"It's not, though," the Forecaster replied; "the proverb is right there,
as well. A gray sky in the morning means that the air is filled with
water drops which are large enough to reflect light of every color.
While this is the same as the gray of evening, the processes that led
to the forming of these drops is quite different. In the day the dust is
heated and the forming of the droplets in the afternoon is due to
cooling. In the night, the condensation is caused by loss of heat
through radiation. Radiation shows that the air above must be dry.
Therefore a gray morning means a dry air above the water drops, and this
means a fine day, for the droplets will soon be evaporated by the rising
sun. The red morning sky declares that the dust particles have been
protected from radiation by a blanket of overlying moisture, the air,
therefore, is saturated to great heights and rain is probable. So you
see, Anton, Mammy's rhyme is right."

"What fo' yo' talk to me against signs," declared Dan'l, putting out his
chest and strutting; "Ah done told yo' them signs am pow'ful good."

"But the sunset colors, sir?" the author of the article asked. "You said
they were due to dust. Just how, sir?"

"Yes, to dust, plain ordinary dust, but dust of the lightest kind," was
the reply. "If you could go up in the air a hundred miles, the sky above
you in the middle of the day would be jet black and the sun would shine
down on you like a great bright-blue ball, without any white glare
around it at all."

"Then it's a blue sun that makes a blue sky!" cried Fred.

"Don't go so fast," the Forecaster warned him. "I want you to think of
the sky, first. It's a dead black, a hundred miles up. Now, at a hundred
miles up, the air is so thin that there's little or no dust, but as you
gradually come down and the air becomes denser, it begins to be able to
buoy up some dust. Boys," he said, breaking off suddenly, "why does a
stick float in water when it falls in air?"

"Because water is denser than air?" guessed Ross.

"Exactly. And why does a bar of iron sink through water and not through
earth?"

"Because the particles of earth won't move aside as easily as the
particles of water, I suppose."

"Not quite, but something that way. So, you see, as the air gets
gradually denser it becomes gradually more able to support particles of
dust, light ones at first, then heavier and heavier, until near the
earth big pieces of dust can be carried in the a