User blog comment:DMUA/D&D Homebrew (Greyfang): CLOUDS!?/@comment-28876888-20181105200127

4.0808e15 m^3 is volume of the proposed cloud (multiplying surface area of Earth in km by 8 km height). Density of Cumulonimbus would be an average of 2 g/m^3 Mass is 8.1616e12 kg Vape Value: 2264705 J/kg We get 1.848e19 Joules, or 4.42 Gigatons of TNT

This is not Tier 5, there was a slight error. Namely in the radius of the earth being millions of kilometers wide. DMUA wants to do the math on his own, but this isn't that high. Sorry.