2 Answers. RAW Fireball appears to ignore full cover, if there is a path fully within range of the effect that can reach the target, as this spell specifically travels around corners.
Do you need to see where you cast fireball?
If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction. So the answer RAW would be, yes. You need line of sight, or, more specifically, an unblocked straight path.
How does the area of effect of a fireball cast in?
If, as Alexis Wilke states, “the fireball is cast with its center on the ground” than you don’t magically have a 20 foot radius semi-circle of fire beneath the ground because the area beneath the ground is not included in the spell’s AoE due to the text I previously quoted.
How big is the radius of a fireball?
The fireball spell is clearly described as a sphere with a 20′ radius. In most case, that is applied at ground level since that’s where most creatures are. (We can actually see an example of the area in: 20 foot square versus a 20 foot circle on a battle mat .)
Can a fireball cast outside water create an obstacle?
You could also consider that water could create an obstacle (PHB 204 – for spells that require a clear path to the target). Following that idea – you could theoretically rule that fireball cast outside water explodes when it hits the water surface. (Rather harsh ruling IMHO)
What happens when you cast the fireball spell in 5e?
I think earlier editions had rules where lightning would spread out in an AOE (area of effect), but as far as I know that doesn’t exist in 5e. This isn’t to say fire would spread in a larger AOE under water, but to show that there have been situations where magic is affected by water.
