One of the easier and totaly cool science projects you can do with your kids!
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There’s a lot going on. There’s convection, of course. For nocturnal surface cooling to produce frost, you have to have calm, cold conditions to cut down on convection. And you have to have clear weather without cloud cover, which shields the surface from the cold of the upper atmosphere. Apparently, the sky has a long-wave transparency window in the mid-range infrared wavelengths. The ground captures solar heat during the day and re-radiates it at night coincidentally on those wavelengths that the sky is unable to absorb. All that heat goes directly to space.
There has been work done to develop materials to assist heat-transfer in air-conditioning, photovoltaic electricity generation, and roofing that absorb sunlight and re-radiate the heat narrowly on those frequencies where the sky is most transparent. Mostly they’re too expensive to use, though.
… so materials are important. Your car’s windshield may be just the thing to maximize nocturnal surface cooling.
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Heat transfer was one of the easier (relatively speaking) college courses. Heat is transfered in only 3 ways: convection, conduction, and radiation.
We all know what convection (gasses) and conduction (solids) are.
Radiation is the magic that is not as understood.
Clear skies radiate heat to space just like when the sun is up we are irradiated by the sun’s rays to warm us.
As has been mentioned, properly sited thermometers are 6 feet above ground. Not taking account for things like cold air drainage, with no wind or temperature inversion at height, warmer surfaces radiate away the latent heat at night.
Cars cool faster than concrete or the bare ground.
So we see frost form with measured air temperatures in the mid or even upper 30’s. It’s not because water is freezing above 32°, it’s because radiation is a surface phenomena.
So this brings me to the easiest analogy I always use when talking about radiation because I’d dare to say we have all experienced it.
In a bonfire or your home fireplace, when you face the fire your skin gets hot. When you turn around in place your face gets cooler. The air around your head is the same temperature. The only difference is the radiation from the fire striking your exposed skin.
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