Vaisesika Sūtra 5.2.11

अपां संयोगाद् विभागाच्चस्तनयिनोः ॥११॥

apāṁ saṁyōgād vibhāgāc ca stanayinōḥ ॥11॥

[अपाम्-संयोगात् (apām-saṁyogāt) - from conjunction of waters; विभागात्-च (vibhāgāt-ca) - and from separation; स्तनयिनोः (stanayinōḥ) - of thunder and lightning;]

Thunder and lightning arise from the conjunction and separation of waters.