Does placing a hand on one’s head suffice in place of a Yarmulke?
It is forbidden for men to sit, walk or say a blessing without a head covering. The following will discuss whether placing a hand on one’s hand is considered a valid head covering in a situation that another covering is not available:
One’s own hand: Placing one’s hand on his head suffices in place of a Yarmulke with regards to being able to sit and walk. [Thus, if one does not have a Yarmulke or other head covering, he is to cover his head with his hand and may then walk for even more than four cubits.] However, he may not say a blessing or any of Hashem’s names if his head is covered merely by his own hand.
Another person’s hand: If another person places his hand on one’s head then one may be lenient to consider it a valid covering [for all matters and one may even recite a blessing with G-d’s name].
 Basra 2:6; Taz 8:3; M”B 2:11; See Measef Lechol Hamachanos 2:21; Yabia Omer 9:1; Piskeiy Teshuvos 2:9; For a thorough analysis on the obligation See Piskeiy Dinim Tzemach Tzedek 1-8
Ruling of Kama: In Kama 2:7 Admur implies that walking four cubits with a revealed head is forbidden according to law. However, in 91:3 Admur rules even walking more than four Amos is merely an act of piety, and so is implied from 46:2. See next footnote.
 Admur Basra 2:6; 91:3
 Based on Admur 93:4; Basra 2:6 as summarized in Ketzos Hashulchan 3:6; and so rules M”B 2:11-12; Kaf Hachaim 2:20 based Misgeres Hazahav
Background: There are two reasons for requiring a skullcap; one is for modesty while the second is [an act of holiness that is necessary] in order to utter G-d’s name, when one comes to say a blessing, or pray. Regarding modesty, covering one’s head with one’s hand suffices. Thus, one may walk more than 4 cubits if he covers his head with his hand. However, when saying G-d’s name, it does not suffice to use one’s own hand as will be explained.
 Admur Basra 2:6 “However, if he places his hand on his head to cover it, it suffices as a recognition, although he may not mention Hashem’s name”; Taz 8:3; M”B 2:11
The reason: As the purpose of the Yarmulke is to cover the normally covered areas and as a sign of modesty, and Jewish identity. Thus, placing a hand on the head suffices as recognition for this matter. [Taz and Basra ibid]
Other opinions: Some Poskim rule that when one is outside, it does not suffice to place one’s hand on one’s head. [Peri Megadim 2 M”Z 5, brought in M”B ibid]
 Admur 91:4; Basra 2:6; M”B ibid; Ketzos Hashulchan ibid; Kaf Hachaim ibid
The reason: One’s hand and body are one entity and the body cannot be used to cover itself. [91:3]
 Admur 91:4
 So is clearly implied from 91:3-4 and is also evident from Basra 2:6 which refers the reader to 91:3, when stating that one’s own hand cannot be used as a covering to utter blessings.
The reason: (As the Rashal ibid is completely lenient) [91:4] This means to say as follows: The Rashal rules that one never requires a head covering inside a house even when reciting a blessing. Hence, in a time of need, one may be lenient to consider another person’s hand a covering. Vetzrauch Iyun why based on this we should be lenient to validate another persons hand even when outside, in which case even the Rashal requires a head covering.