Allāh the Exalted said: {Remember when He overwhelmed you with drowsiness, giving security from Him, and He sent down upon you from the sky, rain by which to purify you and remove from you the evil suggestions of Satan, and to make steadfast your hearts, and plant firmly thereby your feet. [al-Anfāl (8): 11].

This āyah indicates rain water is both pure and purifying. Thus, it can be used for ritual purification from both major and minor states of ritual impurity, and it can be used to remove tangible dirt.

Major ritual impurity is caused by sexual intercourse and wet dreams for instance and requires complete bathing (ghusl) to regain ritual purity.

Minor ritual impurity is caused by answering the call of nature and/or breaking wind. In this case, ritual impurity is regained by just performing ablution (wuḍūʾ).

What follows are the explanations of the scholars throughout history, indicating that rain water is pure and purifying.

Muqātil b. Sulaymān (d. 150)

{and He sent down upon you from the sky, rain by which to purify you}: from impurities and major ritual impurity.

al-Ṭabarī (d. 310)

This was the rain Allāh sent down from the sky on the Day of Badr in order to purify the believers with it for their prayer, because they awoke in the morning on that day in a state of major ritual impurity due to discharging semen or sexual intercourse (janābah in short) without water. When Allāh sent water down on them, they bathed and purified themselves. The devil had whispered to them with what made them sad in terms of them waking up in janābah without access to water, so Allāh removed that from their hearts with the rain.

Qatādah said that the Muslims drank the water, performed ablution with it, and gave it to others.

Ibn ʿAbbās said that Allāh sent down heavy rain on them, so the Muslims drank therefrom and performed ablution.

He also said that the Muslims filled their drinking vessels with the water, bathed from janābah, and gave their riding beasts water to drink. So Allāh made that pure and purifying (ṭahūr).

al-Ḍaḥḥāk said that Allāh sent down rain which filled the valley and so the Muslims drank therefrom and filled their drinking vessels, gave water to their riding beasts, and bathed from janābah.

al-Azharī (d. 370)

al-ṭahūr in the language of the Arabs is that which is pure and that which purifies because it cannot be ṭahūr except if can be purified with, just like waḍūʾ is the water with which ablution is performed, and faṭūr is that by which a fast is broken in terms of what is drunk and eaten.

al-Baghawī (d. 516)

{And He sent down upon you from the sky, rain by which to purify you} from impurities (aḥdāth) and janābah.

Ibn al-Athīr (d. 606)

He states that the word ṭahūr is the water with which one purifies. And ṭahūr water in jurisprudence refers to that which lifts ritual impurity (ḥadath) and removes dirt (najas), because the word pattern faʿūl is from the forms of exaggeration and intensity, so it is as if it has reached the height of purity. [al-Nihāyah fī Gharīb al-Ḥadīth, p. 572 under ṭ-h-r].

al-Nawawī (d. 676)

The scholars of his legal school argue that the word ṭahūr, as it occurs in the Sharīʿah, means ‘purification’. They base this on Allāh’s statement {And We send down from the sky pure and purifying (ṭahūr) water} [al-Furqān (25): 48], and {And He sends down on you, from the sky, water with which to purify you} [al-Anfāl (8): 11], so this explains the intent of the first. What al-Nawawī means is: the first āyah describes rain water using the word ṭahūr, and the second āyah explains what ṭahūr is: it purifies, thus ṭahūr is pure and purifying water. [al-Majmūʿ (p. 62) ed. al-Afkār].

Ibn Kathīr (d. 774)

{in order to purify you with it} means: from minor ritual impurity and major ritual impurity which is the purification of the apparent/external.

al-Shawkānī (d. 1250)

{in order to purify you with it} means: in order to raise from you ritual impurities (aḥdāth).

Jamāl al-Dīn al-Qāsimī (d. 1332)

{And He sent down upon you from the sky, rain by which to purify you} from minor ritual impurity and major, which is the purification of the external. [Same quote as Ibn Kathīr].

al-Saʿdī (d. 1376)

Allāh sent down upon you, from the heavens, rain in order to purify you with it from impurity and filth, and to purify you with it from the whisperings of the devil and his filth.

al-Tafsīr al-Muyassar

Allāh sent down upon them, from the clouds, pure and purifying water, in order to purify them with it from apparent impurities, and to remove from them the internal evil whisperings of the devil.