Very， very high up in the sky there sometimes flows a stream of warm air. When it meets the cold air from the lower level， the cold and warm air mix and become cirrus.

　　Cinus is a kind of very thin cloud. It cannot hide the moon. It only covers the moon with a "thin gauze"， which is made of countless small ice grains. When the moon shines over these ice grains and the light reflects back to the moon， we see a circle of white light around the moon. This means the weather is going to change soon.