Now for the relationships between the constituents of the phrases:

Know that the "or" of "or [the parable] of a violent rainstorm" (the "aw" of "aw ka-sayyibin,") indicates that [the dissemblers] represented in the comparison are divided into two sorts, and is a sign that there is a true correspondence both between the two parables, and between them and [the condition of] the dissemblers, and that there is an indisputable similarity between them. Moreover, the "or" comprises the [intensifying conjunction] "bal al-tarqiyya" [meaning, but rather, no on the contrary, but, indeed], for the second parable is more terrifying. The lack of correspondence between "or [the parable] of a violent rainstorm (aw ka-sayyibin)" and the dissemblers [that is, the dissimilarity between them, because the dissemblres are being likened to the rainstorm] necessitates that the thing [the rain] does resemble is implied (muqaddar). This implied thing is not stated for conciseness and the words are made concise [that is, the purpose of the ellipsis is] to make the meaning prolix, and the prolix meaning is referred to the listener's imagination so he may seek further meanings from the context. Thus by reason of this lack of correspondence [the Qur'an] is as though saying: "or they resemble people who journey through the empty desert and dark night and are struck by many calamities resulting from a violent rainstorm." The replacement of the familiar friendly word 'rain' (matar) with "rain cloud; downpour (al-sayyib)" infers that the droplets of the rain are each calamities (masâ'ib) pelting down on them like bullets finding their mark (tusîbahum), and that [the dissemblers] have nowhere to shelter.

The explicit mention of "(lit. from) in the sky (min as-samâ')" [stating] specifically that the rain comes from the sky alone, is to express generality. That is, the sky is specified to make it absolute (itlaq), as in the verse "There is not an animal [that lives] on the earth, nor a being that flies on its wings;"(6:38) that is, the rain has taken possession of the whole face of the sky. Some Qur'anic commentators concluded from "from the sky" here and from the verse "And He sends down from the sky mountain masses [of clouds] wherein is hail,"(24:43) that the rain falls from the body (firm) of the sky, and some of them even imagined the existence of an ocean beneath the sky. but [the science of] eloquence (or rhetoric) does not endorse this. No, the meaning is "from the direction of the sky," and the sky is specified for the above-mentioned reason. Moreover, [since] the word "sky" is used [generally] for everything above you, the clouds and atmosphere [may also be said to be] the sky.

Verification of the Subject (tahqîq al-maqâm): If you consider [divine] power, [you will see that] all sides are equal [before it]; that is, the rain may fall from any direction.

And if you consider divine wisdom, [you will see that it] establishes the optimum order in things, necessitating the preservation of the general balance, and that it chooses the most direct means. As for the rain, it is [formed] through the condensation of the water vapour dispersed through the globe of the atmosphere, one of the ten constituents of which is this vapour in its depths.