An unusual cloud is a cloud formation or perturbation that leads to weird-looking, and/or often rare sights. These clouds can be made of water droplets or, at higher altitudes, of ice crystals.
In that sense, Sky Oddities isn’t meant to aim at pretty but usual sunsets or menacing but common storm clouds.
Noctilucent clouds are very high-altitude clouds made from ice crystals that can be seen in the morning or evening when the sun is below the horizon.
Because of their altitude, they are lit by the sun that reflects on them, making them bright in a dark sky.
They appear as white or light blue filaments or waves.
Like noctilucent clouds, nacreous clouds are also high-altitude clouds made of ic crystals that are also lit by the sun when the latter is below the horizon. They display iridescences, rainbow-like colors.
Credit: Alan R. Light, NASA Radome, McMurdo Station, Antarctica, 2009.
A Kelvin-Helmholtz instability is a phenomenon that happens when two fluids of different speeds shear in a single fluid. For example, this is what makes the tiny whirlpools that are often seen in rivers.
Since clouds and air are fluids, such instability can happen in the sky, sometimes creating waves or spiral-like clouds.
Credit: Lukas Gallo, South Bohemia region, Czech republic