A.T. Robertson Commentary Revelation 16:8

"And the fourth poured out his bowl upon the sun; and it was given unto it to scorch men with fire." — Revelation 16:8 (ASV)

Upon the sun (επ τον ηλιον). Not εις (into) as in verses 2,3,4. The fourth trumpet (8:12) affected a third of the sun, moon, and stars with a plague of darkness, but here it is a plague of extreme heat.

To scorch with fire (καυματισα εν πυρ). First aorist active infinitive of καυματιζω, late (Plutarch, Epictetus) causative verb (from καυμα, heat), in N.T. only here and verse 9; Matthew 13:6; Mr 4:6. The addition of εν πυρ (in fire, with fire) intensifies the picture.