"Then what are the fires for?"
"Just to heat a very small section of the air on the ground. Don't forget, boys, that a fruit tree ten feet high may have all the fruit on its lower branches, up to five or six feet, absolutely killed off, while the top branches are unharmed."
"How's that?" queried Ross in surprise. "I thought frost came down from on top, and that the higher up you went the colder it would be."
"Not at all," the weather expert answered. "Frost comes from down below. When the air is still and clear, the earth loses heat by radiation. The heat goes up and up and through the air to higher levels, the cold earth cooling the air below. Therefore, on a frosty night, in a region where frosts are rare, or at a time of year when frosts are few, a still clear night will cause a belt of cold air perhaps only a few inches in depth, perhaps ten or twenty feet in height, this belt being several degrees colder than the air overhead.
"Now, Ross, you can see that to light huge fires, with the intention of warming up all the air, would be foolish and unnecessary. All that is needful is to heat this lower cold belt of air, a few feet in depth, and only to heat it the three or four degrees necessary to bring it to the warmth of the air above."
"But suppose a wind comes up and blows the heat away?" asked Anton.
The Forecaster smiled at the question.
"If a wind comes up," he answered, "you wouldn't need to use any heat at all, because the wind would mix the warmer air overhead with the cooler air below and there couldn't be any killing frost."
"But doesn't it cost an awful lot?"
"It costs less than to lose your crop," the weather expert replied. "Usually you can figure that a frosty night will take a gallon of oil per tree, or from twenty to twenty-five cents. In a fruit growing section a grower is unlikely to have more than four or five still, freezing nights a year when his crop may be ruined by frost, so that he will spend a dollar or so per tree in protecting his orchard. As there are few fruit trees which bring in a profit of less than ten dollars during the season, and some a great deal more—according to the nature of the crop—the proportionate expense of heating is small compared with the amount of fruit saved."