The Meath Weather Cam
An Insight into Irish Weather, 24/7
Mid Level Clouds
Altostratus
Appearance Sheet or layer, can usually see the sun through it Precipitation Cloud? Yes
Altostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen shining through them, and they frequently cover the whole sky. They are similar to lower altitude stratus clouds.
Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system and can be found over wide-spread areas. Altostratus clouds are potentially dangerous, because they can cause ice accretion on aircraft. Their altitude is from 6,500-20,000 feet. They are made of ice crystals.
Altostratus Undulatus
| Appearance | Undulated |
|---|---|
| Precipitation Cloud? | Usually no |
The altostratus undulatus is a type of low altocumulus cloud with signature undulations within it. These undulations may be visible (usually as "wavy bases"), but frequently they are indiscernible to the naked eye. These formations will generally appear in the early stages of destabilizing return flows, especially over the southern plains of the United States, when the surface temperature is still relatively cool. The wavy strips of clouds are generally near an inversion surface.
Also referred to as "billow clouds," "windrow clouds," or "wave clouds," variations of the undulatus can be elements that have merged or single elements that have stretched through the sky. They often run parallel, but can also appear to interweave across the sky, especially if dual wave systems are seen (also referred to as biundulatus). Gravity waves aligned normally to the direction of the wind can show wide, near-parallel cloud strips.
Altocumulus
| Appearance | Similar to cirrocumulus, but individual segments are larger and darker. |
|---|---|
| Precipitation Cloud? | No, but usually signifies an oncoming thunderstorm. |
Altocumulus (Alto, "high", cumulus, "heaped") is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection. It is usually white or gray, and often occurs in sheets or patches with wavy, rounded masses or rolls. Altocumulus often are seen preceding a cold front, and their presence on a warm, humid, summer morning frequently signals the development of thunderstorms later in the day. Alto means high and these clouds may cause rain if they are higher up.
Altocumulus Undulatus
| Appearance | Undulated |
|---|---|
| Precipitation Cloud? | No |
The altocumulus undulatus is a mid-level cloud (about 8000 - 20,000 ft or 2400 - 6100 m), usually white or grey with layers or patches containing undulations that resemble "waves" or "ripples" in water. Elements within the cloud (such as the edges of the undulations) are generally darker than those in cirrocumuli and smaller than those in stratocumuli. These clouds may appear both as patches or as covering the sky. The layer of these clouds is generally under about 300 feet (91.44 meters) thick.
As with other altocumuli, the undulatus variety can form in all seasons, announcing an approaching system within the general area (about 100 - 200 mi or 160 - 322 km). They result from wind shear (an abrupt speed or directional shifting in the wind, acutely changing with height). Lines one might see indicate the direction of the shear. In the southern United States, these formations may be the result of tropical or subtropical system outflow in a northern direction. They may also come from a southwestern monsoon making its way across the Western and Plains States.
Altocumulus Mackerel Sky
| Appearance | Clumps and rolls of clouds that resemble mackerel scales altogether |
|---|---|
| Precipitation Cloud? | Usually no but can lead to rainshowers or thunderstorms |
A mackerel sky is an indicator of moisture (the cloud) and instability (the cumulus form) at intermediate levels (2400-6100 m, 8000-20,000 ft). If the lower atmosphere is stable and no moist air moves in, the weather will most likely remain dry. However, moisture at lower levels combined with surface temperature instability can lead to rainshowers or thunderstorms should the rising moist air reach this layer. In the winter it is often said to precede snowstorms and flurries. There is an old saying, "Mackerel sky, mackerel sky. Never long wet and never long dry." The phrase 'mackerel sky' came from the fact that it looks similar to the markings of an adult king mackerel.
Altocumulus Castellanus
| Appearance | Tower-like projections that billow upwards from the base of the cloud |
|---|---|
| Precipitation Cloud? | No, but could signify later precipitation. |
Altocumulus castellanus (ACCAS) is named for its tower-like projections that billow upwards from the base of the cloud. The base of the cloud can form as low as 2,000 metres (6,500 feet), or as high as 6,000 metres (20,000 feet).
Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude lapse rate. They may be a harbinger of bad weather and, if surface-based convection can connect to the mid-tropospheric unstable layer, continued development of castellanus clouds can produce cumulonimbus clouds and thunderstorms.
Altocumulus Castellanus clouds can indicate rough turbulence for aircraft.
The appearance of Altocumulus Castellanus on a sunny morning announce with high probability the formation of thunderstorms in the afternoon.
Altocumulus Lenticularis
| Appearance | Lens-like |
|---|---|
| Precipitation Cloud? | Sometimes |
Lenticular clouds, technically known as altocumulus standing lenticularis, are stationary lens-shaped clouds that form at high altitudes, normally aligned at right-angles to the wind direction. 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. Under certain conditions, long strings of lenticular clouds can form near the crest of each successive wave, creating a formation known as a 'wave cloud'. The wave systems cause large vertical air movements and so enough water vapour may condense to produce precipitation. The clouds have been mistaken for UFOs (or "visual cover" for UFOs) because these clouds have a characteristic lens appearance and smooth saucer-like shape.
Power pilots tend to avoid flying near lenticular clouds because of the turbulence of the rotor systems that accompany them, but sailplane pilots actively seek them out. The precise location of the rising air mass is fairly easy to predict from the orientation of the clouds. "Wave lift" of this kind is often very smooth and strong, and enables gliders to soar to remarkable altitudes and great distances. The current gliding world records for both distance (over 3,000km) and altitude (15,460 m) were set using such lift.
Information and photos on this page are courtesy of Wikipedia
