Mishnah Berurah — משנה ברורה

With one's head uncovered - And certainly it's forbidden make a blessing. And the same applies to learning Torah with an uncovered head. And for this (ie. making a blessing or learning Torah), it doesn't help to cover [the head] with one's hand, because the hand and head are part of the same body and the body can't cover itself. And there are those who rule leniently regarding this in an extenuating circumstance, for example at night when one wants to drink and he does't have a hat/covering on his head, it's enough for him to cover his head with his hand; but it's better to do like most people do - that they pull the sleeve of their shirt over their hand and then use that to cover their head, for that is certainly considered a head covering according to all opinions. The Pri Megadim wrote that one should be careful when putting on his head tefillin, to not recite the blessing with an uncovered head. A toupee made of hair, even those which have a cloth sewed underneath [the hairs], there are those who forbid it in order to avoid the appearance of transgression, for people will say that that is his [actual] hair; and there are those who are lenient.