fact that the
gravel will retain heat and not radiate it, for a much longer time than
grass or green leaves. Dew begins to form upon the grass very soon after
the sun is set because the moment the sun's rays are withdrawn the heat
is rapidly radiated by the blades of grass, which cools the earth under
it and the air above and surrounding it, so that if the air is anywhere
near the moisture saturation point on cooling at the surface of the
ground it will readily give up a part of its moisture, which condenses
in drops upon the blades of grass.

If the night is still and clear and there is much moisture in the air,
the dew will be heavy, but if the night is cloudy there will be little
or no dew formed. The clouds form a screen between the earth and the
upper regions of the atmosphere, which prevents the heat from radiating
to a sufficient extent to form dew. For the same reason no dew will
form under a light covering spread over the ground even at some distance
above it. The covering acts as a screen, which prevents the heat from
radiating to the dew point. From what has gone before it will be seen
that if the atmosphere is not charged with moisture up to the point of
saturation it will require a greater amount of depression of temperature
to cause condensation, and this is why we usually have heavier dews in
June when the air is more highly charged with moisture than we do in
August when it is dry. This also accounts for the ice clouds, called
cirrus, being formed so high up in the atmosphere during dry weather.
There is so little moisture in the air that it requires a very great
difference of temperature to cause condensation to take place, and the
necessary depression is not reached in these cases except at an altitude
of several miles.

Dr. Wells has shown that if we take the reading of two thermometers on a
clear summer night, one of them lying on the grass and the other
suspended two feet above it, we shall find that the one lying on the
grass will read 8 or 10 degrees lower than the one suspended in the air.
If the night is still there will be a cold stratum of air next to the
earth, which will not tend to diffuse itself to a very great degree and
dew will form. If, however, it is cloudy or the wind is blowing there is
rarely any formation of dew. The reason in the former case, as we have
explained, is that the radiated heat is held down to the earth in a
measure, and in the latter case there is a constant change