I tried checking out current weather in Rio with only modest success. As best I can tell, it hasn't rained, but they did have one observation of fog about 4.5 hours ago (don't know how dense). They also reported mist a couple of times, which shouldn't affect visibility enough to impact landings at the airport. However, I can't tell where this weather station is and I don't know where the airport is located. If it's near the water, fog could be more likely.
Time for a little more digging.