Clouds

Extreme-level clouds (Mesospheric)

Noctilucent

Nlc

Appearance: Thin and often very feint layer of wispy pale blue or silvery filaments. On are occasions they can appear green or red. Stars are usually clearly visible through the clouds.

Conditions:

• Location: Latitudes between 50° and 70° north and south of the equator

• Air: Stable airmass, neutral or downwards buoyancy convection, low moisture content, <-120ºC.

• Temporal: Visible during nautical and astronomical twilight during summer months of May – August (northern hemisphere, where they are most common) or November – February (southern hemisphere, where they are less common).

Precipitation: None.

Weather: Typical summer weather.

Forecast: Global climate change of the "Oh no, we're all going to die" variety.

More info: Wikipedia, Crystalinks

Very high-level clouds (Stratospheric)

Nacreous

Psc

Appearance: Thin layer of clouds, glowing brightly with vivid iridescent colours similar to those found in a seashell.

Conditions:

• Location: 60ºN→90ºN (arctic), 60ºS→90ºS (antarctic)

• Air: Stable airmass, neutral or upwards buoyancy convection, very low moisture content, -78ºC to -119ºC.

• Temporal: Visible during civil twilight during winter months.

Precipitation: None.

Weather: Frigidly cold and icy, often with very little moisture in the atmosphere.

Forecast: Ozone layer depletion.

More info: Wikipedia, NamesOfClouds

High-level clouds (Tropospheric)

Cirrus

Ci

Appearance: A wide variety of appearances but generally white, delicate-looking and "feathery". Usually translucent, but dense cirrus (formed from a cumulonimbus) can be opaque. Commonly move slowly in a westerly direction regardless of wind direction at ground level.

Formation:

• General: Tend to form on days with fine weather and light winds at ground level.

• Location: Worldwide, covering approximately 25% of the world at any given time! In polar regions, they are often larger than normal.

• Air: Neutral or turbulent airmass stability, neutral buoyancy convection, small moisture content, -20ºC to -30ºC. Often caused by convection in unstable layers of the upper atmosphere, turbulence caused by strong changes in vertical wind direction.

• Temporal: Occurs all year round, but more so during summertime. Can occur at any time of day, but usually only visible during daylight hours.

• Genitus: Cumulonimbus, Altocumulus.

• Mutatus: Cirrostratus, Cirrocumulus, Altocumulus.

Precipitation: In rare cases, after a thunderstorm Cirrus may produce hail or large water droplets and will then dissipate completely. Occasionally produces snowfall virga.

Weather: Usually fair.

Forecast: Large numbers of cirrus clouds indicate an approaching warm weather front, or upper-air disturbance and troughs of low pressure, signalling the approach of stormy weather. See specific cloud forms for more details.

Notes: When present in the jet stream, Cirrus can travel at over 100mph. However, they'll always appear to be slow moving in comparison to lower clouds even if the lower clouds are travelling much more slowly.

More info: Wikipedia, Clouds-online, Nephology, Pilotfriend

Cirrus
aviaticus

Ci avi

Variable

~5km

Appearance: Long, thin artificial clouds caused by condensation or fumes from airborne objects. Often called "contrails" (if non-persistent) or "chemtrails" (if persistent).

Formation:

• General: ?

• Location: Worldwide, anywhere where aircraft fly

• Air: ?

• Temporal: Non-persistent contrails dissipate within a matter of minutes or hours. Persistent contrails can last for many hours or even days, often seeding extensive cloud cover.

• Genitus: Aircraft, missiles, projectiles

• Mutatus: ?

Notes: Can be used to determine the path taken by an aircraft or projectile.

Precipitation: None.

More info: Wikipedia

Cirrus
castellanus

Ci cas

>6km

Appearance: A rarely seen series of dense lumps, or "towers" of Cirrus, connected by a thinner base. Looks a bit like crenellated battlements on old castles. Often accompanied by a thin veil of Cirrostratus.

Formation:

• Location: Worldwide, but rare.

• Air: Unstable airmass stability, neutral buoyancy convection, small moisture content and, most likely, a steep drop in temperature in the stratosphere.

• Mutatus: Cirrus floccus, Cirrus unicus

Precipitation: May form snowfall virgas.

