Our first Control (Natural Phenomena) build. I spent a lot of time going through canonical
GURPS sources to figure out the game effect of clouds and as far as I can tell, whenever this has been addressed the effects have been quite minor, so I built this on the assumption it would rarely offer a ±1 bonus/penalty. Of course, the spell could have a much bigger impact on aviators, especially early aviators flying by the seat of their pants, but this is tricky enough to pull off that I didn't think it warranted requiring more levels of Control.
Clouds
Keywords: Area (Special).
Full Cost: 59/64/69 points for levels 1-3, plus 19.5 points for each additional level*.
Casting Roll: IQ.
Range: 0.
Duration: 16 minutes, 40 seconds.
You can alter the cloud cover around you in a radius of 0.1 × level miles. This can't darken the sky enough to cause penalties for poor lighting during the day, but can worsen darkness penalties by -1 on a clear night (including twilight hours), or eliminate -1 in darkness penalties from a heavily overcast night.
This is also a Weather spell.
Statistics: Level 1 is Control Cloud Cover 1 (Emanation, -20%; Extended Duration 100×, +80%; Natural Phenomena, +100%; No effect on daytime vision penalties, -15%; Sorcery, -15%) [23] + Control Clouds 2 (Does not increase area, -50%; Emanation, -20%; Extended Duration 100×, +80%; Natural Phenomena, +100%; No effect on daytime vision penalties, -15%; Sorcery, -15%) [36]. Levels 2-3 remove "Does not increase area" from one level at a time [+5/level]. Further levels add Control Clouds (Emanation, -20%; Extended Duration 100×, +80%; Only to increase area, -50%; Natural Phenomena, +100%; Sorcery, -15%) [+19.5/level].
* Calculate the total cost and then round up.