There was a time, not very long ago, when a fair, dry night in winter would have been watched through with dread of the "great white terror."

The last nights of 2015 - before the rains came - were clear, and the stars, even against the city's glare, were bright. A rare full moon hung directly overhead at midnight on Christmas Eve, as sharply defined as if chiseled from a chunk of diamond.

Photo courtesy of USC Digital Library, Dick Whittington Collection

Photo courtesy of USC Digital Library, Dick Whittington Collection

There was a time, not very long ago, when growers would have watched through such a fair night with dread.

Hajime Fukuoka, a candidate for a master's degree in Economics at USC in 1918, explained why in almost poetic terms in his thesis The Lemon Industry in Southern California:

"Great rapid loss of heat ... takes place on quiet, dry, star-lit nights. Under such conditions, the heat in plant tissues passes into space by radiation or by the long, dark heat-rays which pass rapidly through the air without appreciably warming it. As a result, (leaves and fruit) quickly reach a temperature considerably lower than that of the surrounding air. ... Soon, if the night be quiet, the air arranges itself into layers or strata of varying temperature, the coldest and heaviest resting on the ground. These cold layers slowly slip downhill, seeking the lowest levels." More