Africa's Mount Kilimanjaro is a fascinating study in climate. Located near the equator, it has six distinct ecological zones from base to summit, each with its own climate, flora, and fauna. The conditions that characterize each zone are dictated largely by the combination of temperature, altitude, and moisture. It's these conditions that ultimately influence the types of species suited (or not suited) to inhabit each zone — even for a short time.

Temperatures in Earth's troposphere — the lower atmosphere, up to about 9,100 meters (30,000 ft) at the poles and 16,800 meters (55,000 feet) at the equator — decrease rather predictably with altitude. With each 300 meters (1,000 ft) climbed, air cools by about four or five degrees Fahrenheit. As an air mass approaches a mountain, it is forced upward to an area of lower pressure, and it expands and cools as it rises. Once the air temperature falls to the dew point, water vapor in the air condenses to form clouds that drop precipitation onto windward slopes. As the air passes over the summit, conditions change. Because the air now contains less moisture, conditions on the leeward side are drier. As a result, climbers attempting to reach the summit of mountains like Kilimanjaro may experience very different weather conditions, depending on their ascent route.

Every living thing, from single-celled microbes to megafauna, is adapted to thrive in the conditions that characterize the particular ecosystem it inhabits, whether it is a rainforest, high desert, or host life form. Bacteria, such as those that infect human hosts, can only live and reproduce within a certain range of conditions. Temperature is a key determinant. At lower temperatures, molecules move slower. Enzymes that mediate critical biochemical reactions no longer function, and the increasing viscosity of the cell interior can cause cellular activity to slow or cease. By contrast, in warmer temperatures, molecules move faster, enzymes metabolize more quickly, and cells grow and replicate rapidly.

This explains mountain-climber Rob Taylor's final ordeal — the one he endured after being rescued from Kilimanjaro's arctic summit. His descent through warmer zones reinvigorated bacteria whose activity had slowed inside him during his ascent. As a result, his badly broken and infected leg raged with further swelling.

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