Question

This passage is adapted from an article detailing the history of meteorology in the late nineteenth and early twentieth centuries.

For much of the nineteenth century, meteorologists operated under a foundational assumption: the temperature of the Earth's atmosphere decreased at a constant, uniform rate with increasing altitude. This belief, derived from observations made during manned balloon ascents and mathematical modeling of thermodynamics, held that the air would grow progressively colder until it reached the absolute cold of outer space. However, because human balloonists could rarely survive voyages above twenty-five thousand feet due to hypoxia and extreme cold, empirical data from the highest reaches of the atmosphere remained tantalizingly out of reach.

The impasse was broken by the French meteorologist Léon Teisserenc de Bort. Recognizing the limitations of manned flights, Teisserenc de Bort turned to unmanned, hydrogen-filled paper and varnished-silk balloons known as ballons-sondes, or sounding balloons. These balloons carried lightweight, self-registering instruments—barographs to measure pressure and thermographs to record temperature—suspended in wicker baskets. To protect the temperature sensors from the heating effects of direct solar radiation, which had plagued earlier high-altitude measurements, Teisserenc de Bort designed a double-walled cylindrical brass shield that ventilated the thermometer using the balloon's ascent itself.

Beginning in 1896 from his private observatory in Trappes, near Paris, Teisserenc de Bort launched hundreds of these balloons. The methodology was meticulous: as the balloon rose, the decrease in atmospheric pressure caused the gas inside to expand until the envelope burst. A small parachute then deployed, carrying the instrument package safely back to Earth. Crucially, each basket bore a tag offering a monetary reward to any farmer or villager who recovered the apparatus and returned it to Trappes, ensuring a remarkably high rate of recovery.

As the data accumulated, Teisserenc de Bort noticed a recurring, baffling anomaly. At a certain altitude—roughly eleven kilometers (approximately thirty-six thousand feet) over middle latitudes—the steady decline in temperature abruptly ceased. Instead of continuing to drop, the temperature stabilized, hovering around minus fifty-five degrees Celsius, and occasionally even warmed slightly. Suspecting that the heating of the thermometer by solar radiation was still distorting the data, Teisserenc de Bort initially kept his findings quiet. He conducted subsequent launches exclusively at night to eliminate solar interference entirely. To his astonishment, the nocturnal data confirmed the daytime findings: the isothermal zone was a physical reality, not an instrumental artifact.

In April 1902, Teisserenc de Bort presented his monumental findings to the Paris Academy of Sciences. He proposed that the atmosphere was divided into two distinct regions. The lower layer, which he named the 'troposphere' (from the Greek tropos, meaning 'turning' or 'mixing'), was characterized by convective currents, shifting winds, and a constant decrease in temperature with height—the region where weather occurred. The upper layer, which he named the 'stratosphere' (from the Latin stratum, meaning 'layer'), was a region of relative calm where the temperature remained nearly constant and air moved in horizontal layers without vertical mixing.

Simultaneously and independently, German meteorologist Richard Assmann published similar results obtained using rubber balloons, which could reach higher altitudes than silk ones before bursting. While Assmann's use of rubber envelopes represented a significant technical advance, Teisserenc de Bort is widely credited with the discovery due to the sheer volume of his trials—over two hundred and thirty successful balloon flights by 1902—and his brilliant conceptualization of the two atmospheric zones. His work transformed meteorology from a localized, surface-bound study of weather patterns into a three-dimensional science of the global atmosphere.

Based on the passage, Teisserenc de Bort's decision to launch sounding balloons at night was prompted by a desire to resolve which of the following issues?