Named for their smooth, lentil-like appearance, altocumulus lenticularis clouds (pictured above) are believed to be one of the most common explanations for UFO sightings. These spooky forms appear downwind of mountains.
Lenticular clouds are eerie because they seem to hover in place
When moving air encounters an obstacle like a mountain, it is forced to rise up and over it. As the air spills over the other side, the pull of gravity causes it to overshoot a little before resurging back up. It's a bit like a car's suspension bouncing after hitting a speed bump.
A stable air mass will continue to rise and dip for a little while as it travels away from the mountain, setting up an invisible "standing wave".
Lenticular clouds form when the airflow streaming over the mountain is both stable and humid. As it flows upwards and cools, the moisture in the air condenses to form clouds at the crests of the standing wave.
Lenticular clouds are eerie because they seem to hover in place, even in a steady wind. In fact the water droplets are racing through the cloud, carried by the wind. It's just the points where they condense and evaporate that are fixed, creating a well-defined cloud that hangs motionless in the sky.
They are some of the highest and rarest clouds on Earth, but these iridescent specimens are both beauty and beast. As a type of polar stratospheric cloud, they play a central role in the chemical destruction of the ozone layer.
During the coldest winters they can last well into the spring
Named for the French "nacré", meaning "mother of pearl", nacreous clouds form close to the poles in the extreme cold of winter. When air temperatures drop below -83°C, small amounts of moisture in the otherwise dry stratosphere condense into wispy clouds of ice crystals.
Because the clouds form at altitudes of over 15,000m, the Sun keeps illuminating them even when – from the point of view of people on Earth's surface – it is just below the horizon. The ice crystals scatter and diffract light, making the clouds glow with iridescent colours against the dark pre-dawn or post-dusk skies.
Nacreous clouds accelerate the chemical reactions that convert benign chlorofluorocarbons (CFCs) into ozone-destroying chlorine. During the coldest winters they can last well into the spring, trashing the ozone layer all the while.
Asperitas clouds are so unusual, they weren't officially recognised until June 2015. They are the first new type of cloud to be identified in over half a century. They consist of dark, chaotic waves that seem to swirl and tumble haphazardly across the sky.
Our language of clouds dates back to 1802, when amateur scientist and earnest sky watcher Luke Howard delivered a lecture tntitled "On the Modifications of Clouds". Howard classified clouds in terms that remain familiar today: cumulus (heap), cirrus (curl), stratus (layer) and nimbus (rain).
They are most commonly spotted rolling across the North American plains
Since then, a sprinkling of variations and sub-classifications has been added to Howard's work, leaving us with a rich nomenclature that has remained unchanged since 1951. That is, until now.
In 2008, Gavin Pretor-Pinney of the Cloud Appreciation Society proposed a new type of cloud. Members of the Society had been sending in photographs of "the cloud with no name": striking skyscapes that looked like a choppy sea viewed from below. Pretor-Pinney proposed the name asperatus, meaning "roughened" or "agitated".
In June 2015, the name – slightly modified to its noun form, asperitas – was officially accepted into the World Meteorological Organisation's International Cloud Atlas, the reference system used by meteorologists across the globe.
Scientists now want to find out how asperitas clouds form. They are most commonly spotted rolling across the North American plains, chasing convective thunderstorms. Despite their ominous appearance, they tend to dissipate without turning into storms themselves.
One of the rarest and most fleeting formations, this cartoon-like breaking wave is the Holy Grail for many cloud-spotters. It occurs almost everywhere in the world and at all levels of the atmosphere. But it only lasts a few minutes before dispersing without a trace.
These turbulent atmospheric waves are a sign that the air is extremely unstable
The crashing wave pattern is caused when swift, warm air flows over a colder, denser, more sluggish layer. As physicists William Kelvin and Hermann von Helmholtz discovered, the difference in the speeds and densities of these fluids creates a shearing force where they meet, producing undulations in the boundary between the two.
If the speed difference is just right, the tops of the denser ripples can roll up, surge forwards, and tip over in a series of vortices, just like a breaking ocean wave. When clouds happen to form at the horizontal boundary, this invisible air process is briefly made visible.
Though beautiful, these turbulent atmospheric waves are a sign that the air is extremely unstable and may be dangerous to aircraft.
Supercell storm cloud
Supercells are the least common kind of thunderstorm. Their destructive might is second only to hurricanes.
What sets them apart from regular violent storms is a persistent rotating updraft called a mesocyclone, which allows the storm to sustain itself over many hours.
Typical thunderstorms develop from cumulonimbus clouds. These begin as dense, billowing white towers, formed when warm, moist air is carried swiftly upwards by powerful convection currents. As the humid air bubbles up into cooler parts of the atmosphere, the moisture condenses, transforming fluffy cotton-wool clouds into a massive, lumbering rain cloud.
Supercells can produce treacherous winds, tennis-ball-sized hailstones and flash floods
Given enough instability, moisture, and lift, the hefty cloud becomes electrified. When the cloud reaches the top of the troposphere – the bottom layer of Earth's atmosphere, which contains all the weather – a distinctive anvil-shaped thunderhead forms.
