If you said nimbostratus, you are correct.

如果你选择了雨层云，你是正确。

6000 meters, forming icy cirrostratus layers.

6000 米水平层云，形成冰冷卷层层。

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

霍华德认为所有云属于三个不同类别：云、层云和卷云。

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.

在高空中纤薄层云下降并且扩成薄片状，被命名为卷层云。由大量蓬云聚集并扩而成，则命名为层云。

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

有五种主要云类型：层云、云、层云、雨云和卷云，每种云都根据海拔高度细分为不同属。

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.

霍华德体系妙处在于，这些基成分可以自由组合，描述天空中飘过每种形状、每种大小云——一层云、卷层云、雨云等等。

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，所有云属于三种基形式之一，霍德华给它们命名为：卷云，来自于拉丁语中卷须或头发，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，要是携带水汽上升气流无法突破上面比较稳定一层空气，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种不大讨人喜欢，毫无特色，弄得天空阴沉沉云层。