The relationship between elevation and temperature dictates the climate of vast geographic regions. Generally, as elevation increases above sea level, the air temperature progressively decreases. This consistent trend shapes the distribution of plant life and the existence of permanent snow caps on mountain peaks, even in tropical latitudes. Understanding this inverse relationship requires examining the physical properties of the Earth’s lower atmosphere.
The Observed Effect of Increasing Altitude
The cooling effect of altitude is easily demonstrated by comparing the weather in a valley city to that of a nearby mountain summit. On a hot summer day, the valley floor may experience high temperatures, while the mountain peak remains cool enough to require a jacket. This contrast exists because the air’s temperature structure is strongly influenced by the Earth’s surface, which is the primary source of atmospheric heat.
For instance, temperatures can drop by approximately 6.5 degrees Celsius for every 1,000 meters gained in elevation. This temperature gradient explains why high-altitude environments, even in warm regions, often feature tundra-like conditions or year-round snow and ice.
The Scientific Mechanism of Atmospheric Cooling
The cooling is driven by adiabatic cooling, a process directly linked to air pressure and density. The Earth’s surface absorbs incoming solar radiation and then heats the air immediately above it through conduction and convection. This means the atmosphere is heated from the bottom up, making the air warmest near the ground.
As a parcel of air rises, the atmospheric pressure pushing down on it decreases because there is less air mass above it. This reduction in pressure allows the air parcel to expand, which requires energy. This energy is drawn from the air parcel’s internal thermal energy, causing the molecules to slow down and the temperature to drop without any heat being exchanged with the surrounding environment. This process explains why a mountain summit is colder than the base.
Quantifying the Temperature Drop: Understanding Lapse Rates
Environmental Lapse Rate (ELR)
The rate at which temperature decreases with increasing altitude is quantified by the lapse rate, which varies based on atmospheric conditions. The Environmental Lapse Rate (ELR) is the actual measured temperature change in the atmosphere at a specific time and location. It averages about 6.5 degrees Celsius per kilometer in the lower atmosphere, or troposphere.
Dry Adiabatic Lapse Rate (DALR)
Meteorologists often use two theoretical rates, the Dry Adiabatic Lapse Rate (DALR) and the Moist Adiabatic Lapse Rate (MALR), to predict the temperature change of a vertically moving air parcel. The DALR applies to air that is not saturated with water vapor and is constant at approximately 9.8 degrees Celsius per kilometer. This rate is fixed because the cooling is purely due to the work of expansion.
Moist Adiabatic Lapse Rate (MALR)
When an air parcel rises high enough to cool to its dew point, the water vapor inside begins to condense into liquid droplets, forming clouds. This condensation releases latent heat back into the air parcel, which counteracts the cooling effect of expansion. Once condensation begins, the air parcel cools at the Moist Adiabatic Lapse Rate (MALR), which is lower and more variable than the dry rate. The MALR typically ranges from about 5 to 6 degrees Celsius per kilometer. The release of latent heat slows the cooling process, meaning saturated air cools at a slower rate than dry air as it rises.
Local Factors That Modify the Elevation Rule
The general rule of temperature decreasing with elevation can be locally disrupted by terrain and weather phenomena. One significant modification is a temperature inversion, a condition where the temperature actually increases with altitude instead of decreasing. This occurs when cold, dense air sinks and becomes trapped in valley bottoms, often on clear, calm nights, with warmer, lighter air sitting above it.
Geographical features also play a major role, specifically the orientation of a mountain slope, known as its aspect. Slopes that face the sun, such as south-facing slopes in the Northern Hemisphere, receive more direct solar radiation, leading to higher surface temperatures. Conversely, north-facing slopes remain shaded for longer periods, resulting in cooler surface conditions at the same elevation. This difference in solar exposure can cause substantial local temperature variations, sometimes up to 30 degrees Celsius, altering the microclimate compared to the average lapse rate.