What most impressed on Saturday, however, was frost in the metropolitan area, in particular the city of Porto Alegre. It MetSul understanding of what was the longest episode of frost in the capital in decades. Even in the cold wave of historical years 2000, 2007 and 2009 there was no frost as strong as observed today in many places. There are reports of the phenomenon in the balconies of upper floors of some buildings in central districts of the city, within the so-called “urban heat island.” A white blanket covered the grass on the banks of the Arroyo Flood, despite the vicinity of the asphalt and the road beside the stream.
Looks like UHI should really be called USI.
Looks like a good ole fashioned hoar frost to me. High humidity(light fog) freezing on everything.