Question: Is it from the Sunnah when the rain descends for a person to uncover his head and allow the rain to fall on it”?
Answer: “Yes, it is from the Sunnah for a person to uncover something of his body when the rain descends in order for the rain to fall on it. That is not specific to the head. When the rain fell,the Prophet (ﷺ) used to uncover his clothing so the rain could fall on it.
On the authority of Anas ibn Malik radiallahu anhu who said:
A shower of rain fell on us when we were with the Messenger of Allah (May peace be upon him). The Messenger of Allah (May peace be upon him) went out and removed his garment till some of the rain fell on him. We asked him; apostle of Allah! Why did you do this? He replied: Because it has recently been with its Lord”
[Saheeh Sunan Abu Dawood no. 5100]
[Taken from: http://islamancient.com/play.php?catsmktba=11657]
Shaikh Bin Baz rahimahullah said:
“So that proves that it is recommended for a person to uncover something of his body like his forearm or his head so that the rain can fall on it as the Prophet (ﷺ) did. So it is legislated for the Muslim to uncover the turban from his head or the edge of his upper garment from his limb or his forearm so that the rain can fall on it or his shin or whatever resembles that from the things that are permissible to uncover in front of the people, such as the foot or shin or head or hand the likes”
Shaikh Ibn ul-‘Uthaymeen rahimahullah also mentioned it is not from the Sunnah to remain in the rain continuously. The narration says that some of the rain fell on him, not to remain in it. He also said that the jurists said a person can go out into the rain, if he is indoors, in order for the rain to fall on him [See here: http://alathar.net/home/esound/index.php?op=tadevi&id=2294&coid=35507]
And Allah knows best
Faisal Ibn Abdul Qaadir Ibn Hassan