Diameter of a Fireball

[Scott]

A fireball has a 40' diameter as a sphere. What would the diameter be for a hemisphere blast, i.e. you're attacking gound level targets and not shooting it into the sky?

[Scott]

I come up with just over 50' diameter using a formula I found online. Anybody come up with anything different?

[Falconer]

It’s an interesting question. Would the spell be cast at the floor or 4' above? I wouldn’t know how to calculate that. And, to be honest, in actual gameplay I probably wouldn’t bother to even fake it. I’d just (in most cases) figure it out 2-dimensionally, under the assumption that the intent of the spell already takes the ground into consideration. Something like this (breaking it down into 2.5' squares within the usual 10' squares):



That’s an open space. If you’ve got walls converging in the space you just take the squares it would displace and add them on the end in the direction they would go:

[Scott]

I found a calculator online that figures out various properties of different shapes based on other known properties, and a hemisphere blast with 33,000 cubic feet volume would have a 50' diameter.
The same formula comes up with a 40' diameter sphere sphere, which matches the figures in the PH.
Confined areas are easier to calculate, since you can usually just count 33 10' cubes.

[Scott]

Jul 21, 2010 12:34:37 GMT -5 Scott said:

It’s an interesting question. Would the spell be cast at the floor or 4' above?

Aiming at the floor would give you the widest diameter; 4' above would be a small difference, around 3 feet, which would be about 1 1/2 feet difference on each side of the blast. I'd just keep the diameter at 50'.