Tafsir of Al-Mursalat 77:31

(No shade), meaning: no shade at all. It is a second adjective for "shading" (referring to the tripartite division of the smoke), in that it is not a shade meant for protection. It is described as such to indicate that designating it as a "shade" is a form of mockery toward them, and because one might mistakenly assume it provides them with relief; thus, this possibility is negated.

Therein lies an allusion that their shade is not like the shade of the believers. (And [it] does not avail), meaning: it does not protect [them] from the flame, nor is it useful at any time against the heat of the fire. The word yughni (avail) is construed with the preposition min (from) because it implies the meaning of "warding off" or "protecting."

It is well-known that this verse points to a geometric principle, which is that a triangular shape casts no shadow. Reflect upon whether you can grasp that.