This is a list of what I believe to be the top 10 rarest cloud formations. And a brief description of each. No particular order in how ‘rare’ they are though.
1. Nacreous Clouds
These rare clouds, sometimes called mother-of-pearl clouds, are 15 - 25km (9 -16 miles) high in the stratosphere and well above tropospheric clouds. They have iridescent colours but are higher and much rarer than ordinary iridescent clouds. They are seen mostly but not exclusively in polar regions and in winter at high latitudes, Scandinavia, Alaska, Northern Canada. Lower level iridescent clouds can be seen anywhere.
Nacreous clouds shine brightly in high altitude sunlight up to two hours after ground level sunset or before dawn. Their unbelievably bright iridescent colours and slow movement relative to any lower clouds make them an unmistakable and unforgettable sight.
2. Mammatus Clouds
Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air. Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form - a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed.
3. Altocumulus Castelanus
Also known as jellyfish clouds due to their jellyfish-like appearance. These formed around 17,000 ft due to when the rush of moist air comes from the Gulf Stream and gets trapped between layers of dry air. The top of the cloud rises into a jellyfish shape and long tentacles known as “trailing virga” form from rain drops that have evaporated.
4. Noctilucent Clouds
Noctilucent Clouds or Polar Mesopheric Clouds: This is an extroadinarily rare cloud formation that occurs out on the verge of space between 82km to 102 km from the earth’s surface. Noctilucent clouds appear to be luminous yet they reflect the sunlight from the other side of the earth at night, giving them a glowing appearance
5. Mushroom Clouds
A mushroom cloud is a distinctive mushroom-shaped cloud of smoke, condensed water vapor, or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. Volcano eruptions and impact events can produce natural mushroom clouds.
Mushroom clouds form as a result of the sudden formation of a large mass of hot low-density gases near the ground creating a Rayleigh-Taylor instability. The mass of gas rises rapidly, resulting in turbulent vortices curling downward around its edges and drawing up a column of additional smoke and debris in the center to form its “stem”. The mass of gas eventually reaches an altitude where it is no longer less dense than the surrounding air and disperses, the debris drawn upward from the ground scattering and drifting back down (see fallout).
6. Cirrus Kelvin-Helmholtz
Appearing as a slender, horizontal spiral of cloud, cirrus Kelvin-Helmholtz is one of the most distinctive cloud formations. However, it tends to dissipate only a minute or two after forming and, as a result, is rarely observed.
Average height is around 16,500 ft.
7. Lenticular Clouds
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, creating a formation known as a wave cloud.
8. Roll Clouds
A roll cloud is a low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front, or sometimes a cold front. Roll clouds can also be a sign of possible microburst activity. Cool air sinking air from a storm cloud’s downdraft spreads out across the surface with the leading edge called a gust front. This outflow undercuts warm air being drawn into the storm’s updraft. As the cool air lifts the warm moist air water condenses creating cloud, which often rolls with the different winds above and below (wind shear).
9. Shelf Clouds
A shelf cloud is a low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.
10. Stratocumulus Clouds
According to the Sapporo Meteorological Observatory, these low-altitude stratocumulus clouds were rolled into long, distinctive ribbons after becoming trapped in air currents. While it is not uncommon for wind to form such patterns in stratocumulus clouds, photos that clearly show the clouds rolled into strips are rare, says the observatory.