Ground weather: Usually fair.

Forecast: Impending rain or storms.

More info: Wikipedia

Cirrus
<species>
duplicatus

Ci * du

>5km

Appearance: Fairly rare form of Cirrus arranged in superposed layers at slightly different levels, sometimes merged in places.

Species: Most Cirrus fibratus and Cirrus uncinus exhibit this variety. Cirrus spissatus sometimes exhibit this variety.

Notes: They tend to "chase" fair weather.

Pictured: Cirrus fibratus duplicatus.

More info: NamesOfClouds

Cirrus
fibratus

Ci fib

>6km

Appearance: White or silvery clouds with fibrous threads or tufts at the ends. Often seen in two or more layers (duplicatus). They usually appear in small patches.

Formation:

• Location: Worldwide, very common.

• Air: Unstable airmass stability, upwards buoyancy convection.

Precipitation: None.

Ground weather: Usually fair.

Forecast: Thick layers can be a precursor to rain or snow due to an approaching warm front, although this is rare. Thin layers generally indicate that fair weather (eg. Cumulus humilis) will follow, and in countries with tropical weather suggest a sunny day ahead.

More info: Wikipedia, Chitambo

Cirrus
floccus

Ci flo

>5km

Appearance: Small, usually isolated, tufts of pale white cloud, usually with a rugged base. Often seen with snowfall virga.

Formation:

• Location:

• Air: Turbulent airmass stability

Precipitation: Usually virga, although on very rare occasions can briefly cause large rain droplets that reach the ground.

Ground weather: Usually fair.

Forecast: The presence of virga will sometimes give rise to storm cells by forming or contributing to Cumulonimbus.

Notes: Cirrus floccus are the highest and fastest tropospheric clouds, often forming in the jet stream, although they will appear to be moving slowly when viewed from ground level.

More info: Wikipedia

Cirrus
<species>
intortus

Ci * in

>5km

Appearance: Tangled and interwoven filaments (streaks) and wisps of white, pale blue cirrus cloud. Unique to the cirrus genus.

Species: Often associated with Cirrus spissatus.

Formation:

• Location: Worldwide

• Air: Upwards buoyancy convection.

Precipitation: None.

Ground weather: Usually fair.

Forecast: When present in large patches covering most of the sky, stormy weather will likely follow.

Notes: This variety of Cirrus is usually the last to form, using up the last remaining moisture in the stratosphere. The air above them will be incredibly dry.

More info: Wikipedia

Cirrus
<species>
radiatus

Ci * ra

>5km

Appearance: Parallel stripes or bands of pale white cloud, often covering the whole sky. They often appear to converge at a point on the horizon.

Formation:

• Location: Worldwide

• Temporal: Any time of year.

Ground weather: Usually fair.

Forecast:

Notes: Often accompanied by, and sometimes partly merged with, Cirrocumulus or Cirrostratus clouds.

More info: Wikipedia

Cirrus
spissatus

Ci spi

>6km (usually)

>1.5km (sometimes)

Appearance: Opaque threads or whisps, ranging in colour from white to grey with increasing density. Often obscures the sun or moon. Sometimes seen in two or more layers (duplicatus).

Notes: Unlike other Cirrus clouds, these can sometimes link to other, lower-level clouds such as Cumulonimbus, making their base drop well below the high-level part of the stratosphere (as low as 1.5km in some cases).

Formation:

• Location: Worldwide

• Air: Neutral airmass stability, neutral buoyancy convection, small moisture content. Often formed from the upper part of cumulonimbus clouds.

• Genitus: Dissipation of Cumulonimbus capillatus.

• Temporal:

Ground weather: Generally fair, although if present with cumulonimbus there may be rain or snow.

Forecast: Winds from the east travelling south will cause rainfall within 24 hours or less, winds in other directions will cause an overcast sky and often a storm within the next 48 hours.

More info: Wikipedia, NamesOfClouds

Cirrus
unicus

Ci unc

>7km

Appearance: White clouds with "hooks" at one end, often seen in two or more layers (duplicatus). They are generally more thinly distributed than other Cirrus species (for example Cirrus spissatus).

