, meaning 'curl' or 'tuft of hair' -- a direct reference to the cloud's wispy, hair-like appearance. Knowing the origin makes the cloud type easy to recognize: if it looks like a curl of hair stretched across the sky, it's cirrus.
Why Pilots Care
Signals possible approaching weather systems such as a warm front and may precede turbulence or reduced visibility at altitude.
Analogy
Cirrus can look like white brush strokes or fine hair brushed across a blue sky.
Intuition Check
Do not assume cirrus is just any thin cloud. In aviation weather, cirrus specifically means a high, ice-crystal cloud with a wispy or feathery appearance.
Example Sentence 1
On the morning of the flight, thin cirrus streaks were visible high overhead, suggesting a front might arrive within the next day.
Example Sentence 2
The pilot noted increasing cirrus during the preflight weather check and adjusted the route to avoid potential turbulence.