And yet there would be more casualties from Tambora. In addition to millions of tons of ash, the force of the eruption threw 55 million tons of sulfur-dioxide gas more than twenty miles into the air, into the stratosphere. There, the sulfur dioxide rapidly combined with readily available hydroxide gas—which, in liquid form, is commonly known as hydrogen peroxide—to form more than 100 million tons of sulfuric acid. The sulfuric acid condensed into minute droplets—each two hundred times finer than the width of a human hair—that could easily remain suspended in the air as an aerosol cloud. The strong stratospheric jet streams quickly accelerated the particles to a velocity of about sixty miles per hour, blowing primarily in an east-to-west direction. The sheer power of the jet stream allowed the aerosol cloud to circumnavigate Earth in two weeks; but the cloud did not remain coherent.
Variations in the wind speed and the weight of the particles caused some parts of the cloud to travel faster or slower than others, and so the cloud spread as it moved around Earth, until it covered the equator with an almost imperceptible veil of dust and sulfurous particles. It also began to spread north and south, albeit far more slowly. While it took only two weeks for the aerosol cloud to cover the globe at the equator, it was likely more than two months before it reached the North and South Poles.
Rather than a slow, steady broadening of the equatorial cloud into the Northern and Southern Hemispheres, the cloud expanded in fits and starts. As some pieces of the cloud were blown away from the equator, they were quickly caught up in the dominant stratospheric jet streams—which in May blow east to west in the Northern Hemisphere, and west to east in the Southern Hemisphere. The cloud soon began to resemble streamers or filaments, with small portions regularly pushed off the equator and into the middle latitudes in each hemisphere. Eventually, these filaments coalesced into a single, coherent cloud that covered Earth.
And there they remained. Had the aerosol cloud ascended only into the lowest part of the atmosphere, the troposphere, where clouds form, rain would soon have cleansed the ash from the air. But in the more stable stratosphere, conditions mitigate against the formation of clouds of water droplets. The coldest air already is at the bottom of the stratosphere, with warmer air above it, so air rarely rises from the troposphere into the stratosphere. With no rising plumes of warm air to carry moisture into the stratosphere, clouds almost never form; the stratosphere is drier than most deserts. With no clouds, there could be no rain to wash away the stratospheric aerosol veil. Only the slow action of gravity and the occasional circulation of air between the stratosphere and the troposphere could drag the droplets back to the earth. And so the extraordinarily fine sulfur particles from Tambora that reached the stratosphere remained suspended in the air for years, freely transported around the globe by the winds. By the winter of 1815–16, the nearly invisible veil of ash covered the globe, reflecting sunlight, cooling temperatures, and wreaking havoc on weather patterns.