Bhashya
स्तनयित्नुरूपेण वर्षहेतुर्भवति ।कथम्? ऊर्ध्वाभिश्च ऊर्ध्वगाभिः विद्युद्भिः तिरश्चीभिश्च तिर्यग्गताभिश्च सह आह्नादाः स्तनयनशब्दाश्चरन्ति ।तस्मात्तद्दर्शनादाहुर्लौकिकाः-विद्योतते स्तनयति, वर्षिष्यति वै इत्याद्युक्तार्थम् ।अतस्तेज उपास्स्वेति ॥
In plain words
It brings about rain in the form of thunder. How? Upward-moving lightnings and sideways-moving flashes travel together with resounding crashes of thunder. Seeing this, people say: 'It flashes, it thunders, it will rain'—as has already been explained. Therefore, meditate upon fire.
Close to the Sanskrit
It becomes the cause of rain in the form of thunder. How? With upward—going upward—lightnings, and with transverse—gone sideways—along with joyful sounds and rumbling sounds, they move. Therefore, from seeing that, worldly people say: 'It flashes, it thunders, it will rain indeed'—this has the meaning already stated. Hence, meditate upon fire.