Egon, Of course you are right but most welding on mild steel on the guys trailer isn't very special or hi-tech. Depending on the weld parameters, preheat to avoid/relieve stress or avoid warp (purposely induce warping to get desired shape) most welding on a trailer won't be bothered by a few degrees change in ambient, especially as pointed out by my neighbor Okie (no disrespect Okie but I have a friend with a black cocker named Okie) when compared to the temp of the weld.

Wind can be a problem as it displaces the shielding gases generated by the consumption of flux (on a stick or with flux cored wire or from shielding gas supplies like CO, argon, etc.)

Patrick