The rain is the symbol of the descent of Grace or of the higher consciousness which is the cause of the riches, the spiritual plenty.
The descent as of a drizzling rain is a very characteristic and well-known way of descent of the higher consciousness; it brings peace but it also brings all other possibilities of the higher consciousness too and, as you felt the seeds of transformation of the physical consciousness by the coming in it of the seeds of the powers and qualities of the higher Nature.
I am very glad that the experience we have been working to bring to you has come with such force and is increasing. It is the concrete descent of the higher consciousness, which once it settles marks always a definite turning-point in the sadhana. Even if it does not settle with a full stability at once, yet when it has once come with so much strength, there cannot be the least doubt that it will come more and more till it has done its work and is your permanent consciousness. The shower and drizzle, the hold above the head and in the heart, the envelopment, the flaming of Agni within, the sense of firmness and solidity, the Peace and security and devotion, the sense of the Mother's hold are all signs of the descent — eventually it will penetrate everywhere and become something solid and stable occupying the whole consciousness and body.