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12-05-2008, 23:28   #16
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Altocumulus Castellanus (Ac cas)

This cloud formation is the reult of instability in the midlevel regions. This cloud forms when colder air slides across an area of altocumulus cloud. It is a very good indicator for thunderstorms later in the day. With further surface heating during the day the mid level cas grow into cumulonimbus. The first pic below is from a decaying altocumulus late in the day showing virga trails.
The second pic is ML instability also late in the day and ahead of Thunderstorms that moved over to Ireland from Wales later in the evening.


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12-05-2008, 23:29   #17
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Altocumulus Lenticularis

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12-05-2008, 23:31   #18
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Stratus (St)

Stratus (meaning layer) forms in sheets or layers. It occurs when large areas of moist air rises gently in a stable atmosphere to a level where condensation occurs.
A common setup in Ireland.
Also it is the cloud thats asscociated with anticyclonic gloom.



Pic by Lady North

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12-05-2008, 23:32   #19
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Stratocumulus

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12-05-2008, 23:33   #20
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Nimbostratus

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12-05-2008, 23:33   #21
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Orographic Stratus

Orographic clouds are formed when moist air carried on a prevailing wind, is lifted by elevated areas such as a mountain range(upward of 150m) to a level where it condenses. The most common orographic cloud is orographic stratus. This occurs mostly in coastal regions where the air is moisture laden.

Unlike normal stratus which is moved by wind, this cloud remains stationary. The wind flows through the area of condensation, constantly regenerating the cloud as the air rises and dissipating it as the air descends on the other side of the mountain/hill.



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12-05-2008, 23:35   #22
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12-05-2008, 23:36   #23
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Cumulus Humilis

Cumulus humilis(meaning humble) is the weakest form of convection. Usually witnessed in the morning or early afternoon when convection begins or preceding a warm front as fragmented cumulus. This cloud does not represent any significant weather but if conditions are right, can grow into a Cumulonimbus.
Also known as fair weather clouds.









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12-05-2008, 23:36   #24
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Cumulus Mediocris

Cumulus Mediocris which means moderate is created by slightly stronger convection. This cumulus is tall as it is wide. This cloud is often a transitional stage between the lesser humilis and the more developed congested stage.
No precipitation occurs from this cloud.





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12-05-2008, 23:37   #25
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Cumulus Congestus

This is the next stage in the vertical development of a cumulus cloud. Congestus rely not only on convection but instability is required in formation of this cloud. Instability is when the temperature of the surrounding air mass drops with height than normal when cold air sliding over the cloud.
Unlike the lesser cumulus clouds, this congestus is most common for it's heavy showers but without thunder and lightning.




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12-05-2008, 23:37   #26
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Pyrocumulus

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12-05-2008, 23:38   #27
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Cumulonimbus Calvus

Cumulonimbus Calvus or towering cumulus(TCu) represents a transitional stage between cumulus congestus and cumulonimbus incus(anvil).Calvus occurs when convection and instability combine to push the cloud tops beyond the congested stage up to 30,000 feet. Temperature at this level is well below 0C so instead of water droplets, ice crystals are produced giving the brilliant bright white appearance.No anvil is evident at this stage.



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12-05-2008, 23:39   #28
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Cumulonimbus Incus

Cumulonimbus Incus(anvil) is the mature cumulonimbus or thundercloud. These clouds can reach heights of up to 60,000 feet in tropical and sub-tropical regions(30,000 to 36,000 feet in Ireland.This cloud is refered to as "The King of Clouds".
This begins as a simple Cumulus humilis in the morning and can quickly go through mediocris,congestus and clavus stages. To continue to grow, convection must combine with instability to produce a powerful updraft. As long as the air is unstable, the Cb grows and expands untill it reaches the top of the troposphere where the temperature levels off and begins to increase with altitude, this change in temperature has the affect of placing a lid on the updraft and the cloud will rise no further.
The mommentum of the updraft below continues to push up and spread the top out in radial fashion forming the anvil.

From the pic below you can tell the direction of the storm as the anvil is being pushed by the upper level winds.


Pic by Deep Easterly

Next is a small storm over Co.Meath with a small but matured Cb with anvil


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12-05-2008, 23:39   #29
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Cumulonimbus with Pileus

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12-05-2008, 23:40   #30
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Cumulonimbus with Mammatus

Mammatus(Latin for breast) forms on the underside of an anvil of a mature Cb cloud. It is formed from a process called reverse direction convection. What happens is moist warm air rises in the Cb, spreads out at the top of the troposphere. The temp levels off and the air stabilises.

The rising air expands horizontally over cooler cloud free air and creates instability between the two air masses under the anvil, which causes pockets of warm moist air to convect downward. The reverse direction convection is enhanced by the effects of gravity and by precip by the cloud. The process produces near symmetrical protuberances on the underside of the anvil known as mammatus which may cover large areas.

Mammatus is a sign of a thunderstorm reaching maximum growth and intensity and usually a sign of severe thunderstorms. Mammatus may be several miles away from the centre of the storm.



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