Verse 8.12.2

अशरीरो वायुरभ्रं विद्युत्स्तनयित्नुरशरीराण्येतानि तद्यथैतान्यमुष्मादाकाशात्समुत्थाय परं ज्योतिरुपसम्पद्य स्वेन रूपेणाभिनिष्पद्यन्ते ॥ ८.१२.२ ॥

aśarīro vāyurabhraṃ vidyutstanayitnuraśarīrāṇyetāni tadyathaitānyamuṣmādākāśātsamutthāya paraṃ jyotirupasampadya svena rūpeṇābhiniṣpadyante || 8.12.2 ||

2. The air is formless. So also are clouds, lightning, and thunder. All these arise from the sky and assume their respective forms due to the heat of the sun.

Word-for-word explanation:

Aśarīraḥ vāyuḥ, air is formless; abhram vidyut stanayitnuḥ, light clouds, lightning, and thunder; etāni, all these; aśarīrāṇi, are formless; tat yathā, just as; etāni, all these; amuṣmāt ākāśāt, from the sky; samutthāya, arise; param jyotiḥ upasampadya, attain the great light [i.e., are exposed to the heat of the sun in summer]; svena rūpeṇa abhiniṣpadyante, appear in their respective forms [in the rainy season].

Commentary:

Here the idea is that even though we don’t see the Self, it is always within us. The Upaniṣad compares it with air, clouds, lightning, and thunder. In winter the sky is clear. There are very few clouds, and there are no strong winds or storms. But then as summer begins the temperature starts rising, and slowly the clouds gather. Then come the storms with their high winds and lightning and thunder.

Where were the clouds and lightning during winter? They were there, but they were not visible. We see them only when the conditions are right; otherwise they are in a latent form, one with the sky. They appear from the sky, and then they merge into the sun.