This section covers the optical effects caused by ice crystals.
For those living in temperate latitudes displays are usually limited to cirrus clouds.
Displays showing all the phenomena are rare because it needs a very large area of sky to be covered in ice crystals with the right optical properties.
The most spectacular displays are usually seen from very cold climates when the ice crystals surround the observer.
Because some effects are often only visible in a single patch of cirrus they may last for just a few minutes and are easily missed.
The key is knowing what to look for and checking the sky whenever possible.