Area Effect spells and sloping ground

GURPS Magic says about Area Spells, on p. 11, that
"These spells affect an area instead of an individual. They are cast
on a surface - floor; ground, etc., - and their effects extend four
yards (12 feet) up from that surface. (...)"

For the following discussion, a proportional font is recommended.

Assume that the spell caster is standing on a hilltop, as the
following terrain cut-out suggests:

o
T <- spell caster
XXX
XXXXX
XXXXXXX
XXXXXXXXX
XXXXXXXXXXXXXXX <- ground


He now casts a Create Fire spell with a radius of 4 yards (an X being
1 yard wide and long). Since he is necessarily casting this on the
ground according to GURPS Magic (above), will the effect follow the
curvature of the hill, such as this:

~~~
~~~~~
~~~o~~~
~~~T~~~ <- spell caster
~~XXX~~
~XXXXX~
XXXXXXX
XXXXXXXXX
XXXXXXXXXXXXXXX <- ground

Or will it extend into thin air (apparantly contradicted by the spell
having been cast "on a surface"), like this:

~~~~~~~
~~~~~~~
~~~o~~~
~~~T~~~ <- spell caster
XXX
XXXXX
XXXXXXX
XXXXXXXXX
XXXXXXXXXXXXXXX <- ground

Or will it stop where the level ground "stops":

~~~
~~~
~o~
~T~ <- spell caster
XXX
XXXXX
XXXXXXX
XXXXXXXXX
XXXXXXXXXXXXXXX <- ground


Also, does the "cast on a surface" requirement for Area Spells mean
that they are essentially useless on the Plane of Air or in the vacuum
of outer space? (Sans some vehicle or other object to provide a
sufficiently substantial "surface".)

Cheers,
Bent D.
--
Bent Dalager - bcd.RemoveThis@pvv.org - http://www.pvv.org/~bcd
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Bent C Dalager schrieb:

>
> Also, does the "cast on a surface" requirement for Area Spells mean
> that they are essentially useless on the Plane of Air or in the vacuum
> of outer space? (Sans some vehicle or other object to provide a
> sufficiently substantial "surface".)
>
> Cheers,
> Bent D.

seems like they are useless there. you cant cast them in the air on this
plane. so why should you on the plane of air?