GURPSよろず

　In tales of the fantastic, both modern and traditional, the ability to walk on air isn’t as common as true flight.
However, it’s a fair way to represent the capabilities of those who haven’t quite mastered powers that will eventually let them fly. The GM should consider treating 「空中歩行」 as a “placeholder” that the PCs can improve to 「飛行」 with earned points.

■代替案（Alternatives）
The obvious comparison is with 「飛行」 (p. 50), which is far more capable (and twice as expensive). Those capable of limited aerial movement
should compare Super Jump (p. 79).

■追加特別限定（New Special Limitation）s
Take Specific (p. 112) if you can only walk on a particular type of gas.
“Ordinary air” isn’t allowed, but dust, smoke, and water vapor (clouds, steam, etc.) are all legal. This is worth -40% for most substances, but especially uncommon or obscure gases -- like pure hydrogen --might be -60%, at the GM’s discretion.

Lighter Than Air:
　Your ability works by making you lighter than air or gaseous. Wind moves you a yard per second in the direction it’s blowing per 5 mph of wind speed. -10%.

Maximum Height:
　You can’t walk very high off the ground. A 30-foot limit is -10%; a 10-foot limit is -20%; and a 5-foot limit is -25%.

Planetary:
　You push off against a planet’s gravity field or walk along its magnetic field lines. Your ability is useless in the absence of a planet. -5%.

Vertical Only:
　You can only climb straight up, as if using a ladder made of air. You can’t move laterally at all.-25%.

■パワーアップ（Powering Up）
　「空中歩行」 suits almost any power that includes 「飛行」. Elemental powers are especially likely, particularly Air, Earth (with Specific, Dust), Heat/Fire (with Specific, Smoke), Magnetism (with Planetary), and Water (with Specific, Vapor).
「サイコキネシス」 is another good fit --as is a spirit power that lets the user summon sylphs to bear him through the air. 「宇宙パワー」 and divine powers might let gods and their servants walk high above their flocks. 才能 adds to all DX rolls for movement and to avoid or halt a fall.