Lost Radiation to Space
December, January and February is the dry air season at Benares, India. It is the season of continental polar air outflow of Asia time of the year. Thus it is the season of minimum greenhouse warming.
Well, there is a better way to say that. It is the season of greatest terrestrial radiation loss to space! The Earthly greenhouse in question is not one that results in warming but rather a lessening of cooling rate.
In both seasons the cooling rate increases upward from the ground until a maximum of about 2.4° C/day is reached. This occurs at cloud tops. The cooling rate declines above this. (S. Hess Introduction to Theoretical Meteorology, 1979)
We really don't have any greenhouse warming here on earth but rather greenhouse gases put the brakes on radiation loss to space. It is more akin to insulating your house than stoking the fires in your furnace.
Countries dominated by the Asian Monsoon have half a year with ample greenhouse gases, small to modest loss of earth-light out to space, and warm nights. The other half of the year has not nearly as much greenhouse gas (H2O) in the air, earth-light (IR) easily passes out to space, and cool nighttime temperatures are the rule. It is a wet air, dry air thing.
Daytime temperatures don't come into this story much. High daytime temperatures are caused by the sun and moderated by evaporation, if water is available. If the air is dry enough, ice may be made.
Our story on ice making in Benares is all about the time when the sun is below the horizon: night. My report on Benares is extracted from the Transactions of the Royal Society MDCCXCIII:56-58 and MDCCXCIII: 129-131. By J. LL. Williams. The LL. stood for Lloyd in the late 18th century lexicon. The J for Joh-. John was an Esquire at Benares, India. LL.'s paper was read to the Royal Society by William Marsden, Esq. F. R. S. on February 14, 1793 and May 2, 1793 respectively.
LL. Notes that even with temperatures that run from 95° to 100° F in the shade inthe 18th century, locals at Seerore near Benares where LL. lived made lots of ice. Seerorens made their ice on a nearly level plot of some 4 acres. The plot was divided into squares about 5 feet on a side with raised borders. The sunken squares were filled with dry straw or sugar-cane haum. On each bed of hewn and dried standing biomass, broad, shallow pans of unglazed earth were set. The pans, 4 acres of them (~100,000 pans) were then filled with water. This was all handwork as pumps, pipes, valves, hoses and the like were not yet invented. A team of some 300 men, women and children did this work.
The clay pans, being highly porous, wetted throughout and were ready for evaporation. To make it easy to get the ice out when it formed, the insides of the clay pans were rubbed with butter! The pans had to be rebuttered about every 4 hours during the night.
Ice harvesting began around 5 in the morning. If the straw under the pans got wet, no ice would form. The 300 men, women and children had to be ready to replace the straw or reeds during the night if needed. LL. reports that air temperatures at night rarely fell below 40° F. LL. also notes that if there were but the slightest wind, no ice would form. The wind part is easy. On clear, calm nights, air temperatures near the surface increase with height above the ground. Add a little wind and you mix the warm air from aloft with the cold air at the surface and it doesn't get cold enough to permit the ice to form. Fans and helicopters are used to prevent frost in orchards. Keep the air mixed and prevent frost at the surface. The icemakers of Seerore used well water to fill their pans. The pans had to be filled by dusk so that there would be a maximum of hours of cooling. The pans were filled in the afternoon. Under the hot mid-day sun the water in the pans would warm. Not so. Well water at Seerore used in the ice-making season is about 74° F. So during the course of the night, water temperatures in the pan fell 42° F! During the day, while the pans were being filled, evaporation prevented the water in the pans from warming in the 95° F heat. In fact, the temperatures fell from well water temperatures of 74° F to around 68° F. When the clay pans get old, their pores get filled with "gunk" and were not porous enough to permit maximum evaporation. The water in these old pans would warm to 88° F. Pan fillers could tell when the pots were getting too old and needed to be replaced by placing an experienced finger into the water late in the afternoon.