Cirrus clouds form when water vapor turns directly into ice crystals in extremely cold conditions, unlike ground level rain clouds which require extremely cold air close to the ground level.

The traditional theory says that cirrus clouds form when water droplets freeze onto small aerosol particles in the atmosphere, acting as nucleation sites.

However, there are not enough aerosol particles in the atmosphere to account for the number of ice crystals that form in cirrus clouds.

A new study suggests that cirrus clouds form when water droplets freeze onto ice nuclei that are produced by the breakup of larger ice crystals. This process is called "fragmentation." Fragmentation can occur when ice crystals collide with each other or when they are hit by strong winds.

The new study provides a more accurate explanation of how cirrus clouds form. It also suggests that cirrus clouds may play a more important role in the Earth's climate than previously thought. Cirrus clouds reflect sunlight back into space, which can help to cool the planet. They also trap heat in the atmosphere, which can contribute to global warming.