Answered by Sidi Sulaiman Hart
What is the reality behind the hukm of combining whilst raining i.e. can you combine at home, what is rain?
(see Reliance f15.14, page 195)
JOINING PRAYERS BECAUSE OF RAIN
f15.14 It is permissible for a nontraveller to pray the noon prayer (zuhr) and the midafternoon Prayer (asr) at the time of the noon prayer (N: or the Friday prayer (Jumu’a) and midafternoon Prayer at the time of the Friday prayer), and to similarly pray the sunset prayer (maghrib) and nightfall prayer (isha) at the time of the sunset Prayer if:
a) it is raining hard enough to wet one’s Clothing (0: and like rain in this is melted snow or hail);
b) one is praying with a group in a mosque (0: or Other place of prayer);
c) the mosque is far (0: from one’s door, i.e. far by Common acknowledgement (def: f4.5));
d) it is raining when the first prayer begins, when it ends, and when the second prayer begins;
e) and conditions f15.1O(b,c,d) exist
f15. 15 (A: If one arrives during the second of two prayers joined because of rain and does not finish one’s own first prayer before the group finishes their second, then one is no longer entitled to join one’s prayers for rain. It is a necessary condition that one pray at least part of the second prayer with them, though one may hurry through one’s own first prayer alone to catch up with and join them during their second.)
f 15.16 If the rain stops after one finishes the two prayers or during the second one, both prayers are and remain valid.
f 15.17 It is not permissible to join two prayers in the time of the second of them because of rain.
Sulaiman Hart,
Amman, Jordan.