If you said nimbostratus, you are correct.

如果你选择了雨层云，你是正确的。

6000 meters, forming icy cirrostratus layers.

6000 米的水平层云，形成冰冷的卷层层。

Howard believed that all clouds belonged to three distinct groups: cumulus, stratus and cirrus.

霍华德认为所有的云属于三个不同的类别：云、层云和卷云。

There are five primary cloud types: stratus, cumulus, stratocumulus, cumulonimbus  and cirrus, each of which are subdivided into different genera based on altitude.

有要的云类型：层云、云、层云、雨云和卷云，每云海拔高度细分为不同的属。

The beauty of Howard's system was that the basic components could be freely recombined to describe every shape and size of passing cloud--stratocumulus, cirrostratus, cumulocongestus, and so on.

霍华德体系的妙处在于，这些基本成分可以自由组合，描述天空中飘过的每形状、每大小的云——一层云、卷层云、雨云等等。

A high, whispy cirrus cloud that descended and spread into a sheet was named cirrostratus, while groups of fluffy cumulus clouds that joined up and spread were named stratocumulus.

原本在高空中的纤薄的层云下降并且扩散成薄片状，被命名为卷层云。由大量蓬松的云聚集并扩散而成的，则命名为层云。

In fact, all clouds belong to one of three principle types to which Howard gave the names: cirrus, Latin for tendril or hair, cumulus, heap or pile, and stratus, layer or sheet.

实际上，所有的云属于三基本形式之一，霍德华给它们命名为：卷云，来自于拉丁语中的卷须或头发，云，堆或累，层云，层叠或片。

For instance, stratus clouds-those unlovable, featureless sprawls that give us our overcast skies, happen when moisture-bearing updrafts lack the oomph to break through a level of more stable air above, and instead spread out, like smoke hitting a ceiling.

比如，要是携带水汽的上升气流无法突破上面比较稳定的一层空气，就会像烟碰到了天花板那样向外展开，于是就形成了层云一那不大讨人喜欢，毫无特色，弄得天空阴沉沉的云层。