As a person ascends a mountain, the environment changes rapidly, a phenomenon known as vertical zonation. This ascent creates a steep gradient of interconnected environmental factors that determine which forms of life can survive at each elevation. The primary physical forces driving these dramatic shifts are changes in atmospheric pressure, temperature, and solar radiation, which fundamentally alter the climate and biology of the mountain landscape. Understanding these transformations is key to grasping why the summit of a mountain often resembles a polar region, even in the tropics.
Atmospheric Changes: Pressure, Temperature, and Sunlight
The most immediate physical change with increasing altitude is the decrease in barometric pressure, which drops because there is less mass of air pressing down from above. Although the air still contains about \(21\%\) oxygen, the reduced pressure means the partial pressure of oxygen also drops. This results in fewer oxygen molecules being pushed into the lungs with each breath. This thinner air creates the condition known as hypoxia, challenging the respiratory systems of all life forms.
Air temperature also declines consistently with elevation, following the atmospheric lapse rate within the troposphere. On average, the air temperature decreases by approximately \(6.5^\circ\text{C}\) for every \(1,000\) meters (\(3.5^\circ\text{F}\) to \(5.5^\circ\text{F}\) per \(1,000\) feet) of ascent. This cooling occurs because the atmosphere is primarily warmed by heat radiating from the Earth’s surface. Moving farther from that heat source causes the air to become colder.
In contrast to the drop in pressure and temperature, the intensity of ultraviolet (UV) radiation increases significantly with height. This rise occurs because the atmosphere is thinner, meaning there is less air, dust, and water vapor to absorb and scatter the incoming solar rays. UV radiation intensity can increase by approximately \(10\%\) to \(12\%\) for every \(1,000\) meters of gain, posing a threat to organisms who rely on the atmosphere’s filtering effect.
Defining Altitude Zones and Shifting Ecosystems
The combined effects of low pressure, low temperature, and high UV exposure drive the creation of distinct ecological layers, or altitudinal zones. These zones mimic the environmental shifts seen when traveling from the equator toward the poles. They typically transition from lower-elevation montane forests to subalpine zones, then to the exposed alpine zone, and finally to the nival zone of permanent snow and ice. The most visually dramatic boundary is the tree line, which marks the upper elevation limit where trees can grow.
Above the tree line, where the mean monthly soil temperature never exceeds \(10^\circ\text{C}\), trees give way to smaller, hardier vegetation. The extreme cold, high winds, and thin soils force plants into specialized forms, such as low-lying cushion plants or mats. These forms help them retain heat and avoid wind damage. Trees near this boundary often become stunted and deformed, a growth pattern known as krummholz, or “crooked wood.”
Animal life adapts to the harsh alpine environment through various strategies. Mammals often possess specialized coats for insulation or exhibit behavioral strategies like hibernation to survive the long, cold winters. Species at these elevations have evolved physiological mechanisms, such as larger lungs or specialized hemoglobin, to maximize oxygen uptake from the thin air. This allows them to thrive where lowland species would struggle.
The Physiological Impact on the Human Body
The human body reacts immediately to the drop in the partial pressure of oxygen, initiating a complex process called acclimatization. Within the first few hours, the body increases its breathing rate and heart rate to draw in more air and circulate oxygenated blood faster. Over days and weeks, a longer-term adjustment involves the kidneys producing a hormone that stimulates the bone marrow to generate more red blood cells, increasing the oxygen-carrying capacity of the blood.
If the ascent is too rapid, the body may develop Acute Mountain Sickness (AMS), the mildest and most common form of altitude illness, typically occurring above \(2,500\) meters (\(8,000\) feet). Symptoms often resemble a severe hangover, including headache, nausea, loss of appetite, and fatigue. For most individuals, these symptoms resolve within a day or two if the ascent is halted, allowing the body time to adjust.
Failure to descend or continued ascent with AMS symptoms can lead to two severe, life-threatening conditions: High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE).
High Altitude Pulmonary Edema (HAPE)
HAPE involves a dangerous build-up of fluid in the lungs. It is characterized by severe shortness of breath even at rest, a persistent cough, and a rapid heart rate.
High Altitude Cerebral Edema (HACE)
HACE is swelling of the brain tissue. It presents with symptoms like severe headache, profound confusion, and ataxia, which is a loss of coordination that makes walking difficult.
HAPE is the most frequent cause of death related to altitude illness, while HACE is the most severe form. Recognizing the difference between mild AMS and these severe forms is paramount, as the only definitive treatment is immediate descent to a lower elevation. Preventive measures emphasize a slow rate of ascent. They also include never moving to a higher altitude to sleep if symptoms are present and maintaining adequate hydration throughout the journey.