Cirrus

How to recognise it

• Thin white filaments, patches or narrow bands, always separated by blue sky rather than covering it.
• A fibrous, hair-like texture. The Latin name means "lock of hair", and that is exactly what it looks like.
• Often hooked or comma-shaped at one end — the form called Cirrus uncinus, or "mare's tails".
• No grey shading anywhere. Cirrus is too thin to cast a shadow on itself, so it stays uniformly white.

What weather it signals

• On its own, cirrus signals fair weather. It never produces precipitation that reaches the ground.
• Cirrus that steadily increases, thickens and merges into a milky veil usually marks the leading edge of a warm front — rain often follows within 12 to 24 hours.
• A few isolated cirrus streaks in an otherwise blue sky generally mean the settled weather is holding.

How to photograph it

• Keep the sun off to one side. Backlighting makes the ice strands glow and separates them from the sky.
• Include the horizon or a foreground silhouette — an empty sky gives the eye nothing to judge scale against.
• Late afternoon light reveals the fibrous texture far better than harsh midday sun.
• Underexpose slightly so the bright white filaments keep their detail instead of blowing out.

Fun facts

• Cirrus clouds form in air that is often colder than −40 °C and consist entirely of ice crystals, never water droplets.
• The trailing streaks are ice crystals falling out of the cloud and being bent by winds moving at different speeds — you are effectively looking at a drawing of the wind.
• Aircraft contrails are artificial cirrus. When the air is humid enough they spread out and become almost indistinguishable from the real thing.

Easily confused with

Frequently asked questions