Page 71 - James Rodger Fleming - Fixing the sky

and winter storms, are driven by steam power. Heated by the Sun, a column of
moist air rises, allowing the surrounding air to rush in. As the heated air ascends,
it cools and its moisture condenses, releasing its latent heat (this is the “steam
power”) and producing rain, hail, or snow. Espy emphasized, correctly, the
importance of knowing the quantity of vapor in the air, “for it is from the latent
caloric [or heat] contained in the vapor that all the force of the wind in storms is
derived. It is only when the dew-point is high that there is sufficient steam power
in the air to produce a violent storm; for all storms are produced by steam power.”
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His theory was well received by many scientists of his time, including a commit-
tee of the French Academy of Sciences chaired by François Arago. The convective
theory is now an accepted part of meteorology, and for this discovery Espy is
well regarded in the history of science.
Espy strayed from the scientific mainstream when he promoted his idea
that significant rains of commercial importance for agriculture and navigation
could be generated by cutting and burning vast tracts of forest. He believed the
heat and smoke from these fires would create huge columns of hot air, produc-
ing clouds and triggering precipitation, much like the effects of volcanic erup-
tions. He listed five scientific reasons why setting large fires should produce rain:
(1) experiments showed that expanding air cools dramatically, and (2) under
certain conditions of high humidity forms both a visible cloud and significant
amounts of precipitation; (3) chemical principles indicated that the “caloric of
elasticity” (a venerable term for latent heat) released in the condensation of this
vapor is immense, equal to about 20,000 tons of anthracite coal burned on each
square mile of cloud extent. Espy’s convective theory further held that (4) this
release of heat would keep the cloud buoyant, lower the barometer, and “cause
the air to rush inward on all sides toward the center of the cloud and upward in
the middle, thus continuing the process of condensation of vapor, formation of
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cloud, and generation of rain.” Espy derived his final point empirically by col-
lecting observations and testimonials to the effect that (5) air does indeed rush
inward on all sides toward the center of the region where a great rain is falling
and upward into the cloud.
Espy explained that three things can prevent rains from accompanying great
fires: (1) winds, (2) excessive moisture, and (3) stability of the upper levels of the
atmosphere. He released small balloons and tracked their flight in order to get
a sense of the winds, and he used a hygrometer to measure atmospheric mois-
ture and estimate its changes with height. Stability was more of a problem, for as
he observed, in the present state of science, the levity of an upper stratum of air
could not always be known. Correspondents, friends, and even a congressman
laughed at Espy when they heard of his proposal to make rain, but he assured

54 | rain makerS