Brihat Samhita
अच्छिन्नं अवनिगाढं द्युतिमत्स्निग्धं घनं विविधवर्णम् । द्विरुदितं अनुलोमं च प्रशस्तं अम्भः प्रयच्छति च ॥ ३५-०३ ॥
If two rainbows, similar to each other should appear perfect, unbroken, bent towards the Earth, bright, glossy, thick and variegated in colour, there will be abundant rain.
english translation
acchinnaM avanigADhaM dyutimatsnigdhaM ghanaM vividhavarNam । dviruditaM anulomaM ca prazastaM ambhaH prayacchati ca ॥ 35-03 ॥
hk transliteration by Sanscript