It must be quite humid in São Paulo. These instruments are not tightly sealed and therefore receive the same atmosphere as the outside. As the interior portions of the instruments heat up moisture is driven off and collects on the exterior portions that are cooler. Eventually the whole thing heats up and the condensation returns to invisible vapor.
We have high humidity here in North Carolina during the summer. What I do to manage it while parked in my unconditioned garage is to set a jar of Damp Rid in both the closed cabin and closed trunk.