Formation:

• Location: Worldwide

• Air: Unstable airmass stability, upwards buoyancy convection, variable moisture content, -45ºC, warm or occluded front. Winds of 100 – 150mph cause whisps of cloud.

Precipitation: Snowfall virga

Ground weather: Usually fair.

Forecast: Indicates approach of showers, rain, or possibly snow depending on temperature.

More info: Wikipedia, Chitambo

Cirrus
<species>
(bi)undulatus

Ci * un 

>6km

Notes: Not sure this classification is correct - looks more like a Cirrostratus undulatus.

Appearance: Very rare Cirrus clouds with waves running through them. If there are two sets of waves interwoven with each other they are often referred to as "biundulatus".

Formation:

• Location: Worldwide, usually above large open plains (generally flat land).

• Air: Unstable buoyancy convection, variable moisture content.

• Temporal: Usually formed during morning hours and sometimes, usually after a convective thunderstorm, during midday hours.

Precipitation:

Ground weather: Relatively cool or early stages of weather destabilisation.

Forecast:

Cirrus
vertebratus

Ci ve

>6km

Appearance: White clouds in the distinctive form of a spinal column or fish skeleton. Unique to the cirrus genus.

Conditions:

• Location: Worldwide

• Air: Turbulent winds at high altitude. Often found in the jet stream, where air moves in parallel to the main cloud line.

• Mutatus: Cirrus aviaticus

Ground weather: Usually fair.

Forecast: Indication of changing weather.

More info: Wikipedia

Cirrostratus

Cs

>6km

Appearance: Milky, translucent cloud veil of ice crystals, which sometimes causes halo appearances around moon and sun. Often covers the whole sky.

Formation:

• Location: Worldwide

• Air: Stable airmass stability, neutral buoyancy convection, small moisture content, <0ºC

• General: Usually preceded by Cirrus clouds, generally forms when light but increasing winds are present at ground level.

Ground weather:

• Turbulent winds at high altitude. Deterioration of weather conditions within 12 to 24 hours.

More info: Wikipedia, Clouds-online

Cirrostratus
<species>
duplicatus

Cs * du 

>5km

>8km
(topics) 

Appearance: Cirrostratus arranged in up to 12 superposed layers, at different altitudes, sometimes partly merged. Generally the thickest form of Cirrostratus, up to 8km thick, although each layer will usually be only a few hundred metres thick.

Cirrostratus
fibratus

Cs fib

>6km

Appearance: A thin layer of silvery or pale white fibrous veils. Unlike it's nebulosus counterpart, it will not cause halos around the sun or moon.

Formation:

• Location: Worldwide

• Air: ? airmass, ? buoyancy convection, small moisture content, <0ºC.

• Temporal: Most often seen in the morning after sunrise through to the afternoon. In the winter where they can appear at any time of day or night.

Weather: If no other clouds are present, the weather will usually be dry and mild. If other clouds are present, refer to their effects on weather conditions.

Forecast: With falling air pressure, indicates approaching rain or storms within 8 to 24 hours.

More info: Wikipedia, Clouds-online

Cirrostratus
nebulosus

Cs neb

>6km

Appearance: Very common smooth, light-grey, feint or fog-like veil that covers most of the sky. A "halo" may appear around the sun or moon due to light refraction in the ice crystals.

Formation:

• Air: Stable airmass, neutral buoyancy convection, small moisture content, <0ºC.

• Genitus: Cirrus

Precipitation: Does not produce precipitation or virga.

Forecast: With falling air pressure, often accompanied by halo appearance, indicates approaching rain or storms within 8 to 24 hours. This is most common if Cirrus precede them, and Nimbostratus will usually start to form and subsequently lead to a storm.

More info: Wikipedia, Clouds-online, Pilotfriend

Cirrostratus
<species>
undulatus

Cs * un

>5km

>8km
(topics)

Appearance: Quite rare

Formation:

• Location:

• Air: ? airmass stability, ? buoyancy convection, small moisture content, <0ºC, warm front

Precipitation:

Ground weather:

Forecast:

More info: NamesOfClouds, CloudAtlas

Cirrocumulus

Cc

Appearance: High level milky-white cloud with wavelike or fine, dappled cumuliform structure.

Formation: