The atmosphere is a complex, layered system. The mesosphere is the third major layer, positioned directly above the stratosphere and below the thermosphere. This region begins at an approximate altitude of 50 kilometers and extends upward to about 85 kilometers. It is a largely unexplored region where the air becomes exceptionally thin, featuring unique thermal characteristics and atmospheric phenomena.
Defining the Mesosphere’s Location and Boundaries
The mesosphere is defined by its vertical position in the atmosphere, sandwiched between two other distinct thermal layers. Its lower boundary, the stratopause, marks the top of the stratosphere at roughly 50 kilometers where the temperature profile changes direction. The layer extends upward until it reaches the mesopause, a boundary that sits at approximately 85 kilometers and separates it from the thermosphere above.
This altitude range is often referred to as the “middle atmosphere.” The air is already extremely rarefied compared to lower layers. Within the mesosphere, the atmospheric density decreases rapidly with increasing height. The scarcity of gas molecules means that conventional aircraft and weather balloons cannot reach this layer, making it difficult to study directly.
Extreme Cold: The Atmosphere’s Deep Freeze
One of the mesosphere’s most remarkable characteristics is its temperature profile, which makes it the coldest region in Earth’s atmosphere. Unlike the stratosphere below it, temperatures decrease dramatically as altitude increases within the mesosphere. This cooling effect is due to the lack of ozone, which would otherwise absorb solar ultraviolet radiation and generate heat.
At the very top of the layer, at the mesopause, temperatures can plummet to an average of about \(-90^\circ\text{C}\) (\(-130^\circ\text{F}\)). The distance from the warm surface of Earth, which re-emits absorbed solar radiation, also contributes to the intense cold. This extreme temperature minimum forms the boundary between the mesosphere and the thermosphere above it.
Celestial Activity and Unique Clouds
The mesosphere serves as a natural protective shield for the planet, as it is the layer where most incoming space debris is destroyed. As meteoroids enter the atmosphere at high velocity, they experience intense friction with the gases in the mesosphere. This friction causes the meteors to heat up and vaporize, creating the bright streaks of light commonly known as “shooting stars”.
This process leaves behind a continuous supply of microscopic particles, known as “meteor smoke,” suspended at these high altitudes. These particles play a crucial role in the formation of noctilucent clouds (NLCs), which are the highest clouds in the atmosphere. NLCs are composed of tiny ice crystals that form around the meteor smoke particles acting as condensation nuclei. These electric-blue clouds are only visible from the ground at twilight when sunlight illuminates them high above.