From the ethereal to the ominous, our planet’s skies are teeming with strange clouds
- By Ceri Perkins
16 July 2015
Named for their sleek, lentil-like appearance, altocumulus lenticularis clouds (pictured above) are believed to be one of the most common explanations for UFO sightings. Thesis spooky forms show up downwind of mountains.
Lenticular clouds are eerie because they seem to hover ter place
When moving air encounters an obstacle like a mountain, it is coerced to rise up and overheen it. Spil the air spills overheen 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 proceed to rise and dip for a little while spil it travels away from the mountain, setting up an invisible “standing wave”.
Lenticular clouds form when the airflow streaming overheen the mountain is both stable and humid. Spil it flows upwards and cools, the moisture ter the air condenses to form clouds at the crests of the standing wave.
Lenticular clouds are eerie because they seem to hover ter place, even te a stable wind. Te 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 immovable, creating a well-defined cloud that drapes motionless ter the sky.
They are some of the highest and rarest clouds on Earth, but thesis iridescent specimens are both beauty and animal. Spil a type of polar stratospheric cloud, they play a central role te 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 te the extreme cold of winter. When air temperatures druppel below -83°,C, petite amounts of moisture ter the otherwise dry stratosphere condense into wispy clouds of ice crystals.
Because the clouds form at altitudes of overheen 15,000m, the Zon keeps illuminating them even when &ndash, from the point of view of people on Earth’s surface &ndash, 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 very first fresh type of cloud to be identified ter overheen half a century. They consist of dark, chaotic flaps that seem to swirl and tumble haphazardly across the sky.
Our language of clouds dates back to 1802, when fledgling scientist and earnest sky watcher Luke Howard delivered a lecture tntitled “On the Modifications of Clouds”. Howard classified clouds te 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 bot added to Howard’s work, leaving us with a rich nomenclature that has remained unchanged since 1951. That is, until now.
Ter 2008, Gavin Pretor-Pinney of the Cloud Appreciation Society proposed a fresh type of cloud. Members of the Society had bot sending ter 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”.
Ter June 2015, the name &ndash, slightly modified to its noun form, asperitas &ndash, wasgoed 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, pursuing 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 violating wave is the Holy Grail for many cloud-spotters. It occurs almost everywhere ter the world and at all levels of the atmosphere. But it only lasts a few minutes before dispersing without a trace.
Thesis veelbewogen atmospheric swings are a sign that the air is enormously unstable
The crashing wave pattern is caused when swift, warm air flows overheen a colder, denser, more sluggish layer. Spil physicists William Kelvin and Hermann von Helmholtz discovered, the difference te the speeds and densities of thesis fluids creates a shearing force where they meet, producing undulations ter the boundary inbetween the two.
If the speed difference is just right, the tops of the denser ripples can roll up, surge forwards, and peak overheen ter a series of vortices, just like a violating ocean wave. When clouds toebijten to form at the horizontal boundary, this invisible air process is shortly made visible.
However beautiful, thesis veelbewogen atmospheric flaps are a sign that the air is utterly unstable and may be dangerous to aircraft.
Supercells are the least common kleintje of thunderstorm. Their disruptive might is 2nd only to hurricanes.
What sets them chic from regular violent storms is a persistent rotating updraft called a mesocyclone, which permits the storm to sustain itself overheen many hours.
Typical thunderstorms develop from cumulonimbus clouds. Thesis start spil dense, billowing white towers, formed when warm, moist air is carried swiftly upwards by powerful convection currents. Spil the humid air bubbles up into cooler parts of the atmosphere, the moisture condenses, converting 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 &ndash, the bottom layer of Earth’s atmosphere, which contains all the weather &ndash, a distinctive anvil-shaped thunderhead forms.
However, once rain starts falling, it causes a downdraft of cool, dense air. This submerges and flows out of the base of the cloud, strangling the updraft and starving the storm of energy. Spil a result, thesis storms typically dissipate within about 30 minutes of embarking.
Te a supercell, vertical wind shear creates enormous horizontal tubes of spinning air. Thesis tubes get caught te the updraft and tilt upwards into the storm. This sets up a system where the up- and downdrafts become separated, permitting 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, albeit 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 start to form across the entire sky, it’s a good bet that the storm or its remnants will arrive
Thesis clouds form high ter the sky, and the afternoon zon catching their underbellies gives them a dappled, silvery glitter.
Like all good folk sayings, there is some truth to this one. That’s because cirrus clouds &ndash, lean, wispy, collections of ice crystals &ndash, are harbingers of switch. They form from petite amounts of moisture te the air ahead of approaching weather fronts. Spil a gevelbreedte draws nearer, sturdier clouds gather and the weather switches.
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 thrusts past clear, cool air high te the sky. It’s the resistance of the cool air to this maneuverability that causes the ripples.
Ridges of cloud form where water vapour cools and condenses, while troughs of space form where it heats and re-evaporates. When gentle ripples start to form across the entire sky, it’s a good bet that the storm or its remnants will arrive ter just a few hours.
Mammatus clouds are rounded, pouch-like protrusions that drape from the undersides of clouds. Their name derives from the Latin moeder, which means “udder” or “breast”, and they can extend overheen many hundreds of miles of cloud.
Spil the crystals fall back down, they switch from ice to water vapour
Essentially, mammatus are upside-down clouds formed from drowning pockets of cold, moist air. Often seen bulbous from the anvil of a severe thunderstorm or from the ragged clouds te 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 acute differences ter temperature and humidity inbetween the cloud and the underlying air, and pronounced wind shear at the boundary.
One of the most wooing 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. Spil the crystals fall back down, they switch from ice to water vapour.
This cools the surrounding air, causing it to bury. Spil a result, the cloud puffs downwards.
Not an official cloud classification but rather a “supplementary feature”, virga form when rain or snow commences to fall but evaporates or sublimates ter mid-air. They look like a sky total of floating jellyfish, their dangling tendrils being swept away by the wind.
Te a storm system, virga frequently precede mighty rains
When linked 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 ter desert areas, where precipitation quickly evaporates te the warmer, drier air underneath 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.
Ter a storm system, virga frequently precede intense rains when the air below the cloud isn’t yet humid enough to support total precipitation. Spil the rain or snow evaporates away, it cools the surrounding air while enlargening its humidity, until ultimately conditions are ripe for rain to pour.
Also known spil hole-punch clouds, thesis are slots or slashes that emerge te layers of mid-to-high-level cloud, when a patch of moisture all of a sudden starts to freeze and fall.
Fallstreak crevices can grow spil large spil 50km ter just one hour
Fallstreak crevices occur ter layers of cirrocumulus or altocumulus cloud, whose lil’ droplets are much colder than the freezing point of water. Despite their temperature, thesis supercooled droplets remain liquid due to a lack of “seed” particles for ice to grow on. Without thesis seed particles, the droplets voorwaarde 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 thesis very first lil’ crystals are introduced, neighbouring droplets rush to join them, whereupon they quickly grow and commence to fall.
The descending ice leaves a rounded crevice ter the cloud layer, which expands swiftly outwards spil the frozen boundary spreads. Fallstreak fuckholes can grow spil large spil 50km te just one hour.
Like lenticular clouds, they have bot mistaken for UFOs, but they are arguably at their most dramatic at sunset, when they show up like flamy wounds to the sky.