The Time to Observe Salatul-Istisqa

Salatul-Istisqa is permissible when there is a drought and sparse rainfall, or the level of water in the streams and well is low, or due to the river drying up etc. It is recommended (to observe the Salat) after the sun rises and has ascended up to the length of a spear; which is about 20 mins after sunrise, which is the time of Salatul-‘Eid (the Salat observed during the two festivals).
Where to Observe Salatul Istisqa

The Sunnah is for it to be observed on the praying ground and not inside the mosque, in accordance with the Prophet’s ﷺ manner, except when there is need to do otherwise.
SOURCE: https://www.al-feqh.com/en/salatul-i...n-seeking-rain