However, once rain starts falling, it causes a downdraft of cool, dense air. This sinks and flows out of the base of the cloud, smothering the updraft and starving the storm of energy. As a result, these storms typically dissipate within about 30 minutes of starting.
In a supercell, vertical wind shear creates huge horizontal tubes of spinning air. These tubes get caught in the updraft and tilt upwards into the storm. This sets up a system where the up- and downdrafts become separated, allowing the storm to maintain its rage for hours.
Supercells can produce treacherous winds, tennis-ball-sized hailstones and flash floods. All tornadoes are associated with supercells, although not all supercells produce tornadoes.
"Mackerel sky and mares' tails make tall ships carry low sails." So goes an old mariners' rhyme, referring to skies of rippled cirrocumulus clouds that resemble the striped scale pattern of a mackerel.
When gentle ripples begin to form across the entire sky, it's a good bet that the storm or its remnants will arrive
These clouds form high in the sky, and the afternoon sun catching their underbellies gives them a dappled, silvery sheen.
Like all good folk sayings, there is some truth to this one. That's because cirrus clouds – thin, wispy, collections of ice crystals – are harbingers of change. They form from small amounts of moisture in the air ahead of approaching weather fronts. As a front draws nearer, sturdier clouds gather and the weather changes.
Mackerel-type cirrocumulus can be an indicator of warm winds lifting up and flowing out from a distant thunderstorm. The ripples form when humid air at the far-flung edges of the storm system pushes past clear, cool air high in the sky. It's the resistance of the cool air to this motion that causes the ripples.
Ridges of cloud form where water vapour cools and condenses, while troughs of space form where it warms and re-evaporates. When gentle ripples begin to form across the entire sky, it's a good bet that the storm or its remnants will arrive in just a few hours.
Mammatus clouds are rounded, pouch-like protrusions that hang from the undersides of clouds. Their name derives from the Latin mamma, which means "udder" or "breast", and they can extend over many hundreds of miles of cloud.
As the crystals fall back down, they change from ice to water vapour
Essentially, mammatus are upside-down clouds formed from sinking pockets of cold, moist air. Often seen bulging from the anvil of a severe thunderstorm or from the ragged clouds in a storm's wake, they usually indicate that the worst weather has passed.
There are a number of theories about how the clouds develop. They all hinge on sharp differences in temperature and humidity between the cloud and the underlying air, and pronounced wind shear at the boundary.
One of the most convincing explanations relies on "negative buoyancy". Updrafts within storm clouds carry air saturated with ice crystals upwards into air that isn't dense enough to support their weight. As the crystals fall back down, they change from ice to water vapour.
This cools the surrounding air, causing it to sink. As a result, the cloud puffs downwards.
Not an official cloud classification but rather a "supplementary feature", virga form when rain or snow begins to fall but evaporates or sublimates in mid-air. They look like a sky full of floating jellyfish, their dangling tendrils being swept away by the wind.
In a storm system, virga frequently precede heavy rains
Virga – meaning literally "shaft" or "branch" – are associated with many different kinds of cloud.
When attached to the underside of low, gloomy clouds like nimbostratus, they give the impression of the cloud being streaked and smudged towards the ground. The prettiest jellyfish skies occur mostly with high clouds that tend to form individual cloudlets, like altocumulus and cirrocumulus.
Virga are often seen in desert areas, where precipitation quickly evaporates in the warmer, drier air beneath the cloud. High altitude can be another contributing factor, simply because there is more air for the rain to fall through before reaching the ground.
In a storm system, virga frequently precede heavy rains when the air below the cloud isn't yet humid enough to support full precipitation. As the rain or snow evaporates away, it cools the surrounding air while increasing its humidity, until finally conditions are ripe for rain to pour.
Also known as hole-punch clouds, these are holes or slashes that appear in layers of mid-to-high-level cloud, when a patch of moisture suddenly starts to freeze and fall.
Fallstreak holes can grow as large as 50km in just one hour
Fallstreak holes occur in layers of cirrocumulus or altocumulus cloud, whose tiny droplets are much colder than the freezing point of water. Despite their temperature, these supercooled droplets remain liquid due to a lack of "seed" particles for ice to grow on. Without these seed particles, the droplets must be cooled below -40°C before they will freeze.
When an aeroplane passes through the cloud layer, the air that rushes around the wings or propeller tips expands and cools, making the surrounding droplets spontaneously freeze. Once these first tiny crystals are introduced, neighbouring droplets rush to join them, whereupon they quickly grow and start to fall.
The descending ice leaves a rounded hole in the cloud layer, which expands swiftly outwards as the frozen boundary spreads. Fallstreak holes can grow as large as 50km in just one hour.
Like lenticular clouds, they have been mistaken for UFOs, but they are arguably at their most dramatic at sunset, when they appear like fiery wounds to the sky.