The mesosphere is considered the coldest layer of Earth’s atmosphere primarily due to its altitude and the way it interacts with solar radiation.
As we ascend through the atmosphere, the temperature typically decreases with altitude, especially in the troposphere and stratosphere. The mesosphere lies above the stratosphere, extending from about 50 to 85 kilometers (31 to 53 miles) above the Earth’s surface. In this layer, the air is very thin, meaning there are fewer molecules to absorb heat from the sun.
Moreover, the mesosphere lacks significant sources of heat. While the stratosphere contains the ozone layer, which absorbs and warms ultraviolet solar radiation, the mesosphere does not have a similar mechanism to trap heat. In fact, as the mesosphere rises in altitude, the temperature decreases quite dramatically, reaching lows of around -90 degrees Celsius (-130 degrees Fahrenheit) near its upper limits.
In summary, the unique combination of altitude, thin air, and lack of heat retention mechanisms makes the mesosphere the coldest layer of Earth’s atmosphere.