Noctilucent clouds in the night sky

Height of base: Above 200,000 ft

Noctilucent clouds are extremely rare very high clouds seen in the night sky, usually on clear, summer nights. They become visible about the same time as the brightest stars and are usually bluish or silvery, but sometimes orange or reddish. Noctilucent clouds most closely resemble thin streaky cirrus, and are thought to be made of ice crystals. They can only be seen at certain latitudes, between 45°N and 80°N in the Northern Hemisphere.

Contrails or condensation trails from aircrafts

Height of base: Above 20,000 ft

Contrails are frequently seen criss-crossing the sky and are created from condensation trails from aircraft engines. The exhaust of the plane produces a thin trail of water droplets or ice crystals which then expand leaving a fluffy trail where the aircraft has passed.

Kelvin-Helmholtz cloud

Height of base: Above 16,500 ft

An extremely rare phenomenon where a cloud produces a billowing wave pattern. They occur when there is a strong vertical shear between two air streams causing winds to blow faster at the upper level than at the lower levels. They are named after Hermann von Helmholtz and William Thomson Kelvin who were meteorologists who studied turbulent air flow.

Lenticular cloud Lenticular clouds (Photo: Matthew Clark) Enlarge

Height of base: 6,500 - 16,500 ft

These orographic wave clouds form when the air is stable and winds blow from the same or similar direction at many levels of the troposphere. As the wind blows across hilly or mountainous regions, the air undulates in a downstream train of waves. If there is enough moisture in the air, these waves will condense to form the unique appearance of lenticular clouds. The clouds can be seen as far as 60 miles downwind of the hills or mountains that led to their formation and they are believed to be one of the most common explanations for UFO sightings across the world.

Nacreous cloud Nacreous cloud Enlarge

Height of base: 68,500 - 100,000 ft

Nacreous clouds form in the lower stratosphere over polar regions when the sun is just below the horizon. The clouds are illuminated from below and often glow in vivid colours and will often remain visible for a couple of hours after sunset and through the night as they are lit by moonlight. Nacreous clouds form below -78 °C temperatures and so are most likely to occur during the polar winter.