Any garment can be used to cover one’s head, and even an article of clothing made from a material of inferior quality is sufficient. Thus, the Shulchan Aruch rules that straw hats are considered a head covering (Shulchan Aruch 91,4). Unless one has no other choice, loose strands which have not been woven together should not be used as a head covering (Kaf Hachaim 91,19).
Covering one’s head with his hand is not considered a head covering (Shulchan Aruch 91,4). Since the hand and the head are merely two parts of one person, one cannot cover his body with itself. What, then, should a man do if he is stuck without a kippah and needs to cover his head?
If one has no other choice, he may pull down his sleeve to cover his hand. Once his hand is covered he can cover his head with the sleeve of his shirt, which is considered a separate entity although it is being worn, since it isn’t part of the body itself (Mishnah Berurah 91,10). If he only needs his head covered for a short interval, e.g., for a berachah, someone else can put his hand on this person’s head (Shulchan Aruch 91,4).
“Hashem hovers above the head of every Jew. When a person walks four cubits with his head uncovered, the Divine Presence leaves him. Since the Shechinah instils life in a person, once the Divine Presence leaves, a person’s life is in danger” (Zohar, Naso 122d).