clouds = سَحَاب

سَحَاب (sahab) is a singularisable collective noun (i.e. what is known in Arabic grammar as an اِسْمُ جِنْسٍ جَمْعِيّ , literally a collective generic noun), meaning clouds. Because it is singularisable, we can add a ة to the end of it, and we thus form the singular سَحَابَة cloud. If we want to say it rained, because we don’t have the equivalent of a dummy subject it in Arabic, we have to have a subject such as the sky or the clouds, so we would say مَطَرَتِ السَّمَاء , literally the sky rained, or مَطَرَ السَّحَاب the clouds rained. Note that the verb مَطَرَ to rain in Arabic is transitive, unlike the English equivalent, where the object is the person or thing upon which the rain is falling, e.g. مَطَرَنَا السَّحَاب the clouds rained on us.