Nouns ending in -al, -el, -il, -ol, or -eul are usually masculine.
The whole family: -al, -el, -il, -ol, -eul. All masculine.
Is ciel masculine or feminine?
leciel
The rule behind it
96.5% of -al / -el / -il / -ol / -eul nouns are masculine
In context
Aucune étoile n'était visible dans le ciel.
Not a star was to be seen in the sky.
Un nuage blanc flotte dans le ciel bleu.
A white cloud is floating in the blue sky.
Il était en train de regarder le ciel.
He was looking at the sky.