译文:
10种罕见云层
本文所列出的是本人认为最罕见的10种云层，对每一种都做了简单的描述。尽管如此，“罕见”排名并不分先后。
1. 珍珠云层
这种罕见云层又被称为珍珠母云或贝母云层，在离地面15-25公里（9-16英里）的同温层上，远远高过对流云层。它们有着彩虹般的颜色，但比一般的彩虹云高，也更罕见。虽然并不是特有，但珍珠云层大部分发生在极地以及高纬度地区的冬季，比如斯堪的纳维亚，阿拉斯加，加拿大北部。高度稍低的彩虹云则在其他任何地方都可以看到。太阳降下地平线或日出前的两个小时内，光彩云会闪耀在高海拔的太阳光线中。除了不可思议，如同彩虹般亮丽夺目的颜色以外，它也比其它所有低高度云层移动得更加缓慢，因此成为不可错过的难忘景观。
2.乳状云层
乳状云层的结构好像一个个的布袋子，是位于下沉空气中的罕见云层。虽然外表有时让人误以为是坏预兆，但乳状云层实际上是无害的，并不是龙卷风即将来袭的讯号--然而这种误解却普遍存在。事实上，往往在暴风雨侵袭得最猛烈的阶段过后，才能看到这种云层。
3.高积堡状云层
因为其海蜇一般的外表，也被叫做海蜇云层。这种云形成于17，000英尺高度，并取决于大规模潮湿大气何时从海湾涌来并停留在干燥大气层间。云层顶部上升形成海蜇形状，而蒸发后的雨滴形成的那些如同长长触角形状的云则被称为“后缘雨幡”。
4. 夜光云层
夜光云层又被称为极地中层云：这种云层极其罕见，只形成于距离地表82-102公里的大气空间边缘部位。夜光云层虽然看起来很耀眼，但实际上其只是在夜晚反射出地球另一面的太阳光线而已，也因此外表光彩夺目。
5.蘑菇云层
蘑菇云层具有独特的蘑菇形状，由大爆炸产生的烟雾，凝结水蒸气以及各种碎片组成。蘑菇云层通常都与核爆炸有关，但如果有足够大的威力的话，其实任何稍次等级的爆炸也会产生相同效果。火山爆发以及撞击事件发生时也能自然形成蘑菇云层。
蘑菇云层是由造成瑞利-泰勒不稳定性的地面附近大规模低密度热气体突然形成而造成的。大规模气体急剧上升，形成狂暴的漩涡，整个漩涡向下俯冲，卷起大量烟雾和碎片并吸附在漩涡中心，从而形成蘑菇的“茎干”部分。当大规模气体最终到达一定海拔高度后，其密度不再低于周围大气，而是与其密度相等，此时气体散开，从地面被卷起的碎片再次朝地面漂浮散落（参见：放射性尘埃）。
6.
凯文-赫摩兹藤蔓云层
凯文-赫默兹藤蔓云层看起来就好比水平面上一个个纤长的螺旋状云彩，该特点使其成为最独特的云层之一。尽管如此，由于在形成后1-2分钟之内就会消失无踪，这种云层很少能够被看到。其形成平均高度为16，500英尺。
7.荚状云层
荚状云层的学名是荚状层云，外表静止不动，形状如同透镜，形成在高海拔地区，通常位于风向朝右的角度。位于该角度是因为这时稳定的潮湿空气吹向山或山脉，一系列大规模的静止气浪于是在顺风一侧积聚形成，从而形成荚状云层。荚状云层有时会形成在这些气浪的顶部，在这种情况下会出现长长的一排荚状云，这种云层被称做气浪云。
8.涡动云层
涡动云层是位于低空的弧状云层，外表如同水平面上的一只滚筒。它的形成大多都与阵风锋面有关，有时也与寒流有关。与此同时，涡动云层也有可能是微爆气流活动的讯号。雷暴云下降气流扩散在整个天空表面，其最前面的部分被称为阵风锋面，在这种情况下，冷空气会沉积在天空中。这些涌动的冷空气减低了被雷暴上升气流提升的暖空气。冷空气将大气中温暖潮湿的水份凝结成云，随着其上方及下方的不同风向滚动(这一现象被称为风切变）。
9. 架状云层
架状云层位置低，在水平面上如同一个弧状三角形。大多与阵风锋面有关（有时即使没有雷暴，也与寒流有关）。与涡动云层不同，架状云层与其上方的母云层相连（该母云层通常是雷暴云）。因此时常可以见到架状云前端（外侧）向上运动，而架状云下端的运动却如同煮沸的水一般狂暴不安。
10. 层积云层
据札幌气象台观测，这些低海拔层积云层被其周围的大气气流卷成一条条长而奇特的彩带形状。尽管风能够把云吹成这种层积云层的形状并不足为奇，但气象台说，能清晰反映卷条状层积云层的照片十分罕见。