During the day, the sun's rays are reflected onto the cooking
vessel which becomes hot quickly. At night, heat from the
vessel is radiated outward, towards empty space, which is very
cold indeed (a "heat sink").
As a result, the cooking vessel now becomes a small refrigerator.
We routinely achieve cooling of about 29 degrees Fahrenheit
(10 degrees Celsius) below ambient air temperature using this
remarkably simple scheme.
In September 1999, we placed two funnels out in the evening,
with double-bagged jars inside. One jar was on a block of wood
and the other was suspended in the funnel using fishing line.
The temperature that evening (in Provo, Utah) was 78 degrees
Fahrenheit. Using a Radio Shack indoor/outdoor thermometer, a
BYU student (Colter Paulson) measured the temperature inside the
funnel and outside in the open air. He found that the temperature
of the air inside the funnel dropped quickly by about 15 degrees,
as its heat was radiated upwards in the clear sky. That night,
the minimum outdoor air temperature measured was 47.
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