Driving home this evening with the top down, I noticed an interesting phenomenon. It was hot, really hot today, although by the time I was driving home after sunset, the air had started to cool (that wonderful dry, 90ish balmy weather). But every time I drove under a concrete overcrossing, the radiant heat from above was noticeable. That concrete had soaked up the heat during the day, and it was happily radiating in all directions.
So when I was on the open road, my head radiating through dry air to the clear sky, it was probably seeing a sink at a few tens of K, but when zooming under the overpass, looking at a radiator at 320K plus..
That T^4 factor is huge.
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