Answer
Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
We advise you not to swallow any drops of rain as much as you can.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
If you swallow any morsel of food or water deliberately, your fast is nullified. If, however, it happened by mistake or accidentally – against your best of intentions – your fasting is still considered valid.
We are accountable for actions within our control; while we are excused for those we have no control. Allah states in the Qur’an: {Allah does not charge a soul except [with that within] its capacity.} (Al-Baqarah 2:286)
The Prophet (peace be upon him) said, “Allah has excused my community of their lapses and slip-ups, their forgetfulness and that which they are coerced into.” (Ibn Majah)
Almighty Allah knows best.