Shaikh Salih al-Fawzan hafidhuhallah was asked:
May Allah be good to you, Oh Generous Parent. The questioner says: Did the Prophet sallahu alayhi wa salam remove the turban from his head during the beginning of a rainfall or did he remove it during the entire time the rain fell?
At the beginning of the rainfall. He would go out during the beginning of the rainfall and allow some rain to fall on him. Yes”
[Taken from here]
Imam an-Nawaawi rahimahullah said:
وفي هذا الحديث دليل لقول أصحابنا أنه يستحب عند أول المطر أن يكشف غير عورته ليناله المطر
“In this Hadith is evidence for the statement of our companions (i.e. the Shafi’ee Scholars) that it is recommended to uncover some part of the body, other than the awrah, during the beginning of the rainfall in order for some of the rain to fall on it”
[Sharh Saheeh Muslim]
Shaikh Ibn ul-‘Uthaymeen rahimahullah was asked:
Is it from the Sunnah when the rain descends for a person to uncover his head and allow the rain to fall on it?
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 sallahu alayhi wa salam 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 here]
Note: Shaikh Ibn ul-‘Uthaymeen rahimahullah 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]
Faisal Ibn Abdul Qaadir Ibn Hassan
More clarification that shows that the narration can be taken generally include:
2- هل يستحب كشف شيء من الجسد؟
قال الشيخ ابن باز: يكشف المرء بعض الشيء من جسده كذراعه أو رأسه حتى يصيبه المطر، كما فعله النبي صلى الله عليه وسلم. (13-64
Shaikh Bin Baz was asked:
Is it recommended to uncover something of the body (when it rains)?
A person uncovers something from his body like his forearm or head so that the rain can fall on it as the Prophet sallahu alayhi wa salam said
Hafidh Ibn Rajab rahimahullah said:
ونص الشافعي وأصحابنا على استحباب التمطر في أول مطرة تنزل من السماء في السنة .
وحديث أنس الذي خرجه البخاري إنما يدل على التمطر بالمطر النازل
بالاستسقاء ، وإن لم يكن أول مطرة في تلك السنة .
“It was reported from Shafiee and his Companions that the recommendation of uncovering during the rainfall is for the first rainfall that descends from the sky that year. The Hadith of Anas which Bukhari reported verily indicates that uncovering is for when rain falls due Istisqa even if that is not the first rainfall of that year”
[Fath ul-Bari (7/140)]
And Allah knows best