Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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We Went, Provided With
Thermometers, To The Flat Shores Covered With White Sand Which Border
The River Apure.
At two in the afternoon I found the sand, wherever it
was exposed to the sun, at 52.5 degrees.
The instrument, raised
eighteen inches above the sand, marked 42.8 degrees, and at six feet
high 38.7 degrees. The temperature of the air under the shade of a
ceiba was 36.2 degrees. These observations were made during a dead
calm. As soon as the wind began to blow, the temperature of the air
rose 3 degrees higher, yet we were not enveloped by a wind of sand,
but the strata of air had been in contact with a soil more strongly
heated, or through which whirlwinds of sand had passed. This western
part of the Llanos is the hottest, because it receives air that has
already crossed the rest of the barren steppe. The same difference has
been observed between the eastern and western parts of the deserts of
Africa, where the trade-winds blow.
The heat augments sensibly in the Llanos during the rainy season,
particularly in the month of July, when the sky is cloudy, and
reflects the radiant heat toward the earth. During this season the
breeze entirely ceases; and, according to good thermometrical
observations made by M. Pozo, the thermometer rises in the shade to 39
and 39.5 degrees, though kept at the distance of more than fifteen
feet from the ground. As we approached the banks of the Portuguesa,
the Apure, and the Apurito, the air became cooler from the evaporation
of so considerable a mass of water.
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