Lightning and hail are formed in rain clouds due to the accumulation of electrical charges caused by freezing raindrops and contact between droplets and hailstones. The hailstones become negatively charged, while the upper parts of the cloud become positively charged. When the voltage between these two areas reaches 1 billion volts, a spark, or lightning, is discharged from the cloud, sometimes from within the cloud, or sometimes to the ground. This spark forms dazzling lightning strikes, and the sudden increase in electrical charge causes intense heat which leads to the loud noise of thunder.
Everything modern science has established about clouds, and has to say about the causes of thunder and lightning, is in complete accord with the descriptions in the Qur'an.
... He sends down mountains from the sky with hail inside them, striking with it anyone He wills and averting it from anyone He wills. The brightness of His lightning almost blinds the sight. (Quran chapter 24 verse 43)
Or [their likeness is] that of a storm-cloud in the sky, full of darkness, thunder and lightning. They put their fingers in their ears against the thunderclaps, fearful of death... (Quran chapter 2 verse 19)