PaulSpring

Intuitively the conductivity explanation sounds more credible. Also, my experience is that cold damp air doesn't immediately feel colder than cold dry air. The difference is that, over time, it feels like it leeches the warmth out of you quicker, which would make sense if it was conducting heat away faster. Moontanman, you said "I've seen ice condense directly out of the air twice in 40 years of living near the ocean and seeing cold wet air". Where I come from in East Anglia, this phenomenon happens at least half a dozen times every winter. It's called hoar frost.

It's nothing to do with where the CoG is; as you rightly say, angling the wheels doesn't change that. Neither is it related to the width of the wheelbase. This setup is significantly more stable than the same wheelbase if the wheels were vertical. It's simple resolution of moments around the pivot point (the point where one of the wheels contacts the ground). With the angled wheel the sideways force required to tip the wheelchair is 1/cos(theta) times that for the same wheelbase and vertical wheels, where theta is the angle from vertical. So, with a 30 degree angle, the force required is 15% greater. In simple terms, you are using a greater part of the person's weight in opposition to the sideways force.