SUMMARY
The problem is many sided and we must consider the motion of the
air vertically as well as horizontally. Air gains and loses heat chiefly by
convection, and any gain or loss by conduction may be neglected. The
plant gains heat by convection, radiation and perhaps by conduction of an
internal rather than surface character. The ground gains and loses heat
chiefly by radiation. But the whole process is complicated and may not
even be uniform. Frosts generally are preceded by a loss of heat from
the lower air strata, due to convection and a horizontal translation of the
air. Then follows an equally rapid and great loss of heat by free radiation.
There are minor changes such as the setting free of heat in condensation
and the utilization in evaporation, but these latent heats are of
less importance than the actual transference of the air and vapor and the
removal of the latter as an absorber and retainer of heat.
Frosts are recurrent phenomena reasonably certain to occur within
given dates, and, as pointed out above, the cumulative losses are
considerable. Methods of protection to be serviceable must be available
for more than one occasion, for there is no profit in saving a crop on one
night and losing it on the succeeding night. But the effort is worth
while. Consider that the horticulturist regularly risks the labor of many
months on the temperatures of a few hours. An efficient frost fighting
device is in a way the entering wedge for solving problems of climate
control. One may not take a crop indoors, it is true, but there is no
valid reason, in the light of what has been already accomplished, why at
critical periods which may be anticipated, the needed volume of surface
air may not be sufficiently warmed; and the losses which have heretofore
been considered inevitable be prevented.
