مَا فِي السَّمَاءِ قَدْرُ رَاحَةٍ سَحَابًا
Translation: There is not even a palm-sized quantity of cloud in the sky.
مَا: negation.
فِي السَّمَاءِ: prepositional phrase linked to a fronted understood predicate.
قَدْرُ: in this explanatory parsing, postponed nominative subject and muḍāf.
رَاحَةٍ: genitive muḍāf ilayh (a palm-width measure).
سَحَابًا: accusative tamyīz specifying the substance—cloud.
The printed source does not specify the vowels here. The nominative analysis of qadr is one explanatory parsing, not an explicit case assignment quoted from the author.