Rain in the real world and GURPS MAGIC

[Varyon]

Originally Posted by Emerikol

I would consider the rain spell as using what is already available in the atmosphere. Even in a desert there is likely water available if you had the technology to force it out. Or the magic.

Whereas create water is literally creating water from nothing or at minimum pulling it from another plane.

In one of the character vignette teasers leading up to the release of Diablo III, the Wizard (as a young girl) summoned rain to free a village from its drought, but was later driven out when the drought came back worse for her having depleted the region of most of its water in the process. I'd probably interpret a spell that summons rain similarly, particularly if it has the sort of energy efficiency we're talking about here (when compared to something that creates water ex nihilo).

[Tyneras]

Just for fun, if we assume the spell is just sucking water vapor out of the air and not creating any, at standard temp and pressure at 50% humidity that 1 energy rain spell has made a cylinder of air 100 feet high and 220 feet (31m x 67m) in radius bone dry.

Water vapor would naturally diffuse into the drained area, but it's fun for illustrative purposes.

[hal]

Originally Posted by Polydamas

I was going to cite p. B239 as well, but now I am thinking ... could it be that a two-yard radius is represented on the battle map as seven hexes but actually fills another ring of half-hexes? It does seem like "a two-yard radius is a circle four yards across" is the natural meaning.

See page 13 of GURPS MAGIC for 4e, which discusses just that above, in its Box. The money phrase is

"Represent Area spells on a battle map as follows:

The area of effect of a spell cast over a one-yard radius is a single hex. The area of effect of a spell cast over a two-yard radius is a central hex and all adjacent hexes. The area of effect of a spell cast over a three yard radius is a central two-yard area of effect plus the ring of hexes adjacent to that."

[Willy]

That would be a good way to dry things to storage them longer. Many grains, and seeds not to mention meat etc , need to be dried to last long enough to make storage useful, and prevent mold and other problems.

What about a enclosed space , with a siphon at the bottom, fill it with goods, cast rain, let the water vanish via siphon and a valve and that´s it. After a handfull of castings the goods should be bone dry, you only need some time between the castings.

For bulk food stuff the energy ratio per meal would be nearly unbeatable. Same goes for changing fresh cut wood into seasoned firewood.

[Polydamas]

Originally Posted by Willy

That would be a good way to dry things to storage them longer. Many grains, and seeds not to mention meat etc , need to be dried to last long enough to make storage useful, and prevent mold and other problems.

What about a enclosed space , with a siphon at the bottom, fill it with goods, cast rain, let the water vanish via siphon and a valve and that´s it. After a handfull of castings the goods should be bone dry, you only need some time between the castings.

For bulk food stuff the energy ratio per meal would be nearly unbeatable. Same goes for changing fresh cut wood into seasoned firewood.

I would rule that rain and thunder comes from heaven (duh!) So if you cast Rain inside a silo, you hear raindrops plinking on the roof.

[Tyneras]

Rain clouds can be remarkably close to the ground, though I don't think they can actually touch it. Given that the NASA Vehicle Assembly Building, at 160 meters, could have its own weather including rain clouds, a minimum height of 100 yards might seem reasonable for the rain spell. This would also prevent it from being used in all but the largest caverns. A technique or variant that let you have rain at any arbitrary location for greater energy cost would be fitting.

While the penalties for a desert do imply that the water is coming from their air, it would be perfectly logical for the GM to rule it was a very slow, energy efficient summoning from the elemental planes.

[maximara]

Originally Posted by whswhs

If you break the hex up into twelve triangles you can determine that its area is approximately 0.87 times the square of the distance between adjacent hex centers.

So basically the circle is inside the hexagon ie A= 2 * √(3) * R^2

[maximara]

Originally Posted by hal

See page 13 of GURPS MAGIC for 4e, which discusses just that above, in its Box. The money phrase is

"Represent Area spells on a battle map as follows:

The area of effect of a spell cast over a one-yard radius is a single hex. The area of effect of a spell cast over a two-yard radius is a central hex and all adjacent hexes. The area of effect of a spell cast over a three yard radius is a central two-yard area of effect plus the ring of hexes adjacent to that."

This is exactly what is on B239:

"The area of effect of a spell cast over a one-yard radius is a single hex. The area of effect of a spell cast over a two-yard radius is a central hex and all adjacent hexes. The area of effect of a spell cast over a three-yard radius is a central two-yard area of effect plus the ring of hexes adjacent to that." (sic)

None of this changes the fact that the area covered depends on how the circle is placed on the hexagon.

Bill Stoddard seems to be saying the circle is inside the hexagon ie A= 2 * √(3) * R^2

But the way both Magic 4e pg 13 and B239 are written implies the circle is outside the hexagon otherwise it wouldn't effect the entire hex.

Yes, we know the inradius (center to side) is 0.5 yards but is that the radius of the spell's 1 hex area of effect or is it the circumradius (center to vertices)?

If it is the later than that one hex spell is already spilling over into the neighboring hexes (but the amount is so small it can be ignored) and gives us a radius of effect of ~1.05 yards^2 (0.5 = √3/2 x a where a = R, or radius of 0.577)

See Hexagon Calculator for more information on a hexagon than you really need. :-)

[Polydamas]

I am pretty sure that the highest pre-1850 buildings (Western European cathedrals) are less than 100 metres/yards tall inside.

Originally Posted by maximara

This is exactly what is on B239:

"The area of effect of a spell cast over a one-yard radius is a single hex. The area of effect of a spell cast over a two-yard radius is a central hex and all adjacent hexes. The area of effect of a spell cast over a three-yard radius is a central two-yard area of effect plus the ring of hexes adjacent to that." (sic)

None of this changes the fact that the area covered depends on how the circle is placed on the hexagon.

You are quoting selectively. "Represent Area spells on a battle map as follows. The area of effect of a spell cast over a one-yard radius is a single hex. The area of effect of a spell cast over a two-yard radius is a central hex and all adjacent hexes." When I reallized this, I realized that it is not saying what area Area spells affects, it is saying how to represent this area on a hex map. I thought Bill was contradicting the RAW, but he is just ruling that this passage is a simplification for the purpose of battle maps.

[StevenH]

Hexes are a metagaming thing. Actual spells (were they really to exist) wouldn't care about imaginary hexes. Heck, I don't even really use hexes; I use actual distances (Your character has a Move of 5? Here is a piece of string 5" long. That's how far they can go.)
But that could be because I prefer squares on my battle maps anyway. Makes it easier to draw the terrain to scale.

So, spells use circles as radius of effect, but they are represented on the hex-based battle mat as a hex surrounded by other hexes, as appropriate. In the case of area effect spells, Pi*R^2 (times the appropriate thickness, which is usually 2 yards, but that makes no sense in a weather context). Makes the math easier.