Question Three of Chapter Two of the Third Decade
That demons assume bodies from the nature of an element
It is asked why demons, when they assume bodies, assume none but aerial bodies.
To this it is answered that angels do not assume bodies from earth or water because they would not suddenly disappear, nor again from fire because a fiery body would burn the adjoining things which it touched.
Rather, they assume bodies from air, condensing it by divine power as much as is necessary for the formation of the body to be assumed.
And although air, while remaining in its rarity, does not retain a figure or color, nevertheless, when it is condensed, it can be figured and colored, as is evident in clouds.
These things are from Thomas, First Part, question 51, article 2, and Richard in Book II, distinction 8, question 3.