What are clouds? Many millions of little drops of water or frozen crystals suspended in the atmosphere above the surface of a planetary body, and held in place around it by gravity, like interstellar clouds and nebulae in space. On Earth, they are normally made up of water vapour. Dense clouds reflect 70% to 95% of the visible range of wavelengths, which is why they appear to be white, at least from the top.
Water droplets within clouds scatter light efficiently, so that levels of solar radiation decrease as light descends through the cloud cover depth, leading to the sometimes dark appearance at the base of the clouds. Thin clouds can seem to have acquired the color of their background and when illuminated by non-white light, such as sunrise or sunset, can seem multi-
coloured. They can take some awesomely attractive shapes as the following pictures demonstrate.
Shelf clouds are low, horizontal wedge-shaped structures, associated with a thunderstorm gust front, or sometimes with a simple cold weather front. Shelf clouds are always attached to the base of the parent clouds above them.
Roll clouds occur during similar weather patterns as shelf clouds. Cool air sinking from a storm cloud’s downdraft spreads out with the leading edge called a gust front. This outflow undercuts warm air being drawn into the updraft, and as cool air lifts, the warm water condenses creating a cloud which often rolls in harmony with different winds blowing both above and below it.
Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves.
Mammatus clouds are pouch-like structures and a rare example of cloud formations in sinking air. Though they can seem very threatening, these clouds are not harbingers of severe weather to come. In fact, this type of cloud formation is most often seen after a thunderstorm has passed over.
Image: Roberto Miliani
So-called “jellyfish clouds” form at around 17,000 ft up in the atmosphere, due to a rush of moist air coming from the Gulf Stream getting trapped between layers of dry air. The top of the cloud rises into a jellyfish shape and long tentacles known as “trailing virga” form from rain drops that have evaporated. These are amazing to see.
Image: R Clucas
Mushroom clouds form as a result of the sudden formation of a large mass of hot low-density gases near the ground creating what is called a “Rayleigh-Taylor instability”. The rapidly rising volumes of gas result in a kind of whirlpool effect, drawing up a column of additional smoke and debris in the centre to form its “stem”. These clouds form normally through volcanic eruptions or impact events. The cloud reaches an altitude where it stops being more dense than the surrounding air and disperses, the central debris scattering and drifting back down to the ground.
Image: Jeppe salvesen
Nacreous clouds are extremely rare and referred to as mother-of-pearl clouds. They form 9-16 miles up in the stratosphere, well above tropospheric clouds. These structures shine brightly in high altitude sunlight up to two hours after ground level sunset or before dawn. The incredible spectacle of the bright iridescent colours relative to any clouds below them make these cloud formations truly an unforgettable